Bartnikas - Partial Discharges Their Mechanism, Detection and Measurement - 2002

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 9 No. 5,OctoberZOOZ 763

Partial Discharges Their Mechanism, Detection and Measurement

R. Bartnikas lnatitut de Recherche d'KydroQuehec, Varennes, Quebec, Canada

ABSTRACT Different partial discharge (PD) detection and measurement procedures suitable for use on cables, capacitors, transformers and rotating machines are examined and compared. Both narrow and wide bandwidth PD detectors are considered; particular attention is given in regard to their suitability to different types of electrical apparatus and cable specimens under test as well as their applicability to discharge site location and their capability to detect different forms of PD. A rather substantial portion of the discussion is devoted to the use of intelligent machines as applied to PD pattem recognition in terms of either PD pulse-heightldischarge epoch (phase) distributions or discharge pulse shape attributes.

1 INTRODUCTION HE subject of PD or corona discharges, which represents an an- T tecedent term that has been commonly applied to them in the past,

constitutes a field of endeavor which can be traced back to the begin- nings of the twentieth century [14]. While the study of PD may thus be considered as a well developed field, its preeminent importance as a tool for assessing the quality and performance characteristics of HV equipment has been responsible for sustaining a high level of activity in investigations related to its mechanisms, physical and chemical effects, detection and measurement techniques: 15-71, Over the years, the level of investigative effort in the PD field has varied considerably both as re- gards to the type of electrical apparatus under consideration as well as the type of discharge behavioral aspect being examined, e.g. nature and form of the discharge, detection sensitivity, degradation of insulation exposed to PD, discharge pulse quantities recorded such as the apparent charge transfer, pulse repetition rate, energy loss, distributions of pulse- heights, discharge epochs (phase) and pulse separation time intervals, as well as pulse pattern recognition in terms of the sources causing the discharge.

Perhaps nowhere are the different tendencies in PD studies and testing procedures easier to follow and evaluate in their chronological development over last five decades than those applicable to solid extruded dielectric insulated power distribution cables. Tnis is attributable to a large extent to the relatively simple geometry of cables and their transmission line behavior, which greatly facilitate the interpretation of the I'D measurements. While few interpretational difficulties arise in low capacitance lumped HV components such as bushings and capacitors, the detection of PD in capacitors of high capacitance poses substantial difficulties. Discharge detection and its accurate measurement in transformer specimens becomes appreciably more complicated as a result of a more complex transmission line behavior of the coils as well as cou-

pling and resonance effects between the windings. Similar interpretational and calibration difficulties are encountered also with rotating machines, where, in addition to the difficulties inherent with transformer specimens, the magnitudes of the detected pulses may vary appreciably, ranging from low levels generally intrinsic to internal discharges within stator bar insulation to extremely high levels ordinarily associated with slot discharges. Also the question of calibration has not been resolved and there is indeed considerable controversy as to whether or not calibration should be a prerequisite for rotating machine specimens. The intent of this paper is to examine the PD mechanism and its behavioral characteristics and to delineate and compare the I'D detection and measwement procedures that have evolved over the past five decades, which are either currently, or may be, utilized on different HV electrical apparatus and cables.

2 PRELIMINARY CONSIDERATIONS

Oscillograph methods have been employed in the detection of PD in electrical apparatus and cables, following the work reported by Tykociner et ai. in 1933 [8,9]. These techniques respond principally to pulse type discharges; while pulseless glow and pseudoglow discharges can readily occur, their appearance is generally accompanied by the occurrence of pulsed type discharges that can be readily detected so that in the vast majority of cases conventional PD pulse detectors are effective indicators of the presence of PD. However, it should be borne in mind, that pulsed PD detectors may not always indicate the full extent of intensity of the PD present. There are bridge type PD detectors available that respond to both pulse and pulseless discharges 15,101, but their intrinsically lower sensitivity has tended to impede their large scale implementation in the PD area.

Although the fastest rise time limit of pulse type discharges at the

107&9878/2/$3.00 0 2002 IEEE

764 Bartnikas: Partial Discharges

PD site of origin may be established theoretically, historically the fastest measurable rise times, claimed to be recorded experimentally, tended to suggest a monotonically decreasing relationship with the bandwidth capability of the oscilloscopes utilized. With the availability of GHz bandwidth oscilloscopes, it is now generally agreed that PD pulses may have rise times as short as 1 to 2 ns, which should however not be taken tacitly to infer that most discharges do exhibit these rapid rise times. Thus detection of PD pulses at frequencies at bandwidths up to 1 GHz is suitable; but cognizance should be made of the fact that the energy content of PD pulses is a decreasing function of frequency Commercially available conventional PD detectors for routme use on cables, capacitors and transformers are of the narrow band type and are designed to operate within the band of - 30 to 400 kHz; they are charge integrating devices and may be calibrated directly to provide the charge transfers associated with detected discharge pulse in accordance with ASTM method D1868 [ll]. Higher bandwidths are utilized in research related work, where faithful reproduction of the PD pulse shapes is of paramount importance. Also for improved pulse resolution, wider bandwidths are employed on work involving discharge site location in cables (- 20 MHz), rotating machines (800 lcHz to 1 GHz) and bus ducts as well as compressed gas cables (- 1 GHz).

The early PD detection systems employed analog instrumentation. This instrumentation performed adequately well for discharge inception and extinction voltage measurements; with the PD pulse pattems displayed oscillographically on a power frequency time base and calibrated ordinate scale, the charge transfers associated with the discrete discharge pulses could be estimated visually and the approximate phase relationship between the pulses and the applied voltage noted by the observer performing the tests. The availability of crystal controlled pulse counters in the 1950s, allowed the counting of PD pulses per unit time and thereby the determination of the pulse density of discharge patterns 112,141, and permitted as well the development of differential pulse-height analyzers [15] and single channel pulse-height analyzers [16]. The availability of low cost A/D converters led to the commercial introduction of multi-channel analyzers in the 1960s suitable for PD pulse-height distribution analysis [17]. The area of discharge pulse interval and discharge pulse epoch (phase) distribution measurements developed rapidly thereafter in the 1970s [la, 191 and was extended into the practical area with application to rotating machines [ZO]. This was shortly followed by the introduction of computerized techniques for the measurement of PD pulse distributions [21-231.

ne advent of PC computers in the 1980s and their extensive use in the 1990s rapidly altered the approach in the PD pulse distribution analysis area in that the measurement systems shifted away from the hard- ware based instrumentation to software dominated techniques [24-28]. This study area eventually led to investigations on discharge pattern recognition and classification, involving the use of neural network (NN) 129-321 and fuzzy logc [33]. Early studies indicated that the magnitude of a discharge pulse and its epoch or phase of occurrence is strongly influenced by the occurrence of a preceding pulse or pulses (341. This non-Markovian point process was rigorously analyzed using a stochastic approach by van Brunt 1351 in order to elucidate the conditional statistical nature of the discharge mechanism. The obtained results pose some serious questions concerning the effectiveness of PD pattern clas-

sification and recognition as well as any attending statistical treatment of such data to render it more amenable to interpretation. Yet it must be observed that the statistical treatment of PD pattems whether of the pulse-height/phase distribution [36] or pulse shape [30,37] type have yielded some interesting practical results.

The 1990s saw the introduction of rapid response digital circuits for PD measurement applications 138,391. While the use of digital techniques in PD pulse detection, measurement and acquisition has been growing markedly, commercially available PD detectors have retained their separate analog and digital measurement options. In this respect it should be emphasized that the peak PD pulse magnitude determined by the digital system will not generally be the same as the true magnitude determined in real time by the analog circuit because of its dependence upon sampling rate, bandwidth and storage capacity (if the digital system. It should be also pointed out that normally the analog circuitry precedes the digital acquisition system for the purpose of PD signal amplification andtshaping [40]. Also often the PD sensing circuit may be of an analog-digital hybrid configuration 1411. The variety of digital circuits, available and in-use for PD measurements over the last decade has evoked the publication of a position paper by the IEEE Committee on Digital Measurement Techniques [42] and a subsequent paper with invited discussions by experts in the field [43]. We shall devote a considerable effort in the paper towards a critical examination of the various analog and digital techniques either currently in use or with possible future application to PD measurements on HV power apparatus and cables.

3 PD MECHANISMS Proper design, application and deployment of PD sensing and mea-

surement circuits entail a certain degree of cognizance and understanding of the PD processes. It is important to use proper and correct terminology in reference to different forms of PD to maintain clarity in the subject. For example, it is one matter to refer to some PD as ‘streamer- like’ discharges and another matter to refer to the same form of discharge as ‘a streamer discharge’. The streamer discharge theory developed independently by both Rather [44] and Meek [45], considers large gaps, in which the relatively short times of gap breakdown are accounted for by the occurrence of streamer discharges, which propagate rapidly across long gaps due to ionizing photon radiation at the streamer tips or leads. Thus, the use of the term ‘streamer’ by itself, when applied to PD in relatively short gaps or small cavity diameters, introduces unnecessarily a misleading term in the PD lexicon.

PD, when occurring in short gaps, may assume different forms: rapid and slow rise time spark-type pulses, true pulseless glows or pseudoglow discharges [46-52]. All these forms of discharges are cathode emission-sustained discharges i .e . they are essentially Townsend discharges in contradistinction to streamer discharges whose distinguishing features are their independence on cathode emission and their dependence upon photoionization in the gas volume. The classical Townsend discharges are characterized by weakly ionized plasma having a small space charge producing field, which is negligible compared to the externally applied field. Its electron temperature is approximately 104 K and the dominant ionization process is by direct ionization.

IEEE Transactions on Dielectrics and Electrical Insulation Val. 9 No. S,October2002 765

4000, i0.5

(a) -100 0 I00 Time (pS)

Figure 1. Glow discharge in a 0.5 mm gap in nitrogen at atmospheric pressure. (a) voltage across gap photomultiplier current; (b) microavalanche discharge site pattern on ground electrode (after Miralai el ai. [62]).

4000 I I

5' 2000

.a 2 a 0 B & -2000 4

v

a'

>

.- -

-4000 1 , , ~ , ]

Figure 2. Spark (pulse) discharge in a 0.5 nun gap in nitrogen with an oxygen admixture at atmospheric pressure. (a) voltage across gap, photomultiplier current (b) discharge site pattem on ground electrode (after Miralai el ai. [63] ) .

The true glow or pulseless discharge consists of weakly ionizing diffused plasma generally occupying all available interelectrode space. Appreciable space charge formation occurs in both the proximity of the anode and cathode and the discharge process as in the case of the classical Townsend discharge is maintained through cathode emission. A glow discharge is not in local thermal equilibrium and the electron temperature ranges from lo4 to 2x104 K. Direct ionization plays a significant role and step-wise ionization, while negligible at low currents, may become appreciable at currents in the range of 10' mA. The pseudoglow discharge is similar to the pulseless glow in the degree of ionization, electron temperature and particle densities, but exhibits at the same time the presence of minute discharge pulses having features characteristic of spark type discharges. The presence of the minute pulses is readily detected electronically [Ma] or optically by means of a photomultiplier 153,541.

The presence of oxygen within a discharging cavity tends to inhibit

the occurrence of pseudoglow and pulseless glow discharges, because of the electronegativity of the oxygen gas, which reduces the availability of free electrons necessary for discharge initiation and limits the expansion of discharge channel necessary for the formation of glow discharges. This is evident from the very regular pulse-type discharge behavior observed with oxygen [55] as compared to the predisposition of other gases to support atmospheric pressure glow discharges under certain conditions [48]. It also accounts from the predominance of spark or pulse type discharges in air and the transition from a glow discharge process to pulse discharge type behavior in other gases (He, Ar, N?) upon the addition of small trace amounts of oxygen [56,57]. It is also observed that in cavities containing air-like atmospheres, de- pletion of oxygen from the atmosphere and the surrounding surfaces coupled with increases in the conductivity of the walls due to depo- sition of acidic reaction products created by the discharge, will also cause a transition from spark (pulse) to pseudoglow and pulseless glow


discharges to take place 153,541. A gap space undergoing pseudoglow or pulseless glow discharge

can superficially exhibit a uniform glow over its surface, which may not be always uniform in a strict sense, but consists of a multiplicity of light emitting dots that are caused by microavalanches [SE, 591. The arrangement or array of the microavalanche sites, constituting the overall dot pattern, is a function of their density, which in turn is a function of the applied voltage 158-621. The microavalanche dot patterns may be readily observed via a light transparent indium tin oxide ground electrode deposited upon a glass surface, which acts as dielectric barrier to the discharge in metallicidielectric electrode gap. An intensified charge injection device (CID) camera is used to record the discharge site/dot images, employing a voltage-phase resolved synchronizing circuit in conjunction with a pulse generator required to trigger the CID camera shutter 1631. Figure l(a) depicts the voltage waveform across the gap and the recorded light pulse produced by the glow over each half cycle of the applied voltage wave and Figure I@) portrays the resulting microavalanche site configuration pattern. Figures 2(a) and 2(b) illustrate the effect of an oxygen admixture to the nitrogen as the discharge pattern reverts back from a glow discharge to a spark/pulse type discharge, with numerous pulses in evidence in the photocurrent trace; the evidence of some minute pulses in the trace is indicative of a pseudoglow, which OCCUIS concurrently with the large pulse (spark) type discharges. The audio frequency test voltage accentuates the phenomena, which also occur over lower frequencies.

The short gap pulse or spark PD is similar to the pulseless glow and pseudoglow discharges in that it is also a Townsend type discharge, its underlying distinguishing attribute being that it is characterized by a higher degree of ionization and conductivity; but the discharge is still far removed from having achieved local thermodynamic equi- libration. It occurs within a brightly luminous narrowly constricted channel as opposed to the relatively faint emitted diffused glow of a pulseless or pseudoglow discharge; yet it also is sustained by cathode emission. Its electron density is still considerably below cni3. Spark-type PD are commonly classified as rapidly and slowly developing sparks or pulses, which are detected by the external PD sensing circuits as high-amplitude fast rise time and low amplitude slow rise time pulses respectively 151,521. The former are also frequently referred to as 'streamer or 'streamer-like discharges', and the latter as Townsend- type discharges [64-68]. The term streamer-like has been applied to rapid rise-time short-duration pulses, because of its similarity in form to the rapid streamer pulses in long gaps [64,65]. However, there is an important subtle difference in the mechanisms of these two forms of discharge. Since the mechanisms of development of streamer related pulses in long gaps involves ionization wave propagation in a very high field region where the ionization and influx of electrons at the discharge head is produced by a space charge field due to separation of positive and negative charges, the use of the term 'streamer' to denote a rapidly developing discharge pulse in a short gap is misleading. Both rapidly and slowly developing sparks or discharges pulses involve the cathode feedback mechanism of the Townsend type, albeit that in the rapidly developing pulse discharge, the classical ion induced cathode emission process plays a very minor role because of the predominance of the space charge mechanism in the vicinity of the cathode which gives rise

to very intense photoemission at the cathode. In the field of PD measurement, there has been relatively little atten-

tion paid to the detection or measurement of pulseleas glow and pseudoglow discharges [5,6,34,46,47,52,54,69,70]. Traditionally, since the early introduction of oscillographic techniques to PD studies, PD in electrical apparatus have, in the most part, been predominantly detected and measured on electrical apparatus and cables in terms of pulsetype discharges. In retrospect much of this tendency must be attributed to the ease with which pulse-type measurement techniques may be deployed and utilized, particularly more recently with increased usage of signal processing procedures. This inordinate preoccupied tendency with only pulse discharges has resulted in relegating the existence of pulseless glow and pseudoglow discharges to convenient oblivion. Nevertheless, it must be also emphasized that the form of discharge in physical cavities is rarely, if ever, only of the pulseless or pseudoglow type; most frequently, it is found that all these types of discharge, namely pulseless gloM: pseudoglow and pulse or spark type may occur simultaneously over each applied voltage cycle. This then is the redeeming feature of PD pulse detection methods: they are sufficient per se to indicate the presence of PD, the PD inception point and the PD pulse intensity, even though they may not always indicate the full extent of the overall discharge process that may comprise the concurrent occurrence of pulseless glow and pseudoglow discharges to which they fail to respond. A case in point is the dissipation factor (tan d) measurement, which is normally performed on stator bar insulation and oilipaper insulated cables to assess their quality, using either a Schering or a ratio-arm bridge. Frequently, it may be found that the increase in tip-up of the tan 6 value with voltage may not be fully ac- countable by sum of the PD pulse type losses and the dielectric losses, conceivably indicating significant pulseless and pseudoglow discharge loss contribution to the overall tan 6 value.

Attempts have been made to ascertain theoretically the conditions that favor discharge channel expansion i.e. transition from a spark to a glow discharge. Numerical model studies have been carried out on the short gap breakdown to examine the theoretical aspect of discharge channel constriction and expansion in helium, hydrogen and air at atmospheric pressure 149-52,71-75]. Discharges were found to exhibit an increased propensity towards discharge channel broadening when dielectric. surfaces are involved as is substantiated in Figure 3 1751. While initially the electrons are confined to a relatively constricted channel of a radius of - 1 mm at dischargedevelopment timeof2.l0ns, the radius of the channel is seen to broaden to = 7 nun at a time of 4.47 ns. The positive ion density within the discharge channel is found to exhibit similar tendencies. Further calculations made by Nikonov et al. 176,771 show that the discharge channel expansion rate is also influenced by the magnitude of the charge density and its distribution remaining from previous discharges. Experimental data have also demonstrated that accumulation of surface discharge products and the associated changes of surface conductivity (which affect the surface charge distribution) favor the occurrence of glow and pseudoglow discharges.

In view of the current emphasis on PD pulse detection techniques as concerns electrical power apparatus and cables, we shall omit further detailed discussion on pulseless and pseudoglow discharges and devote the remainder of the paper to an in-depth discussion on the sub-


z y - E OS I 0.6 i

0.4

3 0.3 C

8 9 0.2 e, m 0.1

n -

of changes in the breakdown voltage and, hence, the resultant PD pulse magnitudes, which are caused by variations in the statistical time lag i.e. the time required for a free electron to appear in the cavity volume where it is necessary to initiate the electron avalanche required to pro-

--duce voltage breakdown across the cavity. This means essentially that the discharge epoch of a preceding discharge pulse will influence the discharge epoch or phase with respect to the applied voltage wave at which the subsequent discharge will occur. However, in addition the position of the discharge epoch of each subsequent discharges will in turn be also affected by the time of appearance of initiating electron in its own environment. The overall result of the precession of discharge epochs is that each discrete pulse discharge event in a given cavity is controlled to some extent by the time occurrence of its preceding discharge. This stochastic discharge behavior, which has been more recently examined to a considerable depth by van Brunt and von Glahn 138,391, poses Some important ramifications as to the degree of accuracy with which PD patterns may be recognized and related to cavity size and its location in different types of power apparatus and cables.

Radial distance. r (mml

Figure 3, calculated radial distribution of the relative eleckon den. sity, with time as a parameter at ,,,+,int for 0,500 " gap with a,, insulating anode in an air-like mixture at ahnospheric pressure, subjected to a geometrical field of 65.4 kVcd (after N o d and Bartnikas W1).

-

-

-

-

-

-

ject related to PD pulse characteristics and associated test methods. If we consider an idealized cavity occluded within an insulating system that is subjected to a sinusoidally varying applied voltage and further assume that the cavity undergoes only pulse or spark type discharge, then the cavity will discharge when the voltage across the cavity attains its breakdown value. At this point in time or voltage wave phase (discharge epoch) the voltage across the cavity will collapse abruptly either to zero or some finite value normally named the 'residual voltage'. Tne resultant generated voltage step will excite the FD pulse detection circuit and the spark-type generated event will be detected as a discrete PD pulse. Further pulses will be generated along the ascending and descending portions of the sinusoidal voltage wave each time the applied voltage exceeds an integer value of the breakdown voltage value in the two polarities [34]. If the breakdown and residual voltages in the two polarities equal and constant, all detected PD pulse will be of equal magnitude. However, this is not a common occurrence in practice.

The response of PD pulse detectors decreases with increasing rise time of the detected I'D pulses [78]. The rise time of the incident PD pulse front at the PD detector input is determined by the initial PD pulse front rise time at the discharge site and any subsequent degradation of the PD pulse rise time along its transmission path from its site of origin to the PD detector end. The latter effect is of particular importance in specimens which exhibit transmission line behavior, e.g. cables, transformers and rotating machines. However, there are also some important variations within the spark discharge mechanism per se, which may effect significantly the rise time of the discharge pulse formed at the site of its origin as the cavity undergoes successive discharges.

When the PD pulse discharge pattern is viewed superimposed upon the power frequency sinusoidal wave, it is observed to exhibit appreciable instability The amplitudes of the discrete pulses are perceived to undergo substantial fluctuations accompanied by significant displacements of the associated discharge epochs themselves. The phase variations of the PD pulse positions with respect to the applied sinusoidal voltage wave have been referred to as 'precession of discharge epochs' [34,79]. This precession of discharge epochs may be explained in terms

1 AV = 1.9~10" V Pa-!

, , - Total current Ion current

AV= 1 . 1 x104VPa-1

AV = 33x10-5 V Pa- "

0 10 20 30 40 SO 60 70 80 90 100 Time, t (ps)

Figure 4. Calculated breakdown current pulse forms of a 0.5 mm gap in air at atmospheric pressure under negligible space charge conditions, with the overvoltage AV as a parameter. The electron current is given by the difference between the total and ion current values (after Barhikas and Novak [SO)).

The effects of the statistical time lag on the PD pulse shape, magnitude and rise time have been analyzed theoretically by Bartnikas and Novak [SO]. The retarded appearance of the initiating electron in a given cavity means that in the absence of the electron at the point in time when the applied ac voltage becomes just equal to the breakdown voltage of the cavity, breakdown will not occur and the applied voltage w d l continue rising until such time when an initiating electron becomes available. The consequence will be that the breakdown of the cavity will take place at a higher voltage, thereby generating a greater magnitude PD pulse. To illustrate the effect of a protracted statistical time lag upon the magnitude and rise time of the ensuring PD pulse, one must consider the manner in which the PD pulse shape is altered as a function of the overvoltage across a cavity or short gap.

Figure 4 portrays the shapes of calculated breakdown current pulses obtained with a 0.5 nun gap in air under atmospheric pressure at low


values of overvoltage in ascending order of magnitude [SO]. Note that in order to take into account gas pressure effects, the overvoltages are expressed in VPa-' units. At low overvoltages, the peak breakdown current is seen to decrease very substantially with falling overvoltage; the marked reduction in the peak value of the breakdown current is alsd' accompanied by a pronounced increase in the rise time of the pulse. The slow low amplitude current pulses are essentially pulses formed by the movement of ions across the gap, since the electrons, released from the cathode due to ion impact necessary to sustain the Townsend type discharge, are more rapidly swept out of the gap than the slow moving ions.

AV = 2.3~10-4 V Pa-1

Total current Ion current

3 0.5 0

12.8 12.9 13.0 13.1 13.2 13.3 13.4 13.5 13.6 Time, t (ps)

Figure 5. Calculated breakdown current pulse form of a 0.5 mm gap in air at atmospheric pressure at a higher overvoltage under modest space charge conditions (after Bartnikas and Novak [80]).

Hence the contribution of electrons to the breakdown current pulse is relatively small, as indicated in Figure 3. It is thus palpably evident that at extremely low overvoltages across a cavity undergoing Townsend type pulse discharge, the generated pulse magnitudes with inherently long rise times, may render their detection difficult with conventional PD pulse detectors having high lower cut-off frequencies. At higher overvoltages, increasingly more electrons are emitted from the cathode leaving behind the slower moving ions of polarities, causing the accumulated space charge due to the positive ions to augment the ionization rate in the proximity of the cathode. As the positive ion space charge effects become more predominant in the relation to the classical Townsend process of electron emission resulting principally from positive ion impact at the cathode, the pulse form of the current is perceptibly altered.

Figure 5 delineates the positive ion space charge influence, which becomes readily perceptible at ion densities of 10"' ~ m - ~ . In addition with a significant overvoltage across the gap, resultant rise in the electrical field leads to a substantial increase in cathode emission due to photoionization, which results in a breakdown current pulse with a shorter rise time and larger magnitude as compared to the current pulses in Figure 4 obtained under negligible space charge conditions. This occurs as the space charge field attains a value of zz 30% of the external field. Note the protracted ion current tail evinced in the overall pulse current shape, which is caused by the ion drift current and the associated electron current due to electron emission resulting from ion impact at the cathode (following the curtailment of photoemission at the cathode).

1 Time, t (ns)

Figure 6. Calculated breakdown current pulse of 0.5 mm gap in air at atmospheric pressure under intense space charge conditions at high overvoltage (alter Bartnikas and Novak [SO])

Greatly enhanced space charge induced photoionization at the cathode, at applied voltages appreciably above the breakdown voltage, leads to PD current pulses with very much reduced rise times and greatly augmented peak amplitudes as illustrated in Figure 6. Conspic- uously absent from the breakdown current pulse form is the protracted ion tail component, which is prominent at modest space charge densities in the vicinity of the cathode (Figure 5). The relatively miniscule ion current contribution (due to the ion impact phenomena at the cathode) to the overall discharge pulse at high overvoltages is now completely obscured by the very intense electronic current component caused by photoemission at the cathode associated with the pronounced space charge formation in the proximity of the cathode. The PD current pulse form is typical of a large spark-type discharge, whose rise times may range from - 1 to - 10 ns. The probability of occurrence of large magnitude rapid rise time pulses would be expected to be greater in areas of insulation well shielded against cosmic radiation and free of electrical field enhancement sites to reduce the availability of free electrons. As concerns the physical process in the generation of the rapid rise time pulses, it should be noted that at high overvoltage across the cavity, the higher resultant electrical field E increases the ionization frequency v, in accordance with

where ne is the electron density, a the first Townsend ionization coefficient, and p e the electron mobility An enhanced value of y~ leads to a rapid rise in the number of electrons and positive ions within the cavity Since the electron density in the proximity of the cathode is con- strained by the cathode emission influx while the positive ion density is determined by the rate of volume ionization and influx from within the cavity, the positive ion charge, under intense ionization conditions, rapidly exceeds the electron charge thereby giving rise to a very intense cathode field. This results in a reduction in the field across the remainder of the cavity, causing the major portion of ionization to be confined within a thin layer adjacent to the cathode in the pr6sence of a very substantially augmented photoemission. Estimates place the positive ion density beyond 10" which is approximately two orders of magnitude in excess of the electron density Under such conditions, the cathode field may attain values as high as 1.5 MV/cm, i.e. in the region of the field emission threshold; at these fields, formation of thermal emission spots is likely to occur. This may account for the

U" = neaEpe (1)

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 9 No. 5, October ZOO.?

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initiation of electrical trees at the extremities of discharging cavities in which the discharge process is dominated by rapid rise time spark type discharges.

2 . 4

Figure 7. Integral value N of the number of ionizing events (which equals the number of electropositive ion pairs) as a function of the ionization development time of a 5 pm microcavity subjected to an electric field of 270 kV/cm (after Novak and Bartnikas [811).

The statistical time lag, which determines the value of applied voltage at which breakdown ensues across a given cavity, has important practical ramifications in the area of PD detection and measurement, because it influences the rise time, magnitude and shape of the detected PD pulses. Since the appearance or occurrence of free discharge initiating electrons is controlled by cosmic radiation or other sources (such as field emission at field enhancing asperities) at or in the vicinity of the occluded cavities, a single discharge site in a cavity of given size and gas pressure within may generate pulses of different shape, amplitude and rise time depending upon the statistical variations in the appearance or absence of the initialing electrons. In juxtaposition, two identical discharge sites in two similar cavities, located at different areas in a given insulating system, even though under identical stress conditions may produce dissimilar PD pulse shapes and pulse distribution spectra because of a difference in the rate of free electron availability in the two respective cavity locations, This aberrational behavior, coupled with the !act that the response of PD detectors is PD pulse rise time dependent, may result in considerable interpretation difficulties in PD pattern recognition related work.

Often the question arises as to what extent an ionization process may be maintained in microcavities and whether it can lead the development of PD. A recent study of a 5 p m size microcavity indicates that at atmospheric pressure, while an ionization process may be initiated, the number of collisions are by far too few to result in PD development [XI]. Figure 7 shows the number of ionization events DS. the ionization development time in a 5 p m microcavity subjected to an electrical field of 270 kV/cm, a value corresponding to the maximum field gradient at the conductor in HV polymeric transmission cables. Following the release of a free initiating electron at the surface of the cathode, the number of ionization events is seen to increase rapidly with the ionization process attaining full development after an elapse of - 7 11s; the total number of ionization events (electron/positive ion pairs produced) is found to be equal to 7 and is reached after 0.72 ns. Here it should be noted that n a b a l radiation within a miniscule gas space of 5 p m is unlikely to provide a free electron within a reasonably short time necessary to

initiate the ionization process. However, free electrons should become readily available at higher electrical stresses due to Schottky emission or the hot (mobile) electrons within the structure of solid or solid-liquid dielectrics [XZ].

- s i - Positive ions B J

Time. t ($)

Figure 8. Charge transfer due to electrons and positive ions across a 5 microcavity us. the ionization development time at an electric field of 270 kV/cm (after Novak and Bartnikas [Sl]).

Lower elecmnic

J 4

Time. t (s)

Figure 9. Number of excited molecules (upper and lower electronic levels) within a 5 pmmicrocavity us. the ionization development time at an electric field of 270 kV/cm (after Novak and Bartnikas [811).

Figure X depicts the charge transfer associated with electrons and positive ions as a function of the ionization development time in the 5 pm cavity, Compared to even the lower measured levels of the charge transfers due to PD (e.g. - 0.1 pC), the charge transfers involved are indeed minuscule. While in terms of the energy dissipated their effects may be negligible compared to actual PD, in the long term they may produce degradation of the insulating material. The ionization process within the 5 p m cavity results in the production of excited states, whose energies may be sufficient to cause bond scission of polymeric materials which may lead to gradual deterioration of the insulant. This long term process will result in the enlargement of cavities and electrical tree formation with attending PD 1811. Figure 9 portrays the number of molecules excited into upper and lower electronic states as a function


with the consequence that newly produced polymeric cables leaving the manufacturing facility are now free of discharges not only at the operating voltage level, but also at voltages above those levels which can he envisaged to be imposed upon the cables under surge voltage conditions.

HV supply R.F. choke

....

Figure IO. Early schematic circuit arrangement for the measurement of the pulse discharge rate and differential PD pulse-height analysis on short cable specimens cn. 1966 (after Bartnikas 1151).

of the ionization development time in the 5 p m cavity. Here the upper excitation levels denote the energy range between 8.4 to 15.7 eV and the lower levels from 8.4 down to 7.5 eV Since estimates of cleavage of C-C and C-H bonds fall in the region of 10 to 11 eV 184,851 and 7 to 10 eV 186, 871 respectively, the electronic levels of the excited molecules within the 5 p m ionized cavity would thus he sufficient to result in long term damage of the insulating system. Hence, the theoretical analysis sug- gests that while microcavities are insufficient in Sire to Support PD, the ionization events within them that are too minute to register a response on conventional PD pulse detectors, are still capable of producing long term ionization damage in organic insulating systems.

Figure 11. Terminated cable with PD detector and PD pulse height distribution measuring apparatus (after Bartnikas 1961).

? 350, , , . , , . , . , - 10 4 CABLE SPECIMENS

The introduction of polyethylene (PE) extruded cables into the power distribution sector in the early 1950s required a rapid development of PD detection techniques and testing standards to assess the reliability of these cables. Much development work went into characterizing the PD behavior in these cables as well as determining their resistance to PD induced degradation 151. In Figure 10 is delineated a schematic connection diagram of an early arrangement used in the late 1950s and early 1960s by the author for the measurement of the PD discharge rate and differential pulse-height distribution on cables [12-15]. Short cable lengths (shown as CJ, which acted as lumped capacitance specimens at the frequencies of measurement (calibrated using the square pulse generator shown), were used for these investigations. There w'as a general consensus that PD originated from cavities from within the extruded insulation or at the interfaces between the insulation and the semiconducting conductor and insulation shields. It became soon apparent that, in contrast to oil/paper insulated cables, polymeric cables were highly susceptible to PD induced degradation and could not operate in the presence of PD without undergoing eventual failure. Accord- ingly, PD test standards were devised, which required the polymeric cables to he free of discharges at operating voltage, when tested at a detection sensitivity of 5 pC. With the improvement in the extrusion processes of solid polymeric dielectrics and smooth, low contaminant containing extruded semi-conducting conductor and insulation shields, the PD test standards became proportionately more stringent, necessi- tating the polymeric extruded cables to meet the 5 pC sensitivity requirements at voltages substantially above the operating voltage level. With this added requirement an additional safety factor was introduced

s ~ 103 20 - 9 210 0 8

5 1% .e 2 1 Im - 6 0 - ,; m -80 5 s o -ILu ; 'I -30 -,*I1 2

:: I W -20

p m -140 u -150 -3MI

FrrgUUICY

Figure 12. Characteristic impedance and phase angle us. frequency of 25 kV XLPE insulated power cable (after Bartnikas 1951).

ICQHZ iOMi( l 2 0 M 30- u)Mi(l

In view of the foregoing discharge operating constraints, PD tests on newly manufactured polymeric cables are essentially go-nogo type tests in that the cable specimens are rejected if they exhibit the presence of discharges at the prescribed sensitivity and voltage test level or ac- cepted in their absence [88-92]. Long power cable specimens behave as transmission lines and must, therefore, he terminated in their characteristic impedance, if measurement errors due to PD pulse reflection effects are to he obviated 1931. Since routine PD measurements are carried out on newly manufactured cables with the specimens invariably unterminated, pulse reflection errors occur but these are minimized by specifying the so-called LY pulse shape response of the detection circuit whereby the integration errors due to pulse overlap are additive, resulting in increased rather than decreased detection sensitivity, The a response simply refers to a highly damped pulse in which the first peak of oscillation represents the maximum amplitude of the PD pulse [94,95). Additional sensitivity is achieved with suitably designed noise filtering circuits to reduce extraneous interference.

..

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 9 No. 5, October 2002

---t

771

20nsldiv

With the stringent PD specifications in place to eliminate cables at the production site, which are subject to PD, the current effort and activ-

to locate discharges and asses their severity in cables in the field that

installation or through aging developed discharge activity If identi-

pulse-height and discharge epoch distribution patterns, then, in order

specimen under test must be terminated by its characteristic impedance

[96]. The resistor Ro, approximating the magnitude of the characteristic impedance of the cable, is placed in series with a discharge-free capacitor C, which is equal in value to the discharge-free coupling ca- uacitance C,.. The latter acts as a short to the much hieher freauencv PD

0 - ity in the cable area has shifted to elaborate PD measurement schemes

may either be introduced in the course of mishandling during the their

fication of the type of discharge sites is attempted in terms of the PD

5 -20

9 6 4 0

5 3 a a

.-

-80

lO.106 m.id 40"10* Mhld 80110b ICQxlo6 to eliminate pattern errors due to reflections from the far end, the cable

Ro, utilizing a HV termination arrangement as depicted in Figure 11

Frsqucn~y , f (Hz)

Figure 14. Attenuation frequency Of 25 kV XLpE power ble terminated "ith its approximate characteristic impedance of37 R (alter Barmikas [951). Measurement time interval = 60 5. Base line = 470 resolution capacit~ = 1024 channels (a) at c,v b) at 1.6 kV above cIv (c) at 3,6 k" above cIv (d) at 5,6 k" above cIv,

" 1 ,

currents, but presents a high impedance at the power frequency Dis- charges at the cable ends are prevented by the use of oil cups into which the ends of the cable are immersed. At very HV, the oil cup terminations must be replaced by ones containing De-ionized water. Note that with the arrangement shown, the incident PD current pulse at the detector end contains only half the charge content released at the discharge site, since the other half is transmitted to the terminated far end of the cable specimen under test.

. . . . i . . . I . ~ . . . , i . ~ . I . . + . . . . . ..

. . . . . . . . . . . . . , . . . . T ' .~ . . . : . . , . . I . . ' . , , . .

. . ..

. . . . i

Figure 13 demonstrates the rapid degradation in the rise time of a pulse that occurs even over a relative short length of a polymeric distribution cable [95]. In Figure 14 are presented the high frequency attenuation characteristics of the same cable.

When new cable designs are evaluated in terms of their PD pulse height and discharge epoch distribution characteristics for which precise measurements of the discharge pulse amplitude and the actual number of discharge pulse are necessary, it is expedient to employ short cable specimen lengths. With sufficiently short lengths of cable, pulse reflection effects from the far end of the cable do not arise when conventional low frequency narrow band detectors are employed. Figures 15 and 16 portray typical pulse-height distribution curves on two early vintage short lengths of ethylene propylene rubber (EPR) and crosslinked polyethylene (XLPE) distribution cable with applied voltage as a parameter. While these characteristics were obtained utilizing a multichannel pulse-height analyzer, the current approach would be to employ a computerized system shown as an alternative in Figure 11. It should be observed that the data presented in Figures 15 and 16, when normalized to a 1 s measurement time interval, can be used to derive the average PD current, which in such circumstances is given by the integral of the area subtended by these curves. PD pulse interval measurements also may be made readily by means of the technique developed by Bapt et ai. [la, 191. The foregoing type of distribution is now less commonly used, though it provides an important measure of the PD pulse density in terms of the time separation between the discrete pulse. It has complements and supplements the PD discharge epoch or phase distribution, which gives the respective position of the PD pulses with respect to the phase of the applied sinusoidal voltage wave [20].

The widespread usage of computer based techniques in the measurement of PD has provided added flexibility in the treatment of PD test data and associated signal processing techniques. With computer as- sisted techniques, it is readily possible to display simultaneously three- dimensional plots of PD pulse charge transfer, discharge rate or pulse number at the discharge epoch of occurrence of the individual pulse with applied voltage or testing time as parameter. A convenient display of PD pulse data consists of simultaneously obtained plots of the numbers of PD pulses and PD pulse amplitudes as a function of the discharge epoch or phase obtained over a given time interval at a given value of applied voltage. Such PD patterns, portrayed in Figure 17 obtained on


Charge transfer - AQ (6) L

! I , I 3 5 10 20

Charge transfer - AQ (pC) z

b I 1 I I 2.9 3.9 7.8 ' 9.8 Charge transfer - AQ (pC)

0 219 3.'9 7.k 9.8 Charge transfer - AQ (pC) z

z 3 5 IO 20 Charge transfer - AQ (pC)

1 I 1 I I 0 2.9 3.9 7.8 9.8

Charge transfer - AQ (pC) (d

10 20 Charge transfer - AQ (6)

(d)

Figure 15. PD pulse-height distribution characteristics obtained on an early 15 kV XLPE power cable as a function of applied voltage (after Bartnikas [5]). Measurement time interval = 60 s. Base line = 281 channels, resolution capacity = 1024 channels (a) at CIv (b) at 2.0 kV above CIV, (c) at 8.0 kV above CIV.

a PE cable permit not only the determination of energy dissipated by the positive and negative PD pulses but may h some cases provide a possible means for identifying the defects responsible for the observed PD behavior 1281.

Note that the negative pulses occur over the ascending portion and positive pulses over the descending portion of the sinusoidal voltage wave.

Over the last decade or so there has been a considerable increase in PD measurement activities related to PD discharge site location on both distribution and transmission polymeric type cables in the field; some of the same type of work has also involved oil/paper insulated cables. The objective is to remove cables or sections of cable circuits as well as cable joints, which exhibit various degrees of PD activity

Figure 16. PD pulse-height distribution characteristics as a function of applied voltage obtained on an early 25 kV EPR insulated power cable (after Barmikas 1961).

and are judged to be in imminent danger of undergoing failure 1951. The portable PD locating techniques available for solid and solid-liquid type cables can essentially be categorized into probe and non-probe test methods. There are a number of non-probe PD site location methods, most notably the PD pulse polarity correlator 1971 and a number of variations of the time domain reflectometry (TDR) procedures [98,99]. The advantage of the TDR techniques rests in that the measurements may be carried out in-situ on directly buried cables or cables installed in ducts; there are disadvantages, however, in that the TDR methods require tem- porary interruption of service and a portable power supply to energize the disconnected cables. In contrast, while the probe methods may be directly applied on cables under operating conditions, they are by their very nature scanning devices and require accessibility to the cable exterior surfaces. The latter disadvantage may in part be circumvented in the case of splice or joint tests, where it is common practice (particularly in the case of extra HV polymeric transmission cables) to install permanent PD detection probes to monitor any possible development of PD in the joints.

TDR methods employed for the location of PD discharge sites in polymeric cables utilize either low frequency 0.1 Hz voltage sources,

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 9 No. 5, October 2002

1 \ 00, 270°

goo ..-.-

Figure 17. PD pattem associated with a flat cavity in a PE cable at an applied voltage oi 15 kV measured over a 60 min time period (after Gulski [ZS]).

Figure 18. Schematic circuit diagram of PD source locator for cables (after Mashikian el ai. [103]).

which permit the testing of longer cable lengths [lOO], or the usual power frequency SO or 60 Hz voltage sources, which produce a larger number of PD pulses per unit time and thus facilitate electronically the PD site detection,procedure [101-103]. Current TDR methods have been greatly improved by the use of digital techniques and are capable of locating PD discharge sites with acceptable precision. Since discharge site location is determined in terms of the incident PD pulses and the time delay between their multiple reflections, the accuracy and precision of discharge site location is a function of the rise time and pulse width of the discharge pulses, their distortion and broadening as they propagate along the cable as well as the signal-to-noise ratio characteristics of the detection circuit.

A schematic circuit diagram of a TDR type PD site locator developed by Mashikian et al. [101-103] is delineated in Figure 18 in which the cable specimen in-service is disconnected from the remaining power distribution network and energized by means of a 60 Hz power supply The high pass filters utilized are of the RC type with a pass bandwidth of 10 MHz having a lower frequency cut-off to reject the power frequency and its harmonics. The detected PD pulses are transmitted via a buffer amplifier in tandem with an isolation transformer to a digital

773

25

r2 = 1.10 p I, = 2.64 ps 9 20 E

$ I 15

v

.- - e 9 IO %

2 P O a

- $ 5

c

-5 0 2 4 6 8 10

Pulse xparation time (ps)

Figure 19. TDR incident and reflected PD signal haces obtained on an XLPE insulated power distribution cable of 223 m length (after Mashikian el al. [l03]).

oscilloscope, which is interfaced by means of an IEEE 488 bus with a microcomputer equipped with a National Instruments type accelerator board and utilizes Labview IITM software that permits signal processing of the digitally stored discharge pulse data in the oscilloscope. Al- though the algorithm is not specified, there are several digital processing techniques such as deconvolution, linear prediction and maximum likelihood estimation that are amenable for accomplishing the required task [104].

In Figure 19 is represented a typical trace of the incident and reflected PD pulses obtained on a 223 m long XLPE insulated power distribution cable, with the measurements being recorded at the near end of the cable (point n in Figure 18). The discharge site location is taken to be at (1 - Al), so that the first pulse in the TDR trace represents the incident pulse that has traversed a distance (1 - Al) from the site, where Al is the distance to the site from the far end of the cable specimen (point bin Figure 18). Its associated reflected pulse propagates a total distance equal to (A/ + 1) and reaches point a with a time delay of tZ with respect to the incident pulse. The incident pulse and its associated reflected pulse are in turn reflected at the near end a, and then after traveling to the far end bare again reflected, such that upon arriving at the near end a the separation between the incident pulse and its reflected pulse becomes equal to t l . The reflection process of the pulse couplet continues until they are attenuated beyond the sensitivity limit of the TDR detection system. In reference to Figures 18 and 19, the position of the discharge site AI is thus given by

In discharge site location tests on in-service aged cables, it is desirable and appropriate to utilize detector sensitivity levels of S pC, since newly manufacture cables are rejected with discharges at and above this specified level. As the extraneous noise levels in service environments generally exceed this level, suitable noise rejection filters must be incorporated in the test circuitry. Communications related electromagnetic interference occurs at fixed frequencies and is, therefore, most simply removed by band rejection or notch filters. Another simple noise elim-


Concentric neutral jumpper over joint

Conductor olcahle rpecimen T- .- . . . . -. . - cp! S e " m 1 u a m e > h e l d P?

I ~ I CT

C Concentric neutral C 0 > r r I

Figure 20. Discharge site location by means of capacitive probes (after Morin el nl. [107]). (a) Position of capacitive probes with respect to joint. @) Equivalent circuit of capacitive probes and cable specimen: here represents the incremental distributed capacitance of the cable dielectric and C the capacitance of the probe inserted between the concentric neutral and the semiconducting shield.

Probe #1

Probe #2

Chl lOmV Ch2 lOmV Ions Chl

Figure 21. Response of two capacitive probes with probe #2 located a distance of 0.76 m and probe #t 4.5 1.36 m from a joint of two 25 kV XLPE distribution cables containing a discharge source at its midpoint; Vertical scale, 10 mVIdiv., horizontal scale, 10 nsldiv. (after Morin el nl. j1071).

ination procedure is available for the rejection of interference pulses that are generated from switching events which bear a definite phase relationship to the applied sinusoidal voltage wave, such that blanking circuitry may be employed to eliminate all pulses within the applied voltage segments over which the interference pulses appear. When direct operator intervention in the noise filtering approach is not feasible, resort to adoptive digital filtering techniques can he made [102,104]. If the pulse response of a full cable specimen length as well as its attenuation and phase constants are known, the response due to a discharge at any point on the cable may be derived in terms of its transfer function

[102]. Consequently, the pulse response associated with a discharge at any point along the cable specimen may be correlated with the measured noise i.e., the location of the PD site corresponds to that value, wluch yields the maximum cross-correlation coefficient.

As has been mentioned already, the alternative procedure for the 10-

cation of PD sites in solid polymeric and oil-impregnated-paper cables involves the use of scanning probes, which may be of the capacitive [lo51 or inductive [lo61 type. For completely shielded cables only inductive probes are effective. Capacitive probes function well only on unshielded or poorly shielded cable sections, where the shield may be interrupted or damaged or at cable ends and poorly shielded cable joints. Capacitive probes may be also installed permanently under shields in joints to monitor or detect newly initiated discharge activity. It should be also pointed out that acoustical probes which function well with compressed gas cable or bus sections, do not perform well on polymeric or oil impregnated-paper cables that are characterized by high acoustical impedances.

The usual capacitive probe normally consists of a dielectric film capacitor of narrow width with cooper plate electrodes suitably bent to fit the cylindrical contour of the cable specimen. The capacitive probe may be mounted on an insulated rod to facilitate scanning along poorly shielded cable joints. In order to locate the PD sites, it is always necessary to work with two probes placed some distance apart that may be varied to establish whether the fault is between the probes or to either side of the probes. Though tedious, capacitive probes may be installed on distribution cables with concentric neutrals with the two capacitive probes being inserted between the concentric neutral and semiconducting shield of the cable. For cable joints ha\ 'in ' g concentric neutral jumpers, the capacitive probes may be placed as indicated in Figure 20 [107].

If the discharge site in Figure 20 is exactly in the middle of the cable joint, the PD pulse arrival times at the two probes will be equal and the two respective transmitted pulses from the PD site will be of equal magnitude. Had the two probes been inserted under the concentric neutral at 0.76 and 1.4 m on the right hand side away from the cable joint, the situation would be quite different as is demonstrated in Figure 21 for the case nThere the separation distance between the two capacitive probes is 0.61 m. The pulse at probe No. 2, which is 0.76 m from the joint, is seen to arrive before that of the more attenuated pulse at probe No. 1, which is placed 1.4 m from the joint. As has been already dis- cussed, the losses in the semiconducting shields as well as in the solid dielectric of the polymeric distribution cables give rise to substantial attenuation of the magnitude of the transmitted PD pulses; likewise, they are found to reduce the velocity of propagation to - 58% of that of free space.

A single capacitive probe scan of a cable surface is frequently useful as an initial step for detecting gross PD faults at poorly concentric neutral shielded covered cable sections, joints and terminals. For this purpose rf type I'D detection circuitry is employed, whose sensitivity is calibrated in pC but whose output consists of both an indication in dB units as well as an audible noise level that is proportional to the PD intensity. Such a device is depicted in Figure 22; it utilizes two probes: a contact capacitive probe and ungrounded rod probe, which acts as an antenna. The latter probe is employed for ascertaining the overall PD

,

JEEE Transactions on Dielectrics and Electrical Insulation Vol. 9 No. 5, October2002 775

Figure 22. Schematic diagram of rf capacitive probe PD locator designed for application on polymeric power distribution cables (after Morin et ai. IlOS]).

Copper lnsulaling cylinder

b 9

Cable

I I

Figure 23. I'D source location on joints of 275 kV XLPL power trans mission cable (after Katsuta et ai. [lOS]).

activity or ambient noise in a given area, i .e. in the vicinity of cables installed in substations or at openings of man-holes. If such activity is observed, the accessible portions of the cables are scanned with the contact capacitive probe, which is generally found to be effective in locating very intense discharge sources; however, its PD site locating capability is seriously compromised in areas exhibiting high ambient levels of interference. The capacitive probe PD site locating device has a calibrated output A readable directly in dB units, given by the empir- ical relation [lo61

A = K l u A Q k b where I< and bare constants and the value of AQ in pC refers to a train of calibration excitation pulses at a specified repetition rate, i.e. the calibrated dB scale is a function of both the magnitude of the PD pulses as well as their repetition rate.

Since the presence of PD in polymeric type cables under operating conditions cannot be tolerated due to the high susceptibility of the polymer dielectrics to PD induced degradation, discharge onset monitoring of h-service HV power transmission cables is of paramount importance [109,110]. The appearance of PD may necessitate immediate removal of the accessory of the polymeric cable segment in which the I'D source is located in order to avert service interruption due to a high probability of imminent failure at elevated voltages. Figure 23 portrays schematically a PD site locator arrangement, which has been successfully deployed in-service applications on installed 275 kV XLPE power transmission cables. It utilizes a balun circuit, which provides an unbalanced output [109]. The two capacitive probes are applied in the form of a metallic foil over the cable jacket; an insulating ring separates the metallic shield at the mid-point of the joint. Essentially each capacitive probe views one half of the joint and cable length on its respective side. A sensitivity of 2 pC is claimed by the authors.

(3)

Probe - #I

Probe #2 -

Chl 20mV Ch2 20mV lOns Chl

Figure 24. Pulse responses from a discharge site situated equidistant between two inductive probes; horizontal scale: 10 nsldiv., vertical scale: 20 mvldiv. (after Morin et nl. [107]).

C0"trcl P*

Figure 25. Schematic circuit diagram of a commercial ultrasonic PO detector [1081.

Although inductive probes have been deployed for PD site location purposes for several decades [106,111], their application to cables appears to be a relatively recent phenomena [107,112-115]. In shielded cables, the signal of the discharge site is coupled electromagnetically to the concentric neutral and the induced current pulse propagates along the neutral in both directions from the discharge site. The induced voltage pulse magnitude developed across the inductive probe increases with the mutual inductance between the inductive probe and the concentric neutral. In general, ferrite core coils exhibit greater sensitivity and have better frequency response characteristics than multi-turn coils. While higher frequency response inductive coils have better signal resolution characteristics and provide greater accuracy in the location of I'D sites, it is advantageous with longer cable specimens to use inductive probes that respond principally to the lower frequency components of the discharge pulse and can, therefore, 'see' further into the cable specimen. b e d and Srinivas [llS] have described ferrite core probe designs that can sense discharge sites - 600 m away How- ever, for precise discharge site location, inductive coil probes of a few turns [107,112,114] or even a single turn 11131 are required. Figure 24 shows the response of two inductive coil probes of 6 turns each, placed equidistant (44.5 cm) from a PD source [105].

While acoustical methods are relatively ineffective for PD tests on polymeric and oil/paper-impregnated cables, they are ideally suited for PD site location on compressed SFa cables and bus limes. Tnis can be readily accomplished using conventional commercially available ultrasonic detection circuitry depicted in Figure 25 [108]. SF, gas, which


is normally under a pressure of 5 atmospheres transmits ultrasonic signals much more efficiently than under atmospheric pressure [1161, though appreciable attenuation occus at flanges and spacers 11171; consequently, the cables should be scanned in steps between the individual flanges. The acoustical attenuation cy (in dB/cm) between the spacers of a compressed SF6 l i e may expressed by the Kirchhoff formula, which is valid for a tube geometry [116],

27.6x10W5ff? c y = (4)

T where f denotes the frequency in Hz and T is the radius of the cable tube in cm. Even though attenuation is much lower over the audio frequencies (<20 Wz), acoustical tests on SF6 l i e s must be carried at higher frequencies of the ultrasonic regime (e.8. 50 kHz) in order to cir- cumvent the high ambient interference normally encountered over the audio frequency band. Acoustical methods may achieve sensitivity levels of 10 to 25 pC [116], that are substantially less than those of electrical PD detectors, which fall in the range between 0.1 to 1.0 pC. It should be also added, that while acoustical methods can readily detect discharges due to the movement of particles and those initiated at asperities on the surface of the cable conductors, they are quite ineffective in detecting the low level PD pulses which occur within the occluded cavities of the spacer insulators 11161

T -

Figure 26. P D site location and PD level test with an acoustic sensor in conjunction with a narrow band PD detector [116,118].

Coaxial cable or FET pmk Solid dielectric -;;Coaxial coupler

7

cableor FET p m k

Figure 27. Tpes of couplers for high frequency PD measurements on SFa line specimens (after Boggs et nl. [120,122]).

0.1 pC depending on the extraneous noise rejection filter employed, determines at an elevated sensitivity whether discharges are present and the probe is then slid along the exterior of the metallic tube enclosure of the cable to locate the PD sources. Difficulties are encountered with this procedure when the electrically detected discharge levels are low and fall beyond the sensitivity capabilities of the ultrasonic scanning probe. The output across the detection impedance z d may be used also to generate the usual PD pulse distribution functions when measurements are phase synchronized with the power frequency

The nearly dielectric loss-free character of SF6 lines renders them ideally suited for high frequency PD detection techniques. Frequencies in the range from 300 MHz to 1 GHz commonly are utilized in a detection scheme whereby the detection impedance z d and the coupling capacitance C, of the traditional PD detection circuit become incorporated as parts of a high frequency transmission lime to yield the desired frequency response [119-1231. In straight portions of SFa bus ducts or cables, the detection of discharges and their site location is achieved by measuring simultaneously the pulse arrival times at two suitably displaced coupling capacitors along the transmission line. Discharge sites have been shown to be located to within 1 m over straight portions of a duct l i e at sensibility levels as high as 0.1 pC [120,122]. Figure 27 depicts a number of typical PD coupling devices for use with wide band discharge detection systems. While the shown coaxial coupler exhibits a good frequency response, it is characterized by a high coupling loss, which may, however, be eliminated by the use of a solid-dielectric coupler. If a Faraday cage is available to provide adequate shielding for the measuring capacitor, a sheath coupler is also suitable for use with the proviso that an isolated section of the cable sheath is disposable for this purpose.

It was demonstrated that high frequency PD detection techniques may also be employed for continuous PD monitoring on gas-insulated system (GIs) 11201. Since PD measurements in the range from 300 MHz to 1 GHz are well beyond the upper limit of the frequency spectrum of surface corona discharges, interference from overhead transmission l i e s is effectively eliminated. Fixed frequency interference from portable telephone communications and television may be circumvented by suitable filtering techniques. Should the latter approach have short comings, then PD detection may carried out at selected high frequencies, which correspond to the characteristic resonant frequencies of the SFg gas duct structure under test as shown in the detailed analysis by Kurrer et nl. 11241. These groups of resonant frequencies are a complex function of the geometrical configuration of the GIs bus i.e., its overall length, interconnections, T joints and ends of ducts [121-1261: When the PD pulse amplitudes become very large, the number of resonant peaks increases, leading to a very dense frequency spectrum of the resonant peaks.

In closing the discussion on PD measurements on cable specimens some remarks ought to be made in regard to PD site location tests at cable manufacturing facilities. PD site location tests were freauentlv

I ,

A common procedure employed for PD detection and PD site location in SFs power transmission cables, based on a technique developed earlier by Konig [US, 1181, involves the usage of a conventional narrow band PD detector in conjunction with an ultrasonic probe as delineated in Figure 26. The PD detector, whose sensitivity may be as high as

and in some cases routinely carried out on polymeric cables until the early 1970s. As solid dielectric extrusion techniques improved along with the introduction of extruded semiconducting shields that replaced the antecedent carbon-tape shields, the occurrence of PD in newly manufactured cables diminished markedly The most effective technique

IEEE Transactions on Dielectrics and Elech'cal Insulation Vol. 9 No. 5, October 2002 777

for locating rn sites in polymeric extruded insulation and cavities at interfaces between the conductor semiconducting shield and the extruded insulation, involved the use of the so-called Gooding train (1271, whereby a polymeric cable without the outside insulation semiconducting shield was passed through a hollow cylindrical HV electrode situated at the center of a long pipe, containing high resistivity De-ionized water. Tne water column at the ends of the type was grounded via two grounded tanks, thereby causing the cavity or cavities to undergo maximum discharge intensity at the center of the tube. In some installations the Gooding train PD scanning procedure was carried out shortly after the extrusion head as the cable was being extruded. Since the Good- ing train technique could not be applied directly to finished cables with insulation semiconducting shields in place, X-ray techniques were utilized to scan voltage energized cables 11'28,1291. X-ray irradiation of cavities provided the free electrons to initiate and maintain the PD in the cavities, which normally could not have undergone discharge.

5 CAPACITORS Capacitor specimens behave as lumped circuit elements; thus PD

tests on capacitors constitute a simple procedure with the proviso that their capacitance is not too large. Unfortunately, this is not the situation with the vast majority of HV power and energy storage capacitors. If C, represents the major portion of the capacitance of the capacitor, which shunts the series combination of the capacitance of a discharging cavity in series with an extremely small portion of the dielectric, then in terms of the detected peak discharge pulse voltage signal Vd, the associated apparent charge transfer is given by

AQ = AV&, (5) ;:;:-'I 2 - - 5 95w

.e 8 wo 3 2 ' 8 " l

e 7 M o

"-25 -15 -5 o 5 IO I5 Temperature ('C)

Figure 28. PDIV us. temperature characteristic of a 4.6 pF dielectric liquid impregnated plastic foil power capacitor (after Hantouche and Forture (1341).

Evidently, the detected pulse voltage magnitude decreases inversely with the specimen capacitance C,, eventually approaching a situation where PD detection with electrical PD detectors becomes increasingly more difficult when the specimen capacitances begin to exceed much beyond 5 pF. Extraneous noise elimination and reduction in PD tests on high capacitance specimens, using balanced measurement techniques, represents one effective practical means for compensating in portion for this reduced measurement sensitivity 151. Early dielectric liquid impregnated-paper power capacitors were designed for operational stresses of the order of 12 V/Gm and had to comply with IEC Specification 70 (130,1311. The replacement of paper by plastic film

/

0 Phase (degrees) 360

Figure 29. Effect oftemperatureupon the op pulse distribution characteristics of a 4.6 /IF dielectric liquid impregnated plastic-film power capacitor: (a) at PDIV = 1 . 9 8 ~ rated voltage and -25T; @) at PDlV = 2 . 8 2 ~ rated voltage and 15°C (after Hantouche and Forme (1341).

dielectrics permitted to increase the operational stresses to 60 V/pm for the power capacitors; however, this increase was accompanied by more stringent PD level requirements, which stipulated a permissible rn level of only 20 pC relative to that for oil/paper insulated capacitors of 200 pC 1132-1341. This partially reflected the greater concern for the use of plastic films, which tend to undergo more severe degradation in the presence of PD as compared to the well proven high reliability oil,' paper insulating systems. With capacitors that are constructed of several unit capacitors placed in parallel, it is common practice to test the units for PD individually and thus gain sensitivity as opposed to having the entire capacitor itself tested, which represents a specimen of much higher capacitance. As an additional safety feature, efforts are made to design capacitors such that they are free of discharges for at least twice the rated voltage, whose associated charge transfers are = 20 pC. It is interesting that the latter level is four times greater than the 5 pC value utilized as a criterion for the rejection of polymeric power cables. Yet, while the electrical stresses in the polymeric cables are relatively high, they are still substantially less than the stresses at the stress enhancement sites of the metallic foil edges of capacitors [80]. In comparison to cables, the PD charge transfer levels even on low voltage capacitors are found to be relatively high, ranging between 8 and 49 pC [135].

A general characteristic of HV insulating systems is that with aging, the PD inception voltage (PDIV) frequently is observed to diminish very gradually with time as a result of both physical and chemical degradation which take place within the insulating systems. With dielectric liquid impregnated system, the Pniv value is significantly affected by temperature, usually decreasing with falling temperature as the viscos- ity of the impregnant increases and cavities are either created or minute cavities coalesce to form larger macroscopic cavities that commence to ionize and discharge. This effect is demonstrated in Figure 28, which was obtained on a 4.6 pFpower capacitor rated for operation at 3.6 kli


Figures 29(a) and 29@) portray three-dimensional plots of the number of PD pulses and charge transfer as a function of discharge epoch (phase in relation to the applied sinusoidal voltage wave) (1341. It can be discemed from the graphs that the insulating system exhibits a lower PDIV value at -25°C due to the presence of more intense discharges centered around 180"; Figure 29@) obtained at a temperature of 15T reveals a more dense discharge pattern, containing pulses of an appreciably lower intensity

PD pulse distribution patterns may also be used to detect coustruc- tion faults in capacitors. In terms of a PD pulse distribution data bank compiled, Gulski [27] was able to correlate PD pulse distribution patterns on a 220 kV, 10 pF capacitor to establish the existence of a PD fault at a soldering joint between individual capacitor layer packages. HOW- ever, for such correlations to be effective, previous PD pulse distribution data must be available on specific types of PD faults on capacitors having identical construction designs. Acoustical measurements are particularly attractive for PD detection in capacitor specimens of high capacitance. They are immune to extraneous electromagnetic interference and can achieve sensitivity levels better than 20 pC, which are difficult to attain with electrical PD sensors when capacitances >LO fiF are involved [116,136]. The electromechanical transducers or sensors

Figure 30. Schematic circuit arrangement of an induced voltage PD test on a power transformer specimen, including a measurement system for P D pulse-height distribution analysis.

used for detecting acoustic emissions may be constructed of crystals or ceramics, which possess the property of piezoelectricity i .e., the capability of developing electricity upon the application of pressure waves. Rochelle salt, lithium sulfate and ammonium dihydrogen phosphate crystals, barium titanate and lead zirconatelead titanate ceramics are materials that exhibit piezoelectric properties. The piezoelectric crystal and ceramic acoustical sensors may be used within the frequency regime extending from 0.1 Hz to 25 MHz and may be either broad or narrow bandwidth devices [116]. The frequency range within which acoustical sensors are used in the area PD detection is determined by the acoustical transmission characteristics of the power apparatus or cable specimens undergoing test as well as the ambient acoustical interference spectrum at the test site. In insulating systems containing dielectric liquids, which are characterized by low acoustical impedances, a sensitivity level of 1 pC would appear to be achievable under ideal test conditions [116].

It has been demonstrated that ultrasound methods are capable of detecting discharges in capacitors having capacitances as high as 40 pF [116]. The physical size of liquid-filled power capacitors attains sufficiently large dimensions to permit acoustical coupling of the ultrasonic transducers via a film of glycerine to the steel casing of the capacitor. For such large-sized capacitors, acoustical sensors resonant in the frequency regime between 60 to 80 ldlz appear to be most effective. How- ever, with capacitors of reduced physical size casings, a frequency of - 20 ldlz tends to be most suitable. Small capacitors undergoing PD tests are frequently immersed in a mineral oil bath, thereby ensuring good acoustical coupling between the ultrasonic transducers and the test specimen as well as providing effective shielding to ambient ultrasound. Ultrasonic transducers are commonly used for the detection and monitoring of PD signals as well as discharge site location. PD sites are often located at the edges of the capacitor metalized films, where strong electrical stress enhancement takes place; the charge transfers associated with these discharges fall normally between 10 and 20 pC.

Figure 31. Schematic circuit diagram for a discharge test on a small transformer, using a separate 60 Hz H V discharge-free test source with '

additional instrumentation for PD pulse-height and discharge epoch distribution measurements.

6 TRANSFORMERS AND REACTORS

PD measurement and its interpretation on transformers and reactors represent a far more complex and intricate task than that on cables or capacitors. A transformer is an inductive device; as the electrical PD measurements are carried out at the terminals of the transformer, any discharge site within the windings of the transformer is separated from the terminals by a sizable inductance, which appears in parallel with a distributed capacitance and is as well shunted to ground by another distributed capacitance. The PD pulse emerging at the discharge site must travel over a complex LC network prior to reaclung the terminal of the transformer. As the PD pulse propagates along the transformer winding, it is both attenuated and distorted as increasingly its high frequency content is removed or filtered out. In addition, the occmence of resonances, between windings and tums within the windings, can introduce errors into the measured PD quantities should these resonant frequencies fall within the bandwidth of the PD sensing system.

PD tests on the transformers may be performed using either the so- called induced test or by means of a separate independent power frequency voltage source to produce the voltage stress in the insulating system [lll, 1371. In the induced test, the voltage is applied across the low potential winding whereby the voltage stress is impressed between the individual huns and sections of the windings as under normal operating conditions in service. When this test is employed with larger transformers, it is common practice to use the third harmonic (180 Hz) of the power frequency source in order to permit an over voltage test on the transformer without saturating the magnetic core and thereby

-. IEEE Transactions on Dielectrics and Electrical Insulation Vol. 9 No. 5, October2002 779

causing damage to the transformer. Alternatively, a 400 Hz motor generator unit may sometimes be considered acceptable. Since high power transformer are normally equipped with a bushing tap 1111,137-1401 it disposes conveniently with the requirement of a discharge free coupling capacitor; Figure 30 delineates schematically the connection diagram for an induced voltage PD test on a power transformers specimen.

T Figure 32. Schematic circuit diagram lor a discharge test on a reactor equipped with a bushing tap, using a separate 60 Hz HV discharge free transformer.

19659 .............. .__....... . . .... .... .... .... .____ T--.

8.7 17 26 35 Charge transfer, AQ (pC)

Figure 33. PD pulse-height distribution observed over a 2 min interval on a 55 MVA reactor, containing a damaged screen (after Gulski et 01. [141]).

With smaller transformers, the power frequency voltage is generally applied to the HV winding by means of a discharge-free test transformer as portrayed schematically in Figure 31. The transformer insulation is thus voltage stressed between the high potential winding and the low potential winding as well as ground. Note that with this arrangement a discharge-free coupling capacitor C, is required.

When a PD test is performed on a reactor, a separate test transformer must also be employed, but no coupling capacitor is necessary because a bushing tap is provided. Since one end of the reactor coil is grounded, the insulation is stressed between the turns as well as to ground (Fig- ure 32). Figure 33 shows ~ P D pulse-height dishibution obtained on a reactor, using a multichannel pulse-height analyzer; the PD discharge pattern was attributed to a damaged screen (1411.

While PD specifications state a permissible bandwidth < 300 knz in the testing of transformers (1371, a lower flat bandwidth extending from 40 to 200 kHz has been found to provide improved sensitivity

7

J l J Time

Figure 34. PD pulse forms obtained with a 3 mm gap in a transformer mineral oil at atmospheric pressure suijected to 50 Hz ac of 7 kV rms. Upper trace integrated output of a 300 kHz bandwidth PD detector (ordinate scale 20 mVldiv); lower trace negative pulses within a pulse burst output of a 750 MHz k?ndwsidth oscilloscope (ordinate scale 10 mV/div); abscissa scale'l'sldiv for both upper and lower traces (after Pompili et ol. 1152)).

Discharge e p h (radians)

Figure 35. Discharge epoch or phase with respect to the applied voltage wave in a 30 mm transformer mineral oil gap at a 2 kV above the discharge inception voltage at atmospheric pressure. Trace (a): discharge epoch and associated charge transfer variation over eleven consecutive cycles; trace (b): discharge epoch distribution over a 1 s time interval (after Pompili el nl. 11531).

(1421. Attenuation measurements carried out in the same study indicated that the spectral components in the PD pulses closer to 1 MHz are much more attenuated than those below 200 Hz. Tuned PD measurements are not performed on transformers and reactors because of errors introduced due to resonance phenomena arising between turns and windings, which are principally prevalent above 200 kHz.

The filter connected across the detection impedance Z, must remove the power frequency component and its harmonics as well as factory and communication generated interference. The IEEE Standard specifies a filter with a signal attenuation of 60 dB at 15 lcHz and 20 dB at 500 IcHr [137]. Considering that a PD level of 500 pC in oil filled transformers is deemed to be permissible, analog filters may adequately meet the noise rejections requirements in most instances. However, under severe extraneous noise conditions, digital filters of the adaptive


rejection type may utilized. Adaptive rejection filters are essentially mathematical filters: the detected pulses are transformed from the time domain to the frequency domain by means of the fast Fourier transform; in the frequency domain the magnitude of the intense noise frequency components are subtracted and then the noise frequency free signal spectrum is transformed back to the time domain by taking its fast Fourier transform inverse.

Calibration of the PD detection circuits is carried out, as in the case of cable and capacitor specimens, using a small calibration capacitor via which a known charge is injected. The value of this capacitor should be at least 50 pF but should not exceed 150 pF. The square pulse of the generator should be sufficiently wide to prevent overlapping of the simulated PD calibration pulses at the front and the trailing edges of the square excitation pulse. The recommended rise time of the front and trailing edges of the square pulse should be equalized to correspond to 100 ns as recommended in the PD test standards [11,137]. The response of PD detectors is a function of the rise time of the PD pulses and, consequently, failure to maintain the rise and fall times of the excitation or calibrating square pulse constant and equal will lead to calibration errors both with conventional PD detectors and PD pulse-height distri-

record each pulse burst as a single integrated charge pulse [152]. This is demonstrated in Figure 34, which shows the response of a PD detector to a PD pulse burst in transformer oil; the charge transfer associated with the overall pulse burst is 13 pC, so that the charge transfer associated wi!h each discrete pulse within the pulse burst is substantially less and can be estimated in terms of the relative amplitudes of the discrete pulses. The PD pulse bursts in oil are initiated at or in close proximity the ac voltage peaks, so that the discharge epochs or phase position of the pulse bursts will tend to center around the peaks (Figure 3S), i.e. at 90" and 270" in lieu of those of the regular PD discharge patterns that center around the voltage zeros [153]. Since the pulse bursts in liquid dielectrics occur sporadically as opposed to the regularly recurring discharges in the classical cavities in inclusions within the oil/paper systems, their detection is more difficult. It has not yet been established to what extent their presence affects the electrical stability of insulating liquids. In Figure 36 is delineated the experimental arrangement for the simultaneous recording of the integrated pulse burst pulse and the discrete rapid pulse train within the pulse burst in dielectric liquid specimens.

" bution analyzers [S, 76,1431.

Also shown in Figures 30 and 31 are PC computer based systems, which may be used for PD pulse-height and pulse phase distribution analysis in lieu of the sophisticated multichannel analyzer systems. While present PD measurement standards on transformers require only the determination of the PD inception and extinction voltages as well as the maximum PD charge transfer value and its change with time at specified voltage levels, the PD pulse distribution measurement systems may be used to analyze certain PD behavioral features that may bear some relationship to the type of PD faults as well as their location. In the interpretation of PD measurements on solid-liquid insulating systems, it is well to emphasize two distinct discharge mechanisms that may occur in dielectric liquid filled power apparatus. PD within the physically stable macroscopic cavities occluded within the oil-impregnated-cellulose paper fiber interstices or synthetic paper insulation will exhibit the clas- sic discharge behavior in that the positive discharges will occur along the descending portions and negative discharges along the ascending portions of the applied sinusoidal voltage wave. The discharges will thus tend to center on both sides of the voltage zeros or commence just before the voltage zeros Le., - 0" and 180". In addition to the former behavior, discharges can also occur within transient cavities that are momentarily created and disappear abruptly in the liquid impregnant at electrical stress enhancement points such as metallic asperities and protrusions [144].

These cavities, initiated at the electrical field enhancement sites within the dielectric liquid, have a propensity expand due to vapor pressure build-up from within [145-1471. As these cavities grow, the PD process is initiated, resulting in a series of PD pulses of generally increasing magnitude which reflects the increasing size of the cavity until its abrupt collapse due to dynamic instability [147-1511. While the series of rapid discrete PD pulses, now commonly referred to as a pulse burst can be readily recorded using a wide band oscilloscope (750 MHz 1 GHz) with a sampling capability of at least 1 G samples per second, a conventional 300 MHz bandwidth PD detector simply will

Figure 36. Schematic circuit diagram for the simultaneous measurement of the PD pulse burst and the discrete pulses within each pulse burst (after Pompili et fll. [152,153].

Figure 37. Schematic circuit diagram for off-line test using rf current transformer (CT) sensors for P D site location in transformers (after

.Fuhretnl. [170]).

The foregoing examples demonstrate that PD pulse distribution patterns may be employed to ascertain the extent and the nature of discharge activity in inductive power apparatus; they may also be utilized to differentiate between the discharges emanating from within the

\

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 9 No. 5,OctoberZOOZ

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781

transformers and reactors from that of extraneous noise sources. The latter may consist of thyristor pulses, modulated periodic signals, poor electrical contacts or corona discharges from HV leads and be characterized.by distinctly different pulse distribution patterns. The pulse discharge patterns from these interferences are so different from the normal PD pattems, occurring in transformers or reactors, that they may be recognized readily by experienced operators using conventional PD detectors. In a recent study involving large power transformers and reactors (1381, it was shown that with the aid of fractal analysis, changes in the PD pulse distribution pattems could be used to detect gross d e fects that were artificially introduced in the form of

1. an aluminum rod extending from the HV sphere of a trans-

2; a floating shielding electrode, and 3. absence of a shielding electrode on the test object.

In an antecedent study by Gulski (1541, carried out on electrical apparatus using the shape parameters of PD pulse distribution curves, it was shown to be possible to distinguish between defects and actual PD discharges in electrical power apparatus. However, the recognition method used was general in the sense that it only determined whether or not the detected discharge pulses emanated from within the test specimen. The method proposed did not resolve the primordial PD cog- nitory problem, i.e. how many cavities there were involved, what their distribution and location was, or provide definite information on their size.

Some utilities may require that a radio influence voltage (RIV) test be performed. This pointedly expressed preference can he attributed to their extensive interpretive experience accumulated with the RIV test, since the date of its standardization in 1940 1155-1571. Historically, it will be recalled that Quinn [9] concurrently published the first paper, concerning'the use of the resonant circuit for PD detection in transformers, which forms the basis for apparent charge measurements. The apparent charge Qc (in pC) is a fundamental quantity that allows the comparison of discharge intensities between different transformer specimens, which cannot be accomplished using the R I V value in V that constitutes a relative measure of voltage. In the RIV measurement the RCL type detection impedance in the resonant circuit is substituted with a 600 0 resistor and a radio noise meter tuned to a frequency within the range of 0.85 to 1.15 MHz, is employed in lieu of an oscilloscope. The radio noise meter employs suitable weighting circuits to provide an outpnt reading in V quasi-peak values. A transformer specimen is considered to have failed the R I V test if the reading exceeds 100 V. The RIV reading in V is a complex function of the PD pulse magnitude and repetition rate and, as a consequence, does not bear a simple relationship to the measured PD pulse value in pC [158].

The relatively elevated values of 100 V or 500 pC for the permissible PD levels in power transformers are specified for dielectric liquid or oil impregnated insulating systems, which well tolerate moderate discharge levels. This is home out in practice by oil-impregnated-paper insulated cables and oillpaper transformer insulations that have given decades of reliable uninterrupted service under operating conditions. This situation differs appreciably for solid polymeric insulations such as PE and epoxy (without fillers), which tend to degrade relatively

former,

rapidly in the presence of PD. Epoxy insulated instrument and power transformers have been manufactured since the late 1950s and great efforts have been expended to ensure that epoxy transformers are free of discharges and remain so during service. Thus, as in the case of solid dielectric extruded cables, the PD inception voltage; PDIv, becomes the acceptance or rejection criterion for epoxy transformers. Ideally a PD detection sensitivity level of 5 pC, as in the case of polymeric cables, is desirable, though a level between 10 and 20 pC is considered acceptable for epoxy transformers 1159,1611. In an investigation carried out on epoxy transformer coils, Borsi [161] has observed a very substantial decrease of the PDIV value with temperature, which he has attributed to changes in the permittivity with temperature and the development of cracks in the solidified epoxy resin.

There are a considerable number of electrical test methods available for PD site location that may be used on power transformer specimens. However, while the proposed methods have a good theoretical basis and have been proven to be effective when verified on model transformer specimens, one must nevertheless observe that there is a paucity in data showing large scale usage of electrical PD site location methods in practice on actual transformers in the field. In part this reflects the intricacy and complexity of the problem in actual transformers and the associated interpretation difficulties. Perhaps the most widely used electrical method for PD site location on three phase transformers in the past has been based on the comparison of the PD pulse magnitude measured at the terminals of the three respective windings [lll, 162, 1631. In the procedure, the first peak value of an oscillatory pulse at each phase is recorded with a wide band oscilloscope; this peak value of the PD pulse response is due to direct capacitive coupling while the subsequent oscillatory portion of the remaining PD signal is associated with the inductance of the winding. The peak voltage amplitude of the PD pulse front is given by (1111

,

where AV,. represents the actual peak voltage of the discharge pulse, C,7h is the shunt capacitance for each coil section of the transformer, C,, denotes the series capacitance between the successive sections and n, is the number of coil sections between the PD site location and the phase terminal or bushing. The series equivalent inductance along the winding, which appears in parallel with the series capacitance C,,, has a negligible influence because of its much higher reactance. In the fore going PD site location procedure, the terminal of the transformer, which exhibits the highest detected PD pulse magnitude is assumed to be closest to the discharge site or, in the case of a number of discharge sites, to be nearest to the PD site having the highest pulse magnitudes. On the assumption that transformer windings behave as uniform transmission lines over a given frequency spectrum, Harrold and Sletten [164, 1651 have demonstrated that frequency spectra of PD pulses detected at transformer terminals may be utilized to estimate PD site location in windings. The current at the impedances of the HV bushing and neutral terminal are measured in the frequency range extending from 150 kHz to 1.2 MHz and their ratio is plotted us. the percent winding length with frequency as the parameter. The PD site is determined by the point of intersection of the plot of the foregoing line and the percent winding length axis. If there are several discharge sites involved,


then the method will locate the discharge site having the highest intensity Another simple method, which has been developed by Ganger and Vorwerk (1661 on Y-connected transformers and extended by Harrold [167] to A-connected transformers, involves the manipulation of winding connections and the measurement of the PDI\J for each different connection arrangement. The foregoing procedure provides a simple means of expression the ratio of the distance to the fault to that of the overall coil length as a function of the ratio of the PD inception voltages. The only unknown quantity is then the distance to the fault, which can be readily determined from the relationship. The procedure is simple and has been used with considerable success in industry

Tangen [168] used a traveling pulse time-delay technique for the location of PD sources in hansformers; the method was intended for transformer windings characterized by a low series capacitance that results in low magnitude detected capacitively coupled PD signals compared to the peak signal that arrives later at the coil terminal. The difference in the arrival times of the PD pulses recorded at the two opposite ends of the transformer winding is employed to deduce the location of the PD site. This traveling pulse time-delay approach was verified by Theong [I691 and Haroldsen and Winberg [170], using artificial discharge sources. Evidently, the method is compromised when multiple discharge sites are involved.

James ef ni. [171,172] utilized a PD detector at each end the winding in conjunction with a computerized data acquisition system to locate discharge sites in transformers. Capacitively coupled frequency components of the measured time resolved PD signals were extracted by means of digital filtering and followed by further bandpass filtering to ensure that the measuring range fell within the frequency spectrum over which the transformer behaves as a capacitative ladder network. Their measurement scheme was based on the so-called 'valid pulse pair' (pulse separation time criterion) cumulative count as a function of the peak voltage ratio of the pulse pairs. However, the PD sources were simulated in their reported work and no measurement data was reported on practical units in service.

Figure 38. A schematic circuit arrangement for tuned PD measurement on a transformer undergoing offline induced voltage test, using a portable power supply (after Hassig et ni. [175]).

For the purpose of discharge site location in transformers, whether in terms of the amplitudes or the separation times between the transmitted PD pulses, the RCL equivalent circuits for the transformer winding configurations must be known in order to ascertain the PD pulse transmission characteristics. This requires access to transformer design data as well as to actual full size transformer test specimens, so that the frequency regime of the capacitive ladder network can be established and the pulse transmission behavior along the windings analyzed both

theoretically and experimentally. Fuhr ef nl. [173] approached close to the foregoing objective in that their developed technique is capable of effectively locating single PD sites in new transformers on which acquired data, obtained by the injection of simulated PD pulses at various points of the windings and measured at different terminals, is stored for future comparative analysis either after commissioning or while in service of the transformer unit.

The form of the transmitted PD pulses M'as found to be influenced upon filling the actual transformer specimen with oil [173]. As iantic- ipated, the magnitude of the capacitively coupled portion of the PD signal and the resonant frequency of its oscillating component were increased upon the addition of the oil filled tank and the HV bushings, as the shunt and series capacitances (Csh and C,, in Equation (5)) increased upon oil impregnation; the presence of the grounded tank wall also augmented the parallel capacitance of the windings.

The calibration signal, to determine the response of the transformer's winding had a rise time of 100 ns and rf current transformer-type PD pulse sensors were located at the HV and Lv ends of bushings and the neutral. The frequency response of the sensors, which was within the range of 10 kHz to 100 MHz, was beyond the frequency content of the 100 ns excitation pulse rise time. The choice of the upper frequency range of the sensors is puzzling in that it encompasses the frequency regime over which resonance peaks are known to occur. The schematic circuit diagram of the experimental test arrangement is portrayed in Figure 37. A time-encoded signal processing and recognition (TESPAR) system is used in conjunction with a digital oscilloscope. The simulated PD signals are injected at the top, center and bottom and of each winding to which the pulse response of the insulating system is measured at the bushing and a reference pattern matrix is composed by the TESPAR system. Similar data banks can be composed on specific transformer designs, whereby PD site location tests can then be carried out on transformers of the same construction installed in servibe.

The work of Fuhr et 01. [173] was further extended as reported more recently in [174,175], where the authors employed a portable discharge- free power supply to perform off-line PD tests on in-service large power transformers. This isolated the transformer specimens from the remainder of the HV system, thereby eliminating extraneous conductive interference transmitted via the connections and thus permitted PD tests at higher sensitivity (c50 pC). The type of Pwfault inferred and its location in terms of the. test data appeared to agree acceptably well with what was eventually found during the repairs of the large power transformer. For these tests the authors [174,175] employed again rf current transformer sensors, and again made use of bushing taps as coupling capacitors. To improve on the signal-to-noise ratio, they carried out tuned the PD measurements within the range of 200 kHz to 5 MHz, using a spectrum analyzer. However, they do not allude as to how the question of inter-tum and interwinding resonance disturbances were dealt with, other than observing that above 1 MHz, large differences in sensitivity were manifest. For subsequent PD pulse-height/phase distribution measurements a variation of the usual conventional approach described in [I761 was used. The schematic circuit diagram is depicted in Figure 38.

At this juncture, it is perhaps appropriate to make an important aside, concerning tuned measurements. In 1976 Bartnikas and Morin

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 9 No. 5, October2002 783

11771 made use of spectrum analyzers in I'D measurements on power distribution cables; however, it is only recently that the use of spectrum analyzers has gained popularity in PD related measurements. The principle of tuned PD measurements and spectrum analysis is as old as the radio noise meter itself [SI, since the latter as well as the spectrum analyzer are essentially tunable band pass filters; while the radio noise meter uses weighting circuits and provides a quasi-peak reading output, the output from a spectrum analyzer gives the energy content of the signal at various measuring frequencies. Hence, when utilized in the tunable filter mode, the spectrum analyzer will reject much useful information on the characteristics of the PD pulse and may, therefore, also lead to some calibration difficulties inherent with the accurate recording of the PD pulse magnitude and its relation to the charge transfer.

A convenient method to determine whether a transformer has developed or undergoes PD under operating conditions in the field consists in the use of Rogowski coils, which can be readily placed over HV bushings as described by Borsi [178]. If the presence of PD is established and it is deemed necessary to locate the PD sites, then one has the choice of using either the electrical or acoustical PD site location techniques. However, in practical terms, unless the electrical PD site location procedure is well proven with an extensive and unambiguous data bank, the acoustical techniques are in general more effective and more simple to use.

It is to be noted that the acoustical PD site location techniques have already an accumulated history of usage spanning over a period of approximately five decades: they were developed in the 1950s and were first applied to transformers by Anderson [179]. Compared to a gas medium, much less acoustical attenuation occurs in a liquid medium; in an oil-filled transformer, the amount of discharge energy converted to acoustical energy is approximately greater by one order of magnitude [116]. An acoustical triangulation method, devised by Anderson [176], employed three acoustical sensors, which were installed by trial and error at three different locations on the tank of the transformer. The arrival of the first acoustical pulse at one of the sensors was utilized to trigger an oscilloscope, whereby the pulse arrival times from the two remaining acoustical transducers could be compared thereby permitting an estimate to be made of the position of the PD site. Once again it must be emphasized that this procedure is only effective in the presence of a single discharge or a number of discharge sites in which the intensity of one discharge site is predominant. Allan et 01. [180] developed an alternative simpler test procedure, involvinr the use of only

~

two transducers, which are positioned in a line of points along a transformer tank. The position of the sensor, which receives the acoustical signal first is considered to be closest to the discharge fault. Frequently, it is expedient to use both electrical and'acoustical PD detection simultaneously, whereby the acoustical sensors response can be triggered by means of the electrically detected PD pulse [MI]; also computer aided procedures may be helpful in conjunction with acoustical PD site locating measurement techniques [182].

Under elevated ambient noise conditions, Bengtsson et 01. [183] used signal processing procedures to ameliorate the signal to noise ratio. Ultrasound techniques intended for in service transformer tests must be capable of functioning in the vicinity of overhead HV transmission lines and within substations, which are characterized by high levels

of Barkhansen (magnetostriction) noise [116]. Since the frequencies of Barkhansen noise are usually centered at, or in the proximity of, 40 Mz, acoustical PD detection is ordinarily carried out at substantially more elevated ultrasouud frequencies. In order to evade Barkhansen noise generation, Train et ai. [lE4] utilized an ultrasonic transducer with an upper frequency of 200 kHz, whose output was applied to a 10 kHz high pass filter in tandem with an oscilloscope, while Howells and Norton

-[I851 chose to utilize an ultrasound transducer, operating withm the frequency regime of 100 to 150 kHz.

It is to be emphasized that the intensity of the acoustical signal received by an acoustical detector mounted on the surface of a transformer tank is principally determined by the magnitude of the PD pulse at the discharge site and the acoustical wave attenuation characteristics of the windings and theremaining structure of the transformer. In the field of acoustical PD detection, the unit of sensitivity is commonly expressed in pC/mV; for transformers it is usually of the order of 70 pC/mV of the measured ultrasound signal. The level of sensitivity is augmented and the influence of extraneous acoustic noise mitigated if the specimen transformer tank with the attached transducer are submerged in an oil bath; this decreases very substantially the reflections of the acoustical waves at the interface between the enclosure tank and the transducer, resulting in an improved transmission of the acoustical signal. However, acoustical PD detection sensitivity is very greatly reduced for the case when PD sites are located deep within the inner sections of the transformer windings.

Figure 39. Typical PD behavior over a thermal-load cycle of model stator bars subjected to twice the phase-to-ground voltage (16 kV rms) at rated three phase curent and a conductor temperature of 122 C. (a) variation of maximum PD charge transfer and (b) variation of average P D current us. time (after Morin et 01. [187]).

7 ROTATING MACHINES

The use of inorganic mica flakes in epoxy resin impregnated stator bar insulation, imparts substantial resistance to PD to the insulating systems of large power turbo and hydro generators. While it is difficult to eliminate entirely discharges in the insulation systems of HV stators, past experience shows that stators can operate for extended periods of t i e in the presence of PD of relatively elevated intensity as compared to other electrical apparatus. As a consequence, the approach to PD measurement on stators differs considerably from that of other HV equipment in that the emphasis is more on ascertaining the discharge intensity and PD site location in terms of the density and configuration of PD pulse distribution patterns. This is in contradistinction to


..-

Ntimber of load cycles

Figure 40. Maximum charge transfer AQm us. the number of thermal load cycles of a three phase model stator at rated current (1620 A) and twice the rated phase-to-ground voltage (16 kV rms) with a maximum conductor temperature of 122°C (after Morin el al. [187]).

Number of load cycles

Figure 41. Average value of P D current us. the number of thermal load cycles of a three phase model stator at rated current (1620 A) and twice the rated phase-to-ground voltage (16 kV rms) with a maximum conductor temperature of 122T (after Marin el al. 11R71)

the mimosa-like susceptibility of polymeric cables to PD, where conventional wisdom does not tolerate any presence of PD under operating conditions. As has been alluded to previously, this applies also to a somewhat lesser extent to solid type epoxy clad transformers 1159,178, 1861. PD in the stator of a rotating machine may occur within the insulation system of the stator bars themselves, end windings the bar ends at their exit points from the slot sections of the laminated stator core

'and at areas of the bars within the slots themselves where the semiconducting paint over the bar's insulation becomes eroded due to PD beneath the semiconducting paint itself, or abraded mechanically due to the vibration of loosened bars induced by the electromagnetic forces.

may attain magnitudes in the order of - 10' to lo3 pC at the operating voltage, the external discharges at the coil ends, core exits points and within the" eroded and mechanically abraded semiconducting paint regions may reach charge transfer magnitudes to lo5 pC or even higher in.extreme cases. It should he noted also that corona and surface discharge at the end windings of the stator also attain similar levels of discharge magnitude. Thus, generally the most severe degradation of the stator bars results on the outer portions of the bars, where the discharge intensities are substantially more elevated. Notwithstanding,

While the discharges within the insulation of the bars themselves I

the intense discharge activity, the resilience of the bars to dischar e in

pulse intensity data given in Figures 39,40 and 41 obtained on 13.8 kV rated mica paper epoxy impregnated stator bars aged under accelerated multi-stress conditions in a model stator under three phase rated current at twice the rated phase to ground voltage [187]. The bar specimens were load cycled between zero and a full load current of 16211 A; Figure 39 depicts the variation over a typical load cycle of the maximum measured charge transfer in pC and the average value of the total PD current in A. Note that the average PD current is equal to the area under the PD pulse discharge rate us. charge transfer distribution curve determined over a 1 s time interval. Very intense discharge activity is seen to develop with rising temperature; however, it decreases rapidly as the temperature commences to fall when the load current is interrupted.

Figures 40 and 41 illustrate the variation of the maximum charge transfer and average discharge current recorded o v ~ r each load cycle as a function of the number of load cycles. Both quantities continue increasing with the number of load cycles as the PD induced deterioration rate of the insulation increases when subjected to twice its rated external stress under severe thermo-mechanical load cycle conditions. The depression in the rate of rise of the discharge intensity in the proximity of 1200 load cycles is likely caused by the appearance of pseudoglow and true glow (pulseless) discharges, which the PD detection system is incapable of sensing 15,511. The fact that even after 1500 load cycles none of the bars in the model stator failed, notwithstanding evidence of extensive deterioration observed on the surface of the bars while subjected to PD pulses having associated charge transfer levels as high as lZOx103 pC, is indicative of the discharge resistance of the mica paper component to PD despite the high susceptibility of the solidifying epoxy impregnant to PD deterioration. The result also accounts for the reasons as to why rotating machines are frequently observed to operate for decades in the presence of PD.

duced degradation is most remarkable as can he deduced from t a - e PD

Figure 42. Early PD detection system for on-line tests on a generator (after Johnson and Waren 11891).

There are a number of PD detection methods that can be used on rotating machines. A compendium of some of these methods is given in [188]. The early pioneering work on PD detection in rotating machines was done by Johnson and Waren 11891, who detected the PD pulses across the neutral resistor of a generator while in operation as shown in Figure 42. For off-line measurements Johnson [I901 employed the standard PD detection technique with a large discharge-free coupling capacitor connected sequentially on each phase and a separate power supply to energize the phases. He was also the first to comment on the importance and practical significance of slot discharges 11911. Later Emery et nl. 1192,1931 and Timperly 11941 used the same approach as

IEEE Transactions on Dielectrics and Electrical Insulation VOI. 9 No. 5, October2002 785

Diffefential amplifier

to PHA and computorized d a 6 acquisition system

Figure 43. Balanced permanent coupler connections for a water wheel generator (after Bromley and McDermid 12031).

Firvoai

Figure 44. Directionally connected coupler arrangement in a ther mal generator (after Campbell e t n l . [204]).

Johnson and Waren [186], but in lieu of a detection resistance, they employed an rf current transformer sensor between the neutral of the generator and the grounding transformer, and substituted the oscilloscope detector with a radio noise meter. The radio noise meter was tuned to frequencies between 20 and 50 MHz to eliminate the interference from extraneous noise sources. Kurtz 11951 modified the off-line test procedure described by Johnson [190] by replacing the large couplmg capacitance by an appreciably smaller one of N 80 pF and using a resistive detection impedance, which permitted on-line PD measurements on each phase. However, in order to further ameliorate the signal-to- noise ratio and obviate the detection problems with the long oscillatory PD pulses, a series of changes, were introduced into the measurement circuitry first by Kurtz cf 01. 1196,1971 and subsequently by Stone et al. [198-2@21, who utilized delay lines in conjunction with a balanced PD

measurement scheme as portrayed in Figure 43. Here the 80 pF coupler, terminated in 50 R, is installed differentially with one coupler per line end of two parallel circuits per phase. The coupler pairs, C1 and C2,

with the respective bus bar length and coaxial limes are matched in their equivalent electrical length at the input of a differential amplifier, such that the electrical length z + (y/0.65) = T + s/0.65, where 0.65 refers to the reduced velocity of propagation in the PE coaxial cables a s

kcompared to that of free space. The incident interference pulses arriving from the machine with equal times of travel are thus canceled in the differential amplification mode. A PD pulse-height analyzer and a computerized acquisition system in tandem with the differential amplifier provides PD pulse-height and discharge epoch (phase) distribution plots, which when compared with a PD data bank, are used to assess the PD intensity level as well as predict possible causes for the observed discharges and monitor their degradative effects. PD pulse phase or discharge epoch analysis capability is of particular importance in rotating machines as concerns the interpretation of discharge patterns. It permits to determine whether the recorded PD pulse activity is associated with the phase under measurement or if it is coupled from the other two phases, as for example whether it originates from discharges between coils of two different phases at the coil ends. If the PD measurement is carried out in phase A, then the discharge patterns emanating from phase A will be characterized by pulses, which center around the voltage zeros i.e. at 0 and 180" of the ascending and descending portions of the sinusoidal wave of the applied voltage. In the three phase connection, phase B and phase C will be 120 and 240" out of phase respectively with phase A and, consequently, the PD pulse patterns in phases B and C will be displaced by the same amounts with respect to the PD pulse pattern of phase A, thereby permitting to ascertain whether or not discharges are taking place between the phases at the coil ends of the machine. The PD detection system of Stone ef al. [198-202] is a high frequency system, which operates at a bandwidth of 40 to 150 MHz, so that it is not practically feasible to calibrate.its output in terms of pC; its readings are thus relative and are recorded in mV. However, here it should be observed that relative PD readings in mV can be used to compare the PD intensities in rotating machines of similar design. They can also be used effectively to monitor the PD intensity in the same machine in order to observe whether there are any significant changes in the PD activity due to insulation aging. However, there is no fundamental basis from the measurement point of view to permit to assess the level of PD activity in terms of relative units (mv), when a comparison is attempted between two machines of entirely different design and construction; for this purpose pC units must be employed.

The capacitive couplers also may be connected directionally with one coupler per phase at the line terminal and another at least 2 m displaced on the isolated phase bus. In this arrangement, noise rejection is achieved in terms of the time of arrival of the pulse signals from the two couplers. Figure 44 illustrates the directional coupler arrangement, which is used primarily on thermal generators and synchronous condensers [204]. With directionally connected couplers a detection bandwidth of 40 to 350 MHz is employed, requiring microprocessor controlled pulse-height analyzer circuitry described elsewhere [13,206] having signal resolution capability of 3 f i s .

In the case of wide band detection systems, the lower frequency end


0 IM) 200 300 4w 500 m 700

PD pulse magnitude (mV)

1 800 900

Figure 45. I'D behavior of a 333 MVA pump storage generator at 34 and 51T, and approximately the same load. Positive pulses x, negative pulses 0 (after Lloyd et 01. [ZOl]).

b

PD pulse magnitude (mV)

Figure 46. No load hot and hull load hot PD pulse-height distribution characteristics for a 350 MVA pump storage generator. Positive pulses x, negative pulses 0 (after Lloyd et 01. [201]).

of the bandwidth is in some respects more important than the upper end. Not all pulses at the receiving end have rapid rise times, since the latter are degraded to varying degrees as they are transmitted along the windings, which, as cable specimens, due to the use of semiconducting materials for conductor and ground shielding, behave as lossy transmission lines. Also even at the discharge site itself, the rise time of the pulses and their amplitude vary with the overvoltage appearing across the cavity whose magnitude is determined by the statistical time lag, i.e. the time required for a free electron to appear and initiate the discharge. This effect is readily observed with PD pulse pattems of simple cavity cells: magnitudes of the individual pulses are seen to very as precession of discharge epochs takes place i.e., individual discharge pulse positions do not remain fixed with respect to the sinusoidal applied voltage.

A local area network (LAN), either of the ethernet or token ring architecture, commonly is used in laboratories as a high capacity data transmission medium to permit the monitoring of automated experiments from office sites [95]. A WAN allows LAN interconnection in different city locations, so that laboratory tests may be monitored from another city location. Lloyd et nl. [201] have utilized the foregoing arrangement of an ethemet LAN in conjunction with a WAN to monitor the PD activity in generators and machines in remote power stations to determine how the PD behavior changes with temperature and load current. Fig- ure 45 shows that a rapid change in temperature at a given load exerts only a small influence on the PD behavior of a pump storage generator, while a pronounced load dependence is exhibited in Figure 46, which is obtained on another storage generator. The temperdture in these tests was approximately the same under full and no load conditions; the load dependence is believed to have arisen from bar looseness in the slots.

A stator slot-type coupler has been developed by Sedding et 01. [207) specifically for PD measwements on turbine generators in most of wluch there is no circuit-ring bus for half of the winding parallels to permit installation of couplers for effective elimination of noise. Noise generation in turbogenerators arises from shaft~grounding and slip-ring brushes, charging and discharging of the isolated phase bus (connecting the generator to the step-up transformer), PD interference and extraneous noise from the overall power system and other related noise in the power station (e.8. precipitators, welders etc.) all of which have signal intensities far in excess of the PD pulse levels that originate from the turbogenerator itself. The stator slot coupler behaves essentially as an antenna and is based on the directional coupler design principles described by Oliver [208]. Its configuration is rather simple and consists of a ground plane and sense line with 50 Q coaxial cables at each end that provide an output, which is proportional to the PD pulse excitation in the proximity of the sense line. The direction of the PD source is determined by the dual-port of the coupler and the associated instrumentation. The coupler is a high frequency device p150 MHr) so that the detected PD pulses will be those having sufficient high frequency contact. Since the rise time of these pulses deteriorates rapidly as they propagate along the windings, the coupler will respond to pulses originating in its vicinity and will thus be relatively immune to distant generated noise signals; the high intensity noise signals that succeed in reaching the coupler can be readily discriminated in terms o( their distorted pulse shape. Hence, the preferred installation site for the stator slot couplers are bars, which are subject to high electrical stresses, namely those at the line end of each parallel winding. Accordingly, the couplers are placed in the stator slot beneath the wedges.

PD detection on rotating machines is also performed with capacitive couplers connected directly to the.terminals of the machine. There are some variations in the techniques used in so far as the bandwidth, type of couplers, signal processing methods, noise rejection and data acquisition systems are concemed. Bandwidths used generally range from 300 kHz up to 20 MHz and the measurement systems are calibrated in pC. The preferred couplers are capacitive, but occasionally Rogowski coils are employed. The latter are large, non-ferrite coils of substantial diameter, which limits their bandwidth to the ICHz range. They are more popular as on line PD sensors on transformer bushings than on machine terminals where they are more cumbersome to mount.

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The purpose of discharge-free capacitive couplers is to act as power frequency separation filters in conjunction with the series connected detection impedance ;.e., the coupling capacitors act as shorts to the higher frequency PD currents and present a high impedance to the 50/60 Hz sinusoidal voltage at the terminals of the machine [5 ] . The PD detection sensitivity increases with the value of the coupling capacitance; however, the high capacitance couplers are normally confined to lower bandwidth PD detection systems. Thus, while a value of 80 pF is used with detection systems having bandwidths in the range of 40 to 350 MHz, higher capacitance couplers 6 I000 pF are preferred at lower detection frequencies.

.

Figure 47. P D recording system for measurement at terminals ofra- tating machine (after Fruth and Gross [209,2121).

A PD diagnostic system for on-line measurement, utilizing couplers in the range from 100 to 1000 pF in series with an RCL type detection impedance for operation either within the frequency band extending from 2 to 20 MHz or 100 to 800 kHz is described by Fruth et nl. [209- 2161. Their arrangement differs from others utilizing FD pulse phase resolved measurements only in so far as use is made of what in their nomenclature are named 'narrow band or 'wide band' signal conditioning units [216]. The narrow band signal conditioning unit reduces the frequency content of the incoming PD signals, using conventional heterodyne circuitry with a local oscillator analogous to that utilized in a scanning rf probe technique for location of PD sites in cables by Morin et ni. [I071 (Figure 22). This type of demodulation operation is readily performed by a spectrum analyzer, which is frequently employed for such purposes. The wide band conditioning unit omits the local oscillator, because the frequency multiplication function is performed by mul- tiplying the PD signal by itself, i.e. multiplication of two PD pulses in the time domain corresponds to a convolution in the frequency domain. Unfortunately, Fruth and Gross [216] do not provide any performance comparisons between the two types of conditioning units or any details on the shapes of the PD pulses at their outputs. Some signal integration effects are evident in their recorded pulse-phase plots in [209], which show simultaneous occurrences of both positive and negative pulses over the same quadrants. These could be caused by overlapping of long oscillatory PD signals; however, the authors appear to be aware of this problem because they refer to permissible sensitivity offset levels below which the recorded signals should be discarded.

The conditioning units, wzhich in addition to frequency multiplier circuitry also include band-pass filters both at their input and output, are connected directly across the detection impedance Z,, placed in series with the coupling capacitance C, as delineated in Figure 47. The detection impedance 2, consists ofa RCL type circuit or for isolation

purposes an rf current transformer having the necessary characteristics. The output from the signal conditioning units is applied to a PD pulse-phase resolved pattern recording and data acquisition system. Limited FD pattern recognition is carried out with numerical treatment of the data; however, complex PD discharge patterns, as to be expected, still require interpretation of an experienced observe. For remote control and access to data, a token ring type [95] LAN system is utilized. The acquired phase resolved test data indicates that the system, when operated in the bandwidth between 100 and 800 kHz, is subject to interference originating with thyristors and brush sparking; however, with the 2 to 20 MHz bandwidth system, these extraneous disturbances are readily eliminated.

It should be borne in mind that when PD measurements are carried out using wide band and narrow band PD detectors on rotating machines, the measured PD signal response will not only depend upon the bandwidth of the detector but also on the type of machine specimen under test. PD signal propagation in machines is almost as complex as in transformers, the latter specimens having the additional compli- cation of pronounced resonance effects not only between phase coils but also between the numerous tums within each coil. As in the case of a transformer, the pulse transmission takes place over a complex LC network, which in the case of a rotating machine is determined by the nature of the winding (e .g . single turn Roebel bar or multi-turn coil design), end arm configuration, ring bus layout and the length of the stator core. The wide band PD detection system responds well to the fast rise time of the PD pulse front, which is rapidly transmitted along the capacitive ladder network of the machine windings (as is sim- ilarly also the case in transformers). The detected pulse still undergoes rise time degradation and attenuation, because the capacitive ladder network does not consist only of ideal incremental capacitances: the capacitors are shunted by incremental resistances that represent finite dielectric losses of the overall insulating system. The slower portion of the PD pulse waveform, which is of an oscillatory character, is transmitted along the idealized LG transmission line network of the machine coils. It is this signal, which is detected using lower frequency band PD detectors; the exponentially decaying oscillatory PD pulse, if property processed to reduce signal integration effects due to overlapping of adjacent long oscillatory signals, can be used to achieve apparent charge calibration, albeit of a very rudimentary form because of the inductive component of the specimen. The wide bandwidth permanent capacitive coupler FDA system described by Stone ef nl. [200-202] is not readily amenable to standard apparent charge calibration procedures and the measurements are, therefore, recorded in the relative units of mV However, it has a decided advantage in that it operates at very high frequencies: the permanent couplers detect only PD signals originating principally in close proximity on the HV end bars, whereas the noise signals, which enter the winding system at sites further away, are proportionately more attenuated and, therefore, substantially less detectable.

A remark of caution should be made here in regard to the use of the narrow bandwidth conditioning units using the heterodyne principle frequency conversion procedure or tunable frequency spectrum analyzers, whereby a PD signal of center frequency fo and bandwidth 6f is transformed into a low pass signal of an upper frequency f [212].


I

Figure 48. On-line 10 kHz to 10 MHz PD pulse measurement system for diagnostic analysis on turbine generators (after Griinewald and Weidner [219]).

Since the frequency content of a PD pulse diminishes as it propagates along the windings of the machine from its site of origin, its center frequency is altered so that the PD pulse-height distribution patterns ofthe demodulated signals detected at their altered center frequency are also changed. In the presence of numerous distributed discharge sites, the entire frequency spectrum must be considered with frequency peaks at the various values of of giving rise to a different pulse-height distributions requiring separate calibrations. For this reason wideband PD signal conditioning units are preferable [209,212].

In transformers, resonance effects between coils and turns within the coils appear to have been remedied effectively. It has been found by Vaillancourt ef al. [142j that resonance phenomena in transformers predominate at frequencies above 250 H z . IEEE standard C57.113-1991 was altered accordingly to require PD measurements on transformers and reactors to be performed with PD detectors having bandwidths - 200 kHz but not less than 100 kHz, with all measurements reported in apparent charge units. In comparison to transformers there does not appear that much attention has been given to resonance effects in machines, though considerable work has been reported at lower frequencies with measurement data expressed in apparent charge units. However, in such circumstances great care must be exercised to remove the oscillatory portion of the detected PD signal through suitable signal processing or conditioning techniques in order obviate pulse su- perposition and integration errors and improve on the pulse resolution characteristics which are typically poor at low bandwidths.

Low bandwidth detection of PD on machines is common with on line tests and prevalent with off-line tests. On-line low bandwidth tests < 800 !dlz with pulse height and phase resolved capability have been reported by Fruth.and Gross [2091. Wilson 12171 investigated slot discharge induced degradation in stator windings, utilizing a PD detector bandwidth of 300 ldlz. His I'D coupling technique used was interesting in that it consisted of a discharge-free cable with a short under the grounded sheath portion at the end of the cable, which was connected directly to the HV terminal at the generator. A clamp-on rf current transformer of the ungrounded sheath portion served as a detection impedance, with the remainder on the grounded cable constituting the

coupling capacitance. An oscilloscope constituted the display device; evidently, such an arrangement was highly susceptible both to internal and external noise.

Another system functioning between 10 ldlz and 10 MHz in which noise rejection is achieved in terms of the arrival time of PD and noise originated pulses, that has been extensively applied to turbogenerators, is described by Wichmann et nl. [218,219]. Surge caparitors replace the usual coupling capacitors and the PD signals are detected across high frequency resistive impedances Zd; PD detection is also carried across the resistor at the neutral end as indicated schematically in Figure 48, whereby the PD intensity in the overall generator may be measured in addition to that on each phase at the three external terminals of the generator. Also, a I'D pulse sensor is located at the shaft grounding brush. A frequency spectrum analyzer, is employed to determine the frequencies of extraneous noise and internally generated interference.

PD pulse measurements may be carried out by adapting PD sensors to sensing devices already installed in rotating machines for other types of measurements. An interesting variation of PD detection in rotating machines involves the use of rf current sensors on leads of resistive temperature detectors (RTD), with sensors for noise gating situated at various noise sites [220]. The sensors operate over the frequency range extending from 5 to 60 MHz and a narrow band data acquisition system is employed for phase resolved PD pulse height analysis.

Off-line tests on rotating machines are normally carried out during general maintenance periods over which it is possible to examine machine windings for possible discharge induced degradation and determine whether replacement of any aged bars is warranted. The HV stators of the machines are tested with the rotors removed; usually portable 50/60 Hz power supplies are employed for this purpose, although tests may also be performed at 0.1 Hz 1221,2221. Off-line tests permit isolation of the specimen machine from extraneous noise as well as internally generated interference during operation (e.g, excitor interference). Off-line PD tests are commonly carried out with conventional 300 ldlz narrow band detectors, calibrated in apparent charge units in accordance with ASTM Method D1868 1111 and IEC Specification 60270 [223], using the IEEE Standard 1434-2000 [188] recommended calibration pulse rise time of 6 60 ns.

Figure 49. Schematic circuit diagram of off-line PD measurement system with one of the phases of a machine undergoing test.

. IEEE Transactions on Dielectrics and Electrical Insulation Vol. 9 No. 5, October 2002 789

A schematic circuit diagram for off-line PD tests is delineated in Figure 49. Each phase under test is shorted and measured individually with the two other shorted phases connected to ground; hence, the insulating systems of the phase under test are voltage stressed with respect to the grounded stator core and to the other two grounded phases. - However, unlike in the case of on-line tests where the machine coils are subjected to a gradation of voltages from the maximum voltage at the phase terminal to zero across the coil at the neutral, all the bars of the coils of the phase under test in the off-line test are subjected to the same value of the applied voltage. Though some bars are highly overstressed, they experience only a single stress (eleckical), while in the on-line test thermal and electromagnetically induced mechanical stresses are superimposed on the operating electrical stress. Consequently, the absence of thermal expansion and contraction forces as well as electromagnetic vibration forces in the off-line test does not result in a realistic dynamic condition within the slots of the stator: the bars in the slots are now star tionary and do not undergo any longitudinal movement or vertical and lateral vibration within the slots. Thus, the only possible slot air gaps, that may exist where PD may arise with off-line tests, are those which remained in the slots after the load was removed and the machine was allowed to cool.

Returning to Figure 49, the shown frequency spectrum analyzer is principally used to determine whether any resonance peaks are manifest in coils of the same phase or between phases. If the frequencies of the resonance peaks fall within the bandwidth of the PD measuring circuitry, then in order to avert calibration difficulties and spurious results in the test data, suitable stop-band filters must be employed or the detection bandwidth must be altered to exclude the resonance regimes. Off-line tests permit some measurements, which cannot be performed under on-line conditions, such as the determination of the PDIV and PDEV values. Also other important measurements, as for example, plots of the maximum detectable apparent charge transfer AQ,, and the average PD current, I,,, equal to the area under the discharge rate os. charge transfer curve, as a function of voltage may be helpful in ascertaining the degree of aging [187], particularly if such data i s available from the time when the machine was first commissioned. In addition, a measurement of the quadratic rate, defined as the sum of the squares of the individual charge transfers of the discrete pulses over a given time interval divided by that time interval, may be required. Some utilities have used this measure as a quality control index and have much accumulated equipment life data classified in terms of this index [224-226].

The off-line measurements also include I'D pulse-height and pulse discharge epoch (phase) distributions. The later will differ from those carried out during on-lime tests because the vibration effects as well as the contraction and expansion (present during load cycles) are absent during the off-line tests. This will particularly affect the recorded distributions during the on-line tests as concerns phase to phase discharges at the ends, which are 120' apart with respect to each phase. The off-line tests and physical examination of the bars will also senre as a means of either substantiating or disproving of what may have been inferred, conceming the PD induced aging rate or the nature of the PD laults, by various experimental expert systems in terms of the on-line test data.

Cavities occluded with the insulating systems of the stator bars will

be characterized by discharge patterns which tend to center around the voltage zeros i.e., at 0, 180, 360" in phase relationship to the applied voltage wave. This is supported by theoretical considerations, because the cavities breakdown at integer values of the breakdown voltage at

'discharge epochs along the ascending and descending portions of the sinusoidal wave [5,34]. As we have seen from our previous discussions precession of discharge epochs will occur due to the statistical time lag i.e., a given PD pulse will not occur always at the same discharge epoch; this mechanism causes a displacement of discharge epochs of all the subsequent discharges, which in addition are also governed by their own statistical time lags (the times required for a free electron to appear in order to initiate the discharge). The presence of a statistical time lag also implies that a given cavity will not necessarily always breakdown or "fie" as soon as the voltage reaches its breakdown value, but will breakdown at a voltage equal or higher than its actual breakdown voltage. It was demonstrably shown that the rise time of the PD pulse decreases and the magnitude of the charge transfer increases with the amount of overvoltage across the gap 1801. As a consequence, the PD pulse patterns are characterized by an inherent instability: constant movement or precession discharge pulses of varying amplitude on the phase angle scale with respect to the applied sinusoidal voltage waJe. Tt is thus palpably evident that irrespective of the sampling rate of the digital circuitry of the oscilloscope, no two PD pulse patterns even if taken in rapid succession can ever be exactly identical because their difference is intrinsic to the statistically induced anomalous behavior of discharge process per se.

If one of the dielectric surfaces in the cavity is replaced by a metallic surface such as that of the conductor in the interior of the bar or the stator ion core adjacent to a ground wall with its semiconducting layer abraded, the discharge pattern will tend be asymmetrical because the number pulses in the two polarities as well as their magnitude will differ. Nevertheless, the patterns will still center at or in the vicinity of the voltage zeros. The magnitude of the discharges in the slo!s at the ground wall will be substantially greater than those within the insulation of the bar itself, because they necessirily involve larger cavities both in terms of thickness as well as lateral width adjacent to the stator iron core, where the semiconducting layer enveloping the bars has been abraded initially through either mechanical vibration due to electromagnetically induced forces or discharge induced erosion beneath the semiconducting layer.

Two other common sonrces of PD are associated with the end windings of the machine and in the vicinity of the exits points of the bar from the slots. These discharges, which are either of the surface tracking type or surface corona between adjacent phase bars, may have magnitudes that may exceed even those of slot discharges. During off-line tests with the rotor removed, the end windings become readily accessible so that parabolic sensors may be used to locate end winding discharges, which are usually audible at operating voltages. However, a more effective technique involves the use of the so-called corona scope, which is essentially an ultraviolet (UV) radiation detector. It is relatively simple to operate, because the emitted uv radiation is converted to visible light so that the surface discharge at the end windings can be readily pin pointed from a safe distance away from the energized stator winding.

Location of intense slot discharge sites is commonly accomplished


the sinusoidal wave. However, in comparison to the rotor amounted antenna system for on-line location of PD discharges in individual stalor bars [231], the interpretation difficulties associated with the inductive rf probe for off-line tests are relatively minimal.

Figure 50. Schematic diagram of inductive probe for PD site location in generator stators (after Dakinel ni. 12271).

- -

Figure 51. Circuit for the bridge parallelogram method for the combined measurement of pulse and pulseless PD (after ASTM D3382 1691).

by means of rf inductive probe sensors based on a design developed by Dakin et d.[lll, 2271 and extensively used by others [228-230]. The inductively coupled probe is portrayed schematically in Figure 50 and consists of a semicircular ferrite core around which is wound a small winding; the ferrite core coil is connected via a coaxial cable to a quasi- peak reading rf meter or an oscilloscope. The ferrite core coil in conjunc. tion n'ith the capacitance of the coaxial cable constitutes a hmed circuit, normally adjusted for a frequency of 5 MHz, thoughother frequencies have been used to optimize performance for particular machine applications. The ferrite core coil, which is mounted on an insulating rod, must be moved along the entire length of each slot within an energized stator, which poses a serious safety hazard for operating personnel. A careful PD site locating procedure requires compilation of a detailed PD intensity map of the entire stator winding. The interpretation of the data is not a trivial matter, since signal coupling between adjacent bars is involved and the inductive probe does not distinguish between internal PD and those in the slot, though slot discharges tend to he of an appreciably greater intensity. The PD patterns involving slot discharges tend to be dominated by large positive polarity pulse discharges, which occur over the descending portion of the applied sinusoidal voltage wave as opposed to internal discharges, which, if they take place adja- 'cent to the stranded conductor surface. are of a much lower intensitv

Figure 52. Idealized parallelogram trace of the bridge circuit (after ASTM 03382 1691).

If it is desired to ascertain the discharge intensity in toto, i.e. take into account the occurrence of all three forms of PD, namely pseudoglows, true pulseless glow and pulse type, then bridge measurement techniques must be employed in lieu of the PD pulse measurement techniques. A bridge particularly suited for this task is that described by Dakin and Malinaric [228], which has since that time been modified by Povey [IO] and subsequently incorporated as a standard method in ASTM D3382 [69]. As can be seen from the circuit diagram of this bridge in Figure 51, which utilizes the so-called parallelogram method, the ordinate deflection axis of the oscilloscope is coupled to the detector terminals of the bridge while the input to the abscissa axis receives a fraction of the applied voltage from a capacitive voltage divider. Here C , denotes the capacitance of the machine coil specimen and C, represents the discharge-free standard capacitor. The bridge balance is obtained by manipulation of the bridge arm resistance R1 and capacitance C,; the bridge is initially balanced just prior to PD inception by vaging R, and C3 to compensates for the sinusoidal voltage drop across the parallel combination of R4 and C,, which yields a straight horizontal trace on the oscilloscope, whose length is proportional to the applied voltage. When the voltage is raised further to the discharge inception point, the abrupt vertical deflection output applied to the oscilloscope produces a parallelogram trace depicted in Figure 52.

In the parallelogram trace V, designates the peak-to-peak value of the PDIV and V, represents the peak-to-peak applied voltage. Since the height of the parallelogram is given by Oh, the total integrated PD

and are of negative polarity and appear over the descending portion if charge transfer (pulse and pulseless) Q2 is d i k e d hy

,

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Figure 53. Comparison at room temperature of the intemal PD discharge intensity of a virgin specimen bar with that of a specimen bar removed from its slot of a model stator that has been aged for 1500 load cycles at twice rated voltage (16 kV rms) and rated current (1620 A) at a conductor temperature of 122°C (after Barhikas and hlnrin [233]).

Q = DjS, (7)

A = D&,Dh (8)

where S, denotes the vertical deflection sensitivity Since the area of the parallelogram is given by

the energy dissipated by the PD is equal to AS& where S, is the horizontal (abscissa scale) sensitivity Hence, the combined pulse and pulseless power loss P a t the radial frequency w assumes the form

The shorting effect of the cavities by the PD within is manifested externally as an apparent increase in the overall capacitance of the specimen. Its increase is relatively very small relative to the capacitance of the specimen coil or the overall phase and is given by

where D& = VJS,. Equation (9) leads to the expression, which yields an indication of the void or cavity volume in the specimen relative to the soecimen volume 11111

where E' is the real value of the permittivity of thi composite bar insulation and CO is the geometrical capacitance in vacuo, of the bars. The vertical (ordinate) axis sensitivity of the parallelogram bridge S,, is determined, using the standard calibration procedure described in ASTM D1868 [HI. A square pulse generator is employed to inject a known charge via a calibrating capacitance C, and the vertical sensitivity is obtained in terms of the relation

(12) C,AV s, = ~

D:j where DI represents the vertical deflection resulting from the injected charge of C,AV, with AV the voltage pulse magnitude of the square pulse of sufficient width to prevent overlapping of the signal responses at the front and trailing edges of the square pulse. Also the rise and fall times of the excitation square pulse must be identical in order to equal- ize the bridge circuit's positive and negative pulse response, which as in

the case of other PD detectors depends upon the rise time of the calibration pulse as well as on the bandwidth of the PD detection circuit itself. From the practical point of view, it is expedient to obtain calibration with excitation pulse rise and fall times that are withm the range of the minimum and maximum rise times of the actual PD pulses observed.

When PD measurements are performed on individual rotating machine coils or bars, a narrow band (300 Mr) PD detection circuit is normally employed. At relatively low frequencies, the single bars or coils, which are short in length, essentially behave as lumped capacitance specimens, thereby permitting apparent charge calibration to be carried out with relative case. Both new and used bars are evaluated in terms of their PDIV and PDEV values as well as plots of the maximum apparent charge transfer us. applied voltage. The latter approach is particularly useful in ascertaining aging of H\' stator bars. Figure 53 compares the PD behavior in apparent charge units of a new bar with that of an aged bar subjected to 1500 load cycles under accelerated mul- tistress conditions in a three phase model generator at twice the rated lie-to-ground voltage. The PD tests were performed with the two ends of the bars submerged in an oil bath to ensure that the recorded discharge intensity was confined to the straight slot section of the bars Le., discharges occurring between the conductor and ground-wall enclos- ing the insulation. The peak charge transfer measured in pC is seen to be substantially higher and PDIV lower in the aged bar, inferring appreciable discharge induced degradation during the accelerated aging test.

When stator bars are evaluated in terms of the dissipation factor (t,an 6) tip-up, measured as a function of applied voltage, it is desirable to determine whether the increase of tan 6 with voltage is the result of voltage dependent dielectric losses or a consequence of PD. Usually, the tar1 &value may increase with voltage, but occasionally it may exhibit a decrease. A relation between the measured tan6 value and the PD pulse type discharge losses has been derived and is given below [15, 2341

I

_., r 1

where tan 6 represents the dielectric losses in the solid insulating system of the bar specimen, C is the capacitance before and C' the capacitance after pulse discharge onset, V is the rms value of the applied voltage, n3 is the dischargerate (pulsesper second) of thejth discharge of peak voltage V& and V, ( t ) is the instantaneous value of the applied voltage at which the j t h PD pulse occurs. The second term is the PD pulse contribution and it will increase as long as increasing numbers of cavities begin and continue to discharge with increasing voltage. Once all of the cavities are fully ionized and continue to discharge, t a n 6 will commence decreasing with applied voltage as the rate of increase of the term wV2 in the denominator of the second term becomes larger and exceeds the rate of increase in the PD power loss in the numerator. Thus, when all cavities become ionized and undergo discharge, the tan 6 us. voltage characteristic will assume a negative slope. Note that in the calculation of the pulsed PD loss contribution, the relative polarities ofV, and V, ( t ) must be taken into account in Equation (13). Should the dielectric losses predominate and the PD pulse discharge loss be negligible, then a negative slope of the curve will infer the occurence


of a space charge mechanism within the bar specimen that may arise due to ionic contamination in an uncured resin.

8 PD PATTERN RECOGNITION PD pattern recognition, as the name implies, is the ability to rec-

ognize and distinguish between different types of PD sources within the electrical insulating systems of power apparatus and cables and to differentiate them from extraneous interference phenomena. Over many decades in the past, this function and role was effectively ful- filled by expert observers, who possessed this cognitive ability, which they had acquired through personal field experience, which permitted them to discriminate between various discharge phenomena in terms of their pulse density, amplitude and phase distribution patterns [235, 2361. However, with the advent of intelligent machine technology, efforts were undertaken in the early 1990s to automate the cognitive PD test procedures. This effort has grown to such an extent, that now the area of PD pattern recognition constitutes an important and highly specialized subset in PD related studies and deserves, therefore, to be addressed separately

The task of PD pattern recognition may be approached in two separate ways. Computerized techniques may be applied in conjunction with certain statistical parameters that take into consideration the stochastic nature [34,35,38,39, SO] of the PD process; altematively, techniques, which require no statistical preconditioning of the test data, such as NN, may be employed. The advantage of the latter is that they do not require decisions to be made as to which statistical parameters should be used and the degree of bias that should be assigned to the selected parameters. This Section will be devoted to a brief description of the various cognitive procedures that may be utilized to accomplish the task of PD pattem recognition. PD pattern recognition analyzes may be carried out either in terms of PD pulse shape or the I'D pulse-height and pulse-phase (discharge epoch) distributions. The two approaches are inextricably interrelated by the nature of PD pulse discharge mechanism and the reason for selecting the pulse shape approach often centers on the low cost and the simplicity of the test instrumentation involved. The breakdown voltage of the cavity essentially determines the magnitude of the discharge pulse as well as its discharge epoch or phase of occurrence with respect to the applied sinusoidal voltage wave.

When the statistical timelag is long, i.e. the timeof appearanceof the initiating electron is prolonged, the breakdown takes place at a voltage above the nominal breakdown voltage and results in a shorter rise time pulse of large magnitude. Tnis not only alters the shape of the pulse but also causes it to appear latter in the applied voltage cycle, thereby changing the entire discharge sequence as well as the precession rate or movement of the pulses along the phase angle scale and, hence, the relative phase positions or discharge epochs [34]. Thus changes in pulse shape are accompanied by changes in the pulse-height and pulse-phase distributions. However, the pulse shape contains an additional item of information, which pertains or is related to the location of PD sites in the insulating system: as the pulse travels from the PD site to the PD detector, its form becomes increasingly more mutated or distorted (depending upon the transmission medium) due to the attenuation of its higher frequency components. The NN offers many options that render it useful in various tasks of PD pattern recognition because of the unique

structure [237], which consists essentially of an organized topology of interconnected processing elements and is designed for encoding and recalling information. Among these options rests their inherent ability to distinguish distinctive features of PD pulse pattems, as for example involving those of discrimination between different cavity sizes. An NN has arelatively short learning time, and once the learning period is completed, it is able to apply the taught knowledge to make reasonably limited generalizations, even on unknown input PD patterns. Thus, they prevail over other classifiers in that they are flexible and can adapt themselves to different statistical distributions. Furthermore, their response is insensitive to minor variations in the input, i .e. they can render correct decisions when the input deviates from that which they have been taught to recognize. as for instance stochastic variations in the discharge process within the discrete cavities themselves.

NN can be divided into three generic categories, namely those with required supervised training, unsupervised training, and fixed weighting procedures [238]. The latter category's application paradigm is primarily intended for association and optimization work, while the first two categories are suited for classification and are thus applicable directly to PD related investigations. There are three NN within these first two categories that are particularly suited for PD pattern recognition. These NN employ respectively the multi-player perception (MLP), nearest neighbor classifier (NNC) and linear vector quantization (LvQ) paradigms 12391. Both the NNC and LVQ networks are of the unsupervised training type [240], while the MLP network requires supervised training [241]. All three networks were evaluated by Mazroua et al . [242] in terms of their PD pulse form recognition capabilities, using artificial cavities of 1.0, 1.5 and 2.0 mm depths, and were found to perform acceptably well in assigning the correct classification for the three cavities of different size. However, where discrimination was required between smaller cavity sizes, it was found that the learning vector quantization paradigm was distinctly superior. In these cognitive test;, the features or attributes of the PD pulse shape or form used were those of area, decay time, rise time, width and magnitude of the pulse. Gul- ski and Krivda [32,243] also applied NN for PD pattern classification, using PD pulse-height/phase distribution data, but their experimental results were statistically preconditioned. While the cognitive capabilities of the evaluated NNC, LVQ and MLP paradigms are approximately equal, there exists a certain predisposition to favor the use of the MLP paradigm in work related to PD. The MLP paradigm differs from that of the NNC and LVQ techniques in that the structure of the N N can be designed to suit the intricacies of the PD pattern recognition task to be performed e.g. to differentiate between PD phenomena in electrical trees from that of cavities 12441. The general MLP NN structure is depicted in Figure 54 and consists of an input layer, one or more hidden layers and an output layer of neurons or processing elements. Each neuon has many input signals but only one output signal that is applied to every neuron in the next layer. Each connected pair of neurons is associated with an adjustable value that is referred to as the weight. ' h e number of layers and neurons therein is altered by trial and error to optimize the performance of the MLP NN in its recognition task required for the classification of PD patterns. The MLP network is trained, utilizing the back propagation technique [245], i.e. it is provided with both the input patterns and the desired response. The NN proceeds through a series

,

ZEEE Transactions on Dielectrics and Electrical Znsulation vol. 9 No. 5,OctoberZOOZ

of iterations; in each iteration a comparison is made of its own output with that of the desired response and a computation is carried out to determine whether there is a match. If a match is indicated, then no changes are made in the NN structure; however, if there is no match, then the weights are modified by means of the so-called gradient search technique to minimize the mean square difference between the desired response and the actual output. The error function, which must be minimized during the learning step, exhibits a series of local minima in addition to a global minimum. The global minimum is achieved by the gradient search technique, which estimates the weights that correspond to those at which the error surface is lowest. The leaming rate, which controls the width of the steps on the error surface, must not be too rapid in order to avert oscillations in the approach to the global minimum.

793

1npvt Isye, Hiddrr llvcr 0"lP"l Inyer

Figure 54. Architecture of MLP NN. Circles represent neurons, w,, wh and wo are the input, hidden and output layer weights respectively.

0.04

0.02

'0 20 40 60 80 100 120 140 160 180

0.04

0.02

'0 20 40 60 80 100 120 140 160 180

Number of iterations

Figure 55. Cavity us. electrical tree discrimination leaming c w e of an MLP N N (after Mazroua ef ai. [2441).

In reference to Figure 54, the feature or attribute inputs (wt, wh, and wo) to the MLP NN, in the case of PD pulse shape related PD pattern recognition analysis, are those of apparent charge transfer, rise time, fall or decay time, area under the PD current pulse (total charge transfer), the product of the pulse width and apparent charge and the energy. The function of the neurons is to receive all incoming attribute parameters, multiply them by the weights of the connections over which they enter and add the intermediate answers. Multiply the result by the Sig- moid threshold function f , defined by the plot of error us. weight (that contains the global minimum), yields an overall output t given by

z = f(&q + . . . + S,w, - 0) (14)

where SI and S, denote the outputs of the first and last hidden node or neuron respectively, and are described in terms of the input pulse shapes and weights w. Each of these weights is associated with an input threshold 0. The output z in Equation (14) must equal unity for one cavity size (e.g. 1.0 mm) and zero for the other cavity size ( e g 1.5 mm). The error, ei for the ith input pulse shape may be expressed as ,

e, = (di - t i ) - where zi is the actual output and di represents the desired output response. For-all the PD pulse shapes, the mean squared error (MSE) is thus

E = - x ( d i - y)'

(15) 2

(16) I n

i = l

where n is equal to the number of PD pulse shape attributes. To

1. to assign random values in the range [tl, -11 to all the weights and thresholds and to provide the NN with the input and d e sired output pairs;

2. to apply the Sigmoid threshold function and calculate the outputs zi and to initiate the learning process;

3. to adapt the weights by the usage of a recursive algorithm that starts from the output mode back to the first recursive layer.

The back propagation training a lgo r i th of an MLP NN is an itera- tive grading technique that accomplishes the input to output mapping task by minimizing the so-called cost function. The latter represents the MSE between the actual output of the MLP and the desired response. Figure 55 portrays MSE as a function of the number of iterations that are necessary for distinguishing the PD pulse forms associated with electrical trees from those of cavities 12441. The MLP network utilized contained a single output, with a cavity PD discharge being indicated by a unity output and zero output for a PD pulse associated with a tree. The number of neurons in the hidden layer of this particular network was equal to 30. Figure 55 demonstrates the success rate of the NN in its discrimination between the cavity and electrical tree PD sources: after 140 iterations, the MSE between the desired and actual outputs is perceived to be negligible small. A virtual classification of 100% is obtained for both the training and testing procedures without any need for statistical preconditioning.

In the design of MLP NN, the number of inputs and the overall topology of the network is determined by trial and error in terms of the resulting PD pattern classification performance: It is frequently observed that an increase in the number of inputs (attributes) does not necessarily result always in an improved cognitive ability of the NN. Recently, Salama and Bartnikas [246] developed a systematic NN design procedure for determining the appropriate number of inputs and second order neurons or nodes based on an autoregressive (AR) time-series concept.

Since the AR time-series mode1,functions in specific time-steps, it was deemed to be particularly promising for usage on highly time d e pendent PD patterns, for which synchronization of the measurement data (e.$ PD pulse amplitude, rise time and other pulse from attributes) is essential. The AR time-series concept necessitated deployment ofpar- ticular PD features that exhibit a definite trend as a function of a vari- able, such as applied voltage, which influences the overall PD process.

minimize the value of E, the back propagation algorithm is employed


Typically, the hysteresis characteristic of the maximum apparent charge 121)

mental data for this purpose. From the practical point view, utilization (22)

relevance in that it can be derived from meawements carried out by me order of the model is obtained by, calculating the error variance meam of a conventional PD detector in accordance with ASTM D1868 of the estimated output signal x for each model and the model with [l l] or IEC 60270 12231 procedures. If calibration in terms of apparent the least minimum variance is selected. The data treatment procedure charge is not feasible, then Peak Pulse in may can be readily illustrated with reference to a hypothetical hysteresis be as lonE as comoarative measurements are made On the same' characteristic of the maximum amarent charae transfer AQ,, us. V

where L , is expressed in terms of the covariance3atatrix P,

which is by definition, transfer AQm us. the applied voltage can provide the required experi-

of the AQm us. applied voltage curve has the decided advantage of

L, = Pxix [1+ $FPnil,]

P, = 11 - L&Rj P,-l

specimen or specimens of identical design and construction.

sugr I si.g.1

Figure 56. Cascaded M L P NN with double outputs (after Salama and Bartnikas [246]).

In mathematical terms, the AR time series model may be stated as

yt = alYt-l +azl'-z +...+ bl&-l + bzY,_,+. . . +et (17) where Yt represents the input signal at a time t , Yt-l is the input signal at the previous instant, and et denotes the input noise at a time t ; al, uz, bl , and bZ are parameter constants related to AQ,. Thus the vector for yt becomes

where

~ 4 2 3

yt = [$%lT . IS,] + et

[Q,] =[ai ,az , . . . , bi ,bz , . . . 1 (18) and

T [Ye] = [ y t - l , l ' - Z , . . ., K-1, Vt-2,. . . I where represent the parameter and state vectors respectively, and the subscript K defines the instant of the PD pulse event at which the data are being processed, as opposed to subscript t that denotes the time at which the PD data are being sampled. By definition the error at instant K is

E, = yt - +nTB, (19)

Here it must be emphasized that in the foregoing operation, the recursive least square identification procedure must be deployed to identify the AR time-series model parameters, which in essence minimizes the sum of the squares of the identification error Exl 8. The least square " i z a t i o n step yields a model parameter update of the form

8, = 0,-1 i E,L, (20)

.. . I I

in which the parameters a and b denote respectively the highest and the lowest recorded value of AQ,; hence for n data points, the step magnitude on the vertical scale is

(23) a - b

h = ~

n for which the corresponding mean values of the maximum charge transfer AQm at each of these discrete points are given by the set AQ,1, AQ,&z, . . . , AQmn at the corresponding widths of the hysteresis cumes AVl, AV,, . . . , AV, on the applied voltage axis; e.g. at the hysteresis widths of A& and AV,, the values are AQmz = AQ,l+ h, and AQms = AQy,,z + h, respectively.

The order or number of inputs of the NN determined by the AR time series analysis was 3. The NN architecture was arranged in cascaded configuration as delineated in Figure 56, with the indexed output from the first stage I d provided as a weighted quantity Since the NN was designed to discriminate between 1.0 and 1.5 nun depth cavities, the indexed output Id was arranged to indicate zero for a 1.0 nun cavity and unity for a 1.5 mm cavity The number of neurons in the hidden layer of the first stage was fixed at 6, and at 8 in the hidden layer of the second stage with 5 inputs. Note that in order to maintain clarity in the graphic representation of Figure 56, only 7 of the 8 neurons in the hidden layer of the second stage are shown. The success rate of the cascaded output NN in distinguishing between the two cavity sizes attained an exceptionally high level of 94%.

NN topology designs based on the trial and error approach are frequently characterized by relatively low PD pattern recognition rates. Cachm and Wiesmann [247] applied the MLP technique to PD pulse patterns, arising from two or more PD sources. Scale normalization was employed to separate different superimposed PD patterns along their contours on charge transfer us. discharge epoch (phase) plots. Once the PD pattems were separated, the NN classifier was used to examine the patterns individually. A combined PD pulse pattern recognition rate of 79% was achieved, which was still substantially better than the 50% rate reported by Hozumi et nl. [248]. However, irrespective of the NN design approach utilized, the MLP technique applied to I'D pulse pattern recognition has decided advantages. The success achieved in the application of the MLP NN to PD pulse pattern recognition must be attributed to their ability to create clustering shapes that are highly nonlinear. An MLP NN with two hidden layers is capable of forming arbitrary decision boundaries between classes of different cavity sizes, which can be made as smooth as required by selecting an appropriate number of hidden neurons in each layer [239,249-2511. Furthermore, minimizing the mean square error by the back propagation algorithm results in an approximation of the class posterior probabilities, so that the MLP NN estimates the optimal bays decision boundary 1245,2523.

.. ZEEE Transactions on Dielectrics and Electrical Insulation Vol. 9 No. 5, October ZOO.?

In contradistinction, the LVQ paradigm constitutes a piece-wise linear approach [239,253], since it is based on the nearest neighbor rule 1252, 2541 and, consequently, it can only achieve an approximation of the decision boundary Kranz and Hiicker [31,255] made use of the NNC paradigm on PD pulse pattern analysis. In their work on SFh power apparatus under high ambient noise conditions where they employed Fourier transform techniques for feature extraction from PD pulse-height/phase distributions, acceptably good results were obtained with the unsupervised NN. However, in a subsequent extension of the same work they conceded that supervised MLP NN have definite advantages 12561.

The application of fuzzy logic to the classification of PD pulse patterns has much to commend itself because of the nature and the intricate behavior of the PD process. Even under idealized pulse type discharge conditions without any attending complicating effects of simultaneous occurrence of true pulseless glow or pseudo glow discharges and even further assuming that only a small number of cavities are involved, it is difficult to ascertain in terms of the PD pulse pattern whether several discrete cavities or one elongated ellipsoidal cavity with a number of different discharge sites is involved. The interpretation problems are further compounded by variations in the statistical time lag (time required for a free electron to appear and initiate the breakdown of the cavity). Tne larger the statistical time lag, the larger the value of the overvoltage at which a given cavity will breakdown, which will result in larger charge transfers (I'D pulse magnitudes) as well as a shorter rise times of the discharge pulses [go]. This will lead to a precession of discharge epochs, effecting the entire pulse-phase distribution characteristic [34]. An added complexity will arise with power apparatus and cable specimens, in which pulses from various discharge sites will undergo differing amounts of distortion, attenuation and rise time degradation as they travel from their respective discharge sites to the PD detector. It is palpably evident that even identical voids situated at different distances from the detector will not have identical PD pattems not only because of the unequal propagation distances and material media in their paths but also because the availability of free electrons will not be identical at the various discharge sites.

Small Medium Large charge transfer

1

1 2 3 4 5 6 7

Apparent charge AQ (pC)

Figure 57. A membership function for charge transfer associated with PD pdses in a cavity (after Salama and Bartnikas 1331).

Optimization of rignrficant features

795

Figure58. Schemaficdiagramforafuzz~~logicPDpulsepanern clas sification system (after Salama and Bartnikas [33]) .

The foregoing scenario creates a rather fuzzy situation as concerns PD pattern recognition, and so one may beg the question as to why one should not fuzzify the approach to the problem, and apply fuzzy logic for PD pattern classification. It may thus be more expedient to identify PD patterns in more vague or less specific terms, as for instance PD patterns with large or small pulses or in the case of cavities as very large, large, medium and small size cavities, assuming equal gas pressure within the compared cavities. As a consequence of the randomness of the PD phenomena, one is compelled to consider the approximate range of apparent charge transfer and its correlation to the cavity sizes or diameters. In such circumstances, fuzzy logic procedures allow the usage of crisp number sets to furzify the measured real (crisp) values. In a fuzzy logic system, crisp inputs are mapped into furzy sets by means of a hzzyfier [257-2591. An inference engine, which utilizes rules, that have been devised to permit decision making, maps these fuzzy sets into other fuzzy sets that comply with the rules.. The output sets are mapped into crisp sets or real numbers by a defuzzifier. A fuzzy set is described by a membership function, which assumes values in the interval [0,1]; a typical membership function, for charge transfer associated with PD pulses, is portrayed in Figure 57 1331.

A schematic flow chart of information in a fuzzy logic based reasoning process is depicted in Figure 58; note that the membership function forms an integral part of stage 8. It can assume different configurations, which may be determined by numerical organization techniques or self-organization 12601. Fuzzy rules, which play dominant roles both in stages 5 and 8 in Figure 58, are defined to represent certain axioms of PD pulse behavior. They are thus based on expert knowledge of the intricacies of PD pulse behavior and their evaluation in the fuzzy reasoning process constitutes essentially a generalization of the modus ponens and niodus foliens inference process (stages 4 and 5 , Figure 58). Evidently, effective application of fuzzy logic to PD pulse pattern recognition requires considerable knowledge of discharge behavior for the purpose of formulation of the rules and construction of the membership functions. For example, correct classification of 3 cavity sizes (1.0, 1.5, and 2.0 mm depth) in terms of the PD pulse shape attributes requires 25


rules; only for the representation of the apparent charge transfer alone, 10 membership functions for coverage of the low, medium, high and in between charge transfer values are necessary [33]. Commercial pro- grams in fuzzy logic are available and may be applied to PD related

u investigations 12611. ...

Charge magnitude (pC) (a)

eo -0.5

Fractal dimension

Figure 59. Lacunarity us. fractal dimension of 3D PD pulse height/ phase distributions of three highly differing P D defects (A, B a n d C) obtained at 16 kV (after Candela et nI. [266]).

Some work has been reported by Satish and Zaengl[262] in the application of fractal features to the classification of cavities undergoing PD in terms of their PD pulse-height/phase distribution patterns. In their fractal analysis, they employed the geometrical parameters of fractal dimension (a measure of surface roughness of the 3D pulse distribution pattern) and lacunarity (the denseness of the geometrical/fractal surface).

Originally, fractal geometry was developed for analysis of landscape surfaces 12631, but later it was extended to other mathematical modeling applications 12641. In a later study, Krivda et nl. 12651 were able to achieve some further success in the use of fractals in distinguishing between noise and corona sources and artificial cavities. More recently, Candela et al. [266] carried out experiments, using a point-to-plane gap (A), a dielectric-metallic electrode gap (B) and an occluded cavity within epoxy resin (C) as PD sources. The three distinctly different lacunarity us. fractal dimension plots, obtained for the three foregoing PD sources and depicted in Figure 59, demonstrate the recognition capability of the technique. However, it should be observed that the recognition degree indicated by the separation between the three areas in the graph is to a great extent a result of the pronounced differences in the nature of the three PD sources utilized. By applying the fractal data with additional statistical preconditioning data to an MLP NN, the authors 12661 were able to attain a recognition capability of 98%.

In investigations related to recognition of PD pulse patterns, it is common practice to assign statistical operators to facilitate analysis of the PD pulse-height/phase distributions. In the nomenclature of the PD pulse pattern recognition system developed by Gulski [28] and employed by Gulski and Krivda 1321, the term skewness signifies asymmetry of the charge transfer and pulse count distributions with respect to the normal distribution and kurtosis refers to the sharpness of the same distributions with respect to the normal distribution. The so- called cross correlation factor describes the asymmetry of the positive and negative polarity discharge pulse pattern. In addition to another

_1 5 1 1.5 2 2.5

log [charge magnitude (pC)] (b)

Figure 60. Effect of simultaneously occurring P~mechanisms on the Weibull distribution function. (a) PO pulse height distribution; (b) corresponding Weibull plot (after Montanari et al. 12781).

statistical operator, which refers to the number of peaks in the respective distributions, there are 25 other specified statistical operators. The deployment of statistical operators reduces very substantially the required memory capacity of the system. Thus, a total of 29 statistical operators comprises their overall analytical procedure. The PD pulse pattern recognition task is carried out in terms of these statistical parameters either by visual examination by an experienced operator or by expert systems specifically devised by Gulski et al. [28,32,36,141, 267,2681. It is not clear from the tests carried out on electrical apparatus by these authors how their system responds when a multiplicity of discharge faults are encountered. The foregoing statistical parameter analysis technique has also been applied by Bozo ef al. [269,270] in their investigations on PD endurance of thin polymer films. They obtained high classification success rates with NN, whose input data was statistically preconditioned.

Weibull distribution statistical procedures were applied by Contin and Rabach on HV stator windings in an attempt to identify multiple PD sources 12711. Subsequent work by Cacciari et ai. [272] indicated, that when single PD sources are involved, the PD pulse height distribution may be approximated by a two parameter Weibull distribution and that by means of a mixed Weibull model coupled with a goodness of fit test, it is possible to separate several simultaneously occurring PD sources. In addition, Contin et ai. [273,274] were able to demonstrate that the

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 9 No. 5, October ZOO2 797

Weibull shape parameter p was an eigenvalue of the PD pulse source and was independent of its location in the overall insulating system. Mixed Weibull distribution analysis has also been applied to monitor PD induced aging in epoxy-resin insulators by Contin et al. [275]. More recent results reported by Contin et d . [276,2771 indicate that application of a five-parameter Weibull function leads to an effective recognition of PD sources in terms of their respective shape factors, even when they are occurring simultaneously. The shape factor p appears to be a more effective criterion for PD pulse pattern recognition than the skewness and kurtosis parameters usedin the statistical pattern analysis procedure developed by Gulski 1281. Figure 6U(a) shows a pulse-height distribution curve of a stator bar after 1425 h of aging; in (b) is given its corresponding Weibull plot, whose curvature at the upper end of the charge transfer scale is attributed to another PD mechanism [278]. It is further demonstrated that the shape factor /3 of the same PD source is identical, irrespective of whether the Weibull probability analysis is carried out in the terms of the PD pulse-height distribution or PD pulse shape characteristics. This result may be questioned, however, because with distant PD sites, the transmitted pulses will undergo distortion and their shape will differ substantially from those that would be recorded were the same PD sites located close to the PD detector.

multi-source PD patterns

i

i 3-level wavelet decomposi t ion

reconstruction of vertical & horizontal wavelet coefficients

to yield H & V images

1 magni tude and phase averaging of H & V

images I

i feature vector

formulat ion

Figure 61. A PD pattem classification system employing the wavelet transformation approach (after Lalitha and Satish [280]).

task. Recently there appeared two independent contributions [279,280], which have introduced the use of wavelets in the classification of PD pulse patterns. Wavelet transformation is characterized by its two vari- able approach of time-frequency or space-frequency, which lends itself readily to application of non-periodic irregularly recurring PD pulse type phenomena. Carminati et a!. [279] described an approach in which NN and fuzzy logic are combined in conjunction with wavelet transform to identify PD induced aging phenomena, which have been shown to affect the PD pulse form (principally a decrease in rise time and charge transfer) [281]. They were able to ascertain the degree of PD induced aging in terms of contour plots involving the pulse rise time with aging time as a parameter. Lalitha and Satish I2801 employed wavelet analysis techniques in conjunction with a NN to distinguish the PD pulse-height/ phase distribution patterns of highly differing PD sources, namely those of point corona, surface and cavity type discharges. A schematic flow diagram of their I'D pattem classification technique is depicted in Fig- ure 61. In another investigation, also involving classification of gross artificial defects, Ming and Birlasekaran 12821 were able to achieve sat- isfactory PD source identification rates with statistically preconditioned wavelet analysis procedures.

Notwithstanding the considerable effort expended in the area of PD pattern recognition using intelligent machines, current practice still re- lies heavily on human expertise in the identification and location of PD sources in electrical apparatus and cables 1283-2851. The associated instrumentation costs for PD pattern identification are still relatively high but they can be substantially reduced by having the pulse peak-detect hold and A/D conversion functions embedded in a microprocessor circuit boards [284]. This approach also decreases the signal processing time interval to 5 ps from the value of 8.3 /IS specified by Bartnikas in [286].

9 DISCHARGE DEGRADATION EFFECTS

Most insulating materials are organically based materials, which if subjected continuously to PD over long periods of time will eventually deteriorate and undergo ultimate failure. The degradation process is both chemical and physical in nature, and the actual failure mechanism may assume either an electrical, mechanical, thermal character or a combination thereof. Exposure of organic insulating materials to PD causes physical erosion due charged particle impact [5,48,287-2911 on the cavity walls; this is accompanied by chemical deterioration as the hydrocarbon molecules undergo bond scission. Various gases are formed within the cavity as a result of the attending reactions occurring between the oxygen withm the cavity's atmosphere and chemically activated surfaces of the cavity's walls. The gas pressure within the altered atmosphere of the cavity changes and that together with the changes of the wall surface conductivity alters the discharge behavior within the cavity [281]. The foregoing variables lead to a rather erratic discharp process, which is further compounded by statistical time lag I . " effects that cause the discharge mechanism itself to assume a stochastic behavior as overvoltages develop across the cavity, thereby altering the rise time, magnitude and pulse shape of the PD pulses and in turn leading to changes in the pulse discharge sequence and a precession of discharge epochs [34]. It is thus palpably evident from the foregoing

It is now for some time that researchers in the PD area have been utilizing wavelet techniques, which are especially effective in retriev- ing low magnitude PD signals under intense ambient noise conditions. Commercial software, already available, can be applied directly to this


considerations of internal cavities that PD discharge patterns are intrinsically unstable by their very nature and are, therefore, characterized by discharge evanescence and changes in its intensity with time that may be either gradual or abrupt. Consequently, it is not astounding, that aging tests carried out under PD conditions indicate that PD patterns, observed at the commencement or at various times during the progress of the tests, frequently bear little relationship to the PD pattern form recorded just prior to breakdown. I'D pulse intensity (pulse count and magnitude) cannot be always taken as a certain cogent indicator of impending failure: the discharge intensity may either increase or decrease with time as the failure event is approached. For example, in the author's experience, it has been generally found that with internally occluded voids, as in the case of solid epoxy insulated transformers, the discharge intensity may tend to gradually decrease with time until just prior to breakdown, one or several extremely large r D pulses may appear, presumably indicative of the tracking-type breakdown streamer propagation across the insulation thickness. PD intensity increases towards breakdown are normally observed with slot discharges in rotating machines as slot gaps at the ground wall of the bar grow in size or large delamination regions develop within the stator bars themselves. The subject of PD induced degradation and aging is indeed a vast area of endeavor and it would be difficult to treat it in detail here. However, the reader is referred to a series of monographs, which deal in depth with its numerous facets in [5,95,29&292]. The discussion in this Sec- tion will thus be primarily confined to discharge induced aging as con- cems its effects on the nature of the rD, their pulse form, detection and measurement. Here it may be of interest to note that discharge induced failures have been now recognized and reported for nearly a century [293,294]. As early as 1957, Reynolds [295] reported the discovery of oxalic acid crystals on PD degraded plastic surfaces and McMahon etnl . [5, 2961 in their studies on mechanical stress and discharge induced crack- ing phenomena on PE suggested that the formation of conductive oxalic acid crystals on the walls of occluded cavities within PE may account for the evanescence characteristics of PD pulses within these cai' rities ' as discharges are shorted by their conductive walls. Thus there is a direct feedback mechanisms between PD behavior and the PD induced degradation i.e., PD give rise to physical and chemical degradation within the cavities and then the resulting degradation modifies ipso facto the form and intensity of the discharge process itself. There may be instances when breakdown of the insulation occurs very suddenly and unexpectedly at a time when the PD pulse pattern appears relatively stable; sudden development of thermal instability may be a plausible explanation for such an event, if the heat generated from the conduction losses &thin the accumulated degradation products can no longer be effectively dissipated by the surroundings of the discharge site. The intricate and complex relationships between discharge intensity and the ensuing degradation rate of insulating materials have eluded thus far many efforts in the past, whose aim was to assess insulation life in the presence of discharges [5,53,54,281,287-292,296-3081. Cavity inclu-

In contradistinction, vented cavities as, for example, air gaps between the ground walls of stator bars and their respective slots, are considerably less influenced by discharge products which tend to be partially expelled. If one considers cross-linked PE (XLPE) cable insulation, then one is essentially dealing with occluded non-vented cavities. Oxidative degradation is the usual process of polymer degradation and the oxygen is available either from the limited atmosphere within the cavity as well as through some absorption and diffusion of oxygen into the polymer, which is a function of temperature and the extent off crosslinking as well as structure of the polymer. Amorphous regions tend to degrade more rapidly because they are more predisposed towards oxygen absorption. The degradation reaction is also promoted by the PD processes, which constitutes an efficient generator of ozone and atomic oxygen both of which are very chemically reactive. The first step in oxidative degradation involves the formation of a polymer radial R as a result of bond scission initiated by the bombardment of ions and electrons and photon radiation from PD, [305,306]

R H + R t H ' The newly formed radical may then react with molecular oxygen to form a peroxide (ROO),

which in turn may lead to the formation of a hydroperoxide (ROOH),

(24)

R' + o2 + ROO

ROO + RH i ROOH+ R

(25)

(26) The radicals formed in Reactions 24 to 26 may react further in ter-

minating type reactions, 2ROO i ROOR + 0 2

(27)

2R t R-R (25)

ROO + R + ROOR and

'00 r-----+-

0- 5 3200 2800 '.is00 1700 1600 1500

I

Wavenumbers ( c m ~ l )

Figure 62. M I R infrared spectra of XLPE specimen exposed to PD at twice the PDIV value in dry air for (a) 0 h; (b) 6 h; (c) 12 h; (d) 18 h (after GamezGarcia et ai. [306308])

Reaction 27 represents a crosslinking reaction, assuming that it occurs intermolecularly, while the peroxide ROOR in Reactions 25 and 26 is stable, provided it is not decomposed at high temperatures. The peroxy intermediates in Reactions 24 to 26 also can be involved in func- tionalization reactions with without cleavage of the polymer chain,

;ions within the insulating systems of electrical apparatus and cables, which are completely enclosed, must be considered as non-vented cavities. In such cavities, the gas atmosphere is static and, as a consequence, the created gases and wall deposits are not expelled from the cavity so that the chemical reaction products affect the nature of the discharge.

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 9 No. 5, October2002 799

These can lead to a variety of oxidation products, notably C OH (alcohol), C=O [carbonyl,-ketone or aldehyde), C-0-C [ether), COOH (car- boxylic acid), COOC (ester) and epoxide [305].

Figure 62 depicts multiple internal reflection (MIR) infrared spectra of a XLPE specimen, obtained in dry air with PD exposure time as a parameter [306-308]. Readily discernible changes in the transmit- tance level occur at 3425 cm-' (hydroxyl, -OH) and 1637 cm~l [vinyl, C=C) while a very pronounced change in the spectrum is observed- at 1715 cm-' [carbonyl, C=O). It is found that in XLPE, the additives used play a very significant role in the formation of surface degradation products when XLPE is exposed to the action of PD. Crosslinking of PE is achieved through the use of dicumyl peroxide [309] and the ensuing crosslinking reaction is accompanied by the formation of decomposi- tion products. The latter, which comprise acetophenone, cumyl alcohol, and methyl styrene and cumene diffuse eventually out of the polymer bulk onto its surface. At the surface these by-products react chemically in the presence of PD, resulting in surface deposits which are different from those that would form with PE surfaces only

L I Time 100 nsidiv

Figure 64. Typical PD current pulse form with ion cment tail of an associated charge transfer of 8.7 pC, characteristic of discharges in 0.5 mm air gap with an epoxy resin electrode undergoing discharge within a test period extending from 180 to 8W h (after Hudon et nl. [541)

70 , I

10

0

Exposure time (h)

Figure 63. Mean apparent charge transfer above 20 pC associated with pulse type discharges occurring within a 0.5 mm air gap with an epoxy resin electrode as a function of PD exposure time (after Hudon et 01. [54])

Of particular practical interest are PD aging effects observed with epoxy surfaces exposed to discharges, because epoxy is extensively used in conjunction with mica as an insulant in stator bars, with silica fillers in solid-type transformers and in spacers in compressed gas cables. Stator bar insulation is unique vis- -vis insulation in solid-type transformers and cables in that, unlike in these where specifications require complete or near absence of discharges it frequently operates in presence of discharges and at times under rather intense discharge conditions. Its redeeming feature is that epoxy is used in conjunction with mica, which is a highly discharge resistant inorganic material. When epoxy resin is exposed to PD, its surface, as in the case of XLPE undergoes modification in several stages. Hudon et ai. [54,281] found that in a non-vented cavity initially the discharge process is characterized by large discharge pulses of the spark-type during which time the

degradation products on the epoxy surface assume the form of liquid droplets. As the PD pulse magnitudes diminish with time and transition occurs from a pulse to a pseudoglow and true glow pulseless regime, the droplets are replaced by crystals. The dimensions of these crystals continue growing over the pseudoglow and pulseless discharge regime until ultimate breakdown ensues. The discharge behavior is similar in both air and nitrogen atmospheres, indicating that the oxygen required for the observed chemical reactions is supplied from the atmosphere within the gap [when available) and by oxygen from within the molecular structure of the epoxy resin itself. Analysis of the droplets reveals the presence of a mixture of acids, consisting of formic, gly- colic, glyoxalic and nitric acids; the crystals are identified by meam of Debye-Scherrer X-ray diffraction analysis as consisting of hydrated oxalic acid. Venting of the gap or cavity does not appear to alter appreciably the amounts of degradation products formed. electron spectroscopy for chemical analysis (ESCA) indicates a rise in the level of oxidation; amounts of all oxidized groups comprising COOH, COOR, COH and C-0 are perceived to exhibit substantial increases with respect to the C-C and C-H peak at 285.0 eV The effects of chemical degradation of the epoxy resin surface exert a very profound influence of on the PD behavior itself, which is the cause of the induced degradation. Figure 63 portrays the variation of the mean value of the charge transfer equal or exceeding 20 pC (average of 100 measurements) as a function of the PD exposure time. Beyond 320 h, the charge transfers fall below the 20 pC; and a typical PD pulse form characterizing the PD regime of small charge transfers down to 0.5 pC over the testing time scale from 180 to 800 his depicted in Figure 64. It can be recognized that the pulse form represents a classical Townsend discharge in which secondary emission is sustained by ion impact at the cathode i.e., the waveform evinces, the protracted ion conduction current tail. The magnitude of the recorded charge transfers associated with PD pulse type discharges is found to continue diminishing beyond 800 h until no further PD pulses can be detected.

Photomultiplier measurements demonstrate that discharges exist even after PD pulse type discharges cannot any longer be detected.


0 k ' 200 ' 4kJ ' 400 ' 800 ' lobo

Exposure time (h)

Figure 65. Photocurrent emission from a discharge within a 0.5 mm air gap with an epoxy resin electrode as a function of PD exposure time (after Hudonet nl. [281])

Figure 66. lntew spark-type PU pulse with an associated charge transfer of 5W pC in a metallic-epoxy resin electrode gap observed at the beginning of a long term aging test (after Hudon et ai. [54])

Moreover, even over the time scale over which sporadic PD pulses of very low charge transfer magnitude (- 0.5 pC) may be still intermit- tently detected, their number or intensity is not sufficiently large to account for the overall recorded photomultiplier current. From this behavior, one must deduce that low intensity PD pulses occur concurrently with pseudoglow and true pulseless glow discharge even before all remnant small PD pulses disappear entirely. Figure 65 shows the photomultiplier output as a function of testmg time. Note that above 800 h, no PD pulses are detected and, consequently, in that region the photomultiplier cment results entirely from pseudoglow and pulseless discharges. Here the photocurrent is of the order of 0.8 A, which is still substantially above.the dark noise current of 1 pA of the photomultiplier employed.

Large intensity intermittent discharge pulses may be occasionally observed in short air gaps at the commencement of aging tests. These occur before the surface of the epoxy resin becomes increasingly more conductive due to the accumulation of acidic degradation products on its surface. Such a pulse is portrayed in Firmre 66 and it represent

applied here, because a short gap is involved and the term streamer would be misleading since rapid rise-time large magnitude discharges in short gaps are also Townsend type discharge in that the discharge is sustained by photoemission at the cathode due to intense space charge build-up. Thus the feedback mechanism is again emission at the cathode, but now it is due to photon impact in lieu of ion impact as in the classical Townsend discharge case. Here it well to be again adamant in the usage of correct terminology and reiterate once more that the term streamer discharge refers to a rapid propagation of a breakdown event in a long gap as a streamer propagates by means of photoionization in the volume of the gap [44,45].

Slot PD behavior in stator bars, which employ mica-epoxy bonded insulating systems, differs appreciably from the behavior of non-vented short gaps and internal cavities within the insulation system of thebars. Slot discharges involve much larger charge transfers and the discharge products, which, while they can accumulate on the walls of cavity inclusions, tend to be in part abraded by vibration of the bars that may become loosened should the wedges fail to retain them rigidly within the slots and dispelled to a limited extent by forced cooling gas currents. However the exterior surfaces of PD degraded bars within the slots, is covered generally by a white powder, which may still contain some of these conductive acid degradation products. The large pulse discharges in the slots are likely to be accompanied by significant amounts of pseudoglow and pulseless discharges; visual examination of the surface discharge phenomena at the coil ends of machine windings would suggest the presence of glow discharges.

Figure 67. Pu and dielectric loss behavior of 15 kV oil-impregnated- paper power cable subjected to an aging test at IO kV above its rated voltage (cn. 1960, after Barmikas 13101).

In comparison to solid polymeric insulation, oil or dielectric liquid impregnated paper insulation is much less susceptible to PD degradation. Some of the v e v early oil/paper cables, which were known to have internal discharges have operated for long periods of time before being replaced by newer and more discharge-free design constructions. Mineral oils with aromatic constituents and synthetic oils with gas in- hibitingadditives absorb gases under electrical stress; even in the absence of electrical stress, some physical gas absorption occurs in the oils, which further impedes the formation of ionizable cavities within the solid-liquid insulating system. In addition extra HV cables are pressurized to maintain the gases in an absorbed state within the oil. The remarkable performance of the oil-impregnated-paper insulating system is demonstrated in Fiwre 67, which depicts test data obtained on

. I - " a form typical of spark type discharges. The term spark discharge is an early 15 kV rated oil-impregnated-paper lead sheath covered cable

,

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 9 No. S,October2002 801

that was subjected to an accelerated voltage agmg test at 10 kV above its rated value [310]. The presence of continuous PD is indicated by the PD pulse discharge rate and the relative average discharge current us. time characteristics. The dissipation factor ( tan 6) value, which is less sensitive to PD, reveals two intense flare-ups in PD pulse activity over the testing time scale. The PD activity peak, are seen to subside in the presence of the lowered PD activity. The increase in PD activity is attributed to cavity enlargements or new cavity formations due to evolving gases at increased voltage stress. Readsorption of the gases by the non-saturated aromatic constituents of the oil as well as polymer- ization or cross-linking of the oil, which causes formation of waxes that fill-in the voltage stress created cavities, are the primary factors responsible for the 'self healing' behavior evinced by the oil/paper insulating system.

B Numberofswitchingsurges(10Wpr~lep)

Figure 68. PD inception stress at 50 H z of a HV bushing us. the number of 1000 switching surge steps, measured over a one second time internal; (a) with 12 h impulse stress-free rest period and (b) without 12 h impulse stress-bee period; 0.5 mm lnsulation thickness (after l'ompili et ni. [3111)

Figure 68 illustrates the gas absorption capability of a mineral oil, used as an impregnant in a HV bushing which was subjected to a series of switching surges [311]. The number of switching surges is seen to result in a reduction of the PD inception voltage, however, if the insulation is given a rest period between the subsequent series of impulses to permit the physical gas re-adsorption by the oil to proceed, then the reduction in the PDIv is substantially reduced.

10 CONCLUSION Current narrow bandwidth type PO test specifications on newly

manufactured cables, capacitors and transformers are of the go-no go type in that they require complete absence of discharges in polymeric insulated cables and permit only moderate upper discharge level magnitudes for capacitors and large oil-filled transformers. The primary concern in go-no go type measurements is that of sensitivity, High sensitivity levels are particularly difficult to achieve with high capacitance specimens; thus with capacitor specimens having high capacitance, balanced-type PD measurements are preferred to improve the signal-to-noise ratio. With pressurized gas cables, which behave essentially as low loss transmission limes wideband PD measurements techniques are preferred with which lugh signal-to-noise ratios are achievable. In the area of equipment maintenance, discharge site location in soliddielectric extruded cables is normally accomplished using me-

readily accessible, rf probes of either the capacitive or inductive type are utilized, depending upon whether or not cables are shielded or have exposed unshielded sections such as splices. In capacitors and transformers (with the exception of discharges occurring deep in the winding structures), acoustical PD measurement techniques are found to be more effective in the location of PD sites. Frequently, both electrical and acoustical procedures are employed jointly; such approach is most popular in low dielectric loss compressed gas cables where large bandwidth PD detection techniques are normally utilized, which unlike solid dielectric cables, are characterized by low acoustical impedances. PD site identification requires the deployment of PD pattem recognition techniques, based either on PD pulse-height/discharge epoch (phase) distributions or PD pulse form analysis. Intelligent machines may be applied for this taskandare found to performadequately wellin relatively simple cases as in distinguishing between discharges in cavities within the insulating systems and gross defects such as poor grounds or common sources of extraneous interference. However, the interpretation and identification of complex discharge patterns still necessitates the services of experienced observers. Rotating machine insulation commonly operates in the presence of PD discharges, whose intensity under certain conditions may attain substantially elevated levels. As a consequence, the approach to PD measurement on rotating machines differs appreciably from that on other electrical apparatus and cables in that it is essentially designed and implemented to monitor the discharge activity This task can be carried out by measurement of PD pulse-height and discharge epoch (phase) distributions, maximum charge transfer, discharge current, and during off-line tests, PD inception and extinction voltages, as well as t a n 6 tip-up; it represents an area of endeavor where considerable effort has be expended in the interpretation of the PD pulse patterns in assessing both the PD induced aging rate as well in the identification of the location and nature of the PD sources. The most effective approach appears to center on the accumulation and subsequent analysis of field PD data obtained over regular test intervals on the same machine as well as on other m a c h e s of similar design, a task in which the expert observer plays a critical role. Most effective means for locating discharge sites involves the use of the classical Dakin-type rf inductive probe with which each slot is scanned with the rotor removed and the stator energized; however, this procedure does not take into account any possible vibrational displacements of the bars while the machine is in operation under full load conditions. Surface discharge site location at the end windings of the rotating machine is carried out also with the rotor removed. Surface discharges are readily detected in terms of the ultraviolet (UV) radiation emitted by the discharges, using remote directional UV detection devices. PD pulse measurements on polymeric cables, capacitors and transformers are normally performed using narrow bandwidth instrument and the PD quantities are recorded in apparent charge units expressed in pC. Cal- ibration in pC units becomes more difficult with inductive specimens such as transformers and rotating machines. However, there is an important subtle difference between these two types of specimen. While the inductance effects in transformers are generally more pronounced than in rotating machines, the PD pulse density (number of discharges per cycle) in transformer specimens tends to be very substantial less

'

dium bandwidth pulse reflectometry; altern&vely i i t h cables thit are than that in rotating mackes .


This redeeming feature minimizes overlap of adjacent oscillatory PD pulse transients, thereby ensuring a more meaningful measure of the apparent charge in terms of the PD pulse magnitudes. In rotating machine specimens, the presence of a relatively large number of pulses per power frequency cycle leads to a reduced time interval between the adjacent pulses resulting in increased PD signal integration and consequently, to increased error in the recorded pC value. Calibration in apparent charge units becomes extremely difficult at very high PD measurement frequencies, and it is for this reason that high frequency PD test methods on rotating machines measure PD quantities only in relative units QV).

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Manuscript was received on 28 December 2001, in final form 251une 2002.

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