1068 EEE Transactions on Power Delivery, Vol. 7, No. 3, July 1992

DEVELOPMENT OF VOLTAGE

Ginzo Katsuta Atsushi Toya

A METHOD OF PARTIAL DISCHARGE DETECTION IN EXTRA-HIGH CROSS-LINKED POLYETHYLENE INSULATED CABLE LINES

Keiichi Muraoka

The Tokyo Electric Power C o . , Inc. Tokyo, Japan

Abstract - Deterioration in the insulation performance of extra-high voltage XLPE cables is believed to be attributable to the deterioration caused by partial discharges. In our study, after using an XLPE cable to investigate the behavior of partial discharges under various adverse conditions, we succeeded in developing a highly sensitive new method of measuring partial discharge in XLPE cable lines. Partial discharges in a 275 kV XLPE cable live line have been measured using this newly developed method. As a result, a detection sensitivity of 1 pC has been achieved.

1. INTRODUCTION

In order to meet power demand in the center of large cities, electric power has been directly transmitted from suburban substations using cross-linked polyethylene (XLPE) insulated cables laid in cable tunnels[ll. After completion of long-distance high voltage cable lines, it is desirable to check insulation performance initially and routinely during operation.

After manufacture of XLPE cables by the "dry curing process", extra-high voltage (EHV) XLPE cables are generally provided with metal sheaths and laid in water-tight cable tunnels. Thus, the deterioration in the insulation performance of the cable is presumed to be mainly attributable to partial discharges (PD). In order to develop a method of detecting PD causing the deterioration in cable insulation, the following are needed;

(1) An understanding of PD behavior generated in

(2) Development of a highly sensitive method of PD cable insulation.

measurement in live cable linesf2.31.

Takeshi Endoh Yasuo Sekii Chuki Ikeda Member

Hitachi Cable, Ltd. Hitachi , Japan

of a 215 kV long-distance XLPE cable line. It was found that the new method improved detection sensitivity by 2 to 3 orders of magnitude and was effective in evaluating the cable's insulation performance.

2. BEHAVIOR OF PD AT DEFECTS IN MODEL XLPE CABLES

2.1 Causes of Insulation Deterioration in XLPE Cables

Our major goal in evaluating the insulation performance of XLPE cable lines was to detect insulation deterioration and to predict insulation breakdown (BD). The following are regarded as major causes of XLPE cable BD:

(1)Voids and foreign materials in the insulation layer

(2)Protrusions of the inner and outer semi-conductive layer [ 4 , 5 1

(3)Water treeing

In the case of existing BD tests, the BD channels have for the most part been damaged by current flowing through these channels, and it is difficult to identify the causes of the BD. We previously focused our attention on the phenomenon of electric tree development in the defective area which accompanies PD, and developed a method of pre-breakdown PD (PBPD) detection which makes it possible to isolate the defective area by shutting down power just before BDI6,71.

Fig. 1 is an example showing that the PBPD method permits identification of the cause of XLPE cable ED. This example in Fig. 1 clearly shows an electric tree

Fig.1 An example of a BD path due to foreign material.

In this study, we first investigated the behavior generated by metal impurities in the insulation layer of PD occurring at defects in XLPE cables. We then of an XLPE cable. As a result of ED testing of various developed a new method of measurement and developed XLPE cables, employing the PBPD method, it has been technology capable of detecting PD in live cables from confirmed that foreign materials and protrusions of both sides of the separated polyvinyl chloride (PVC) the inner or outer semi-conductive layer cause BD. jacket of insulation-type straight-through joints (IJ) . The new method has been used to measure the PD 2.2 PD Characteristics of Samples Containinq Metal

In order to investigate the BD caused by foreign 91 SM 341-8 PWRD A paper recommended and approved materials, an experiment was performed using a by the IEEE Insulated Conductors Committee of the cable-like electrode configuration model. The model IEEE Power Engineering Society for presentation cable used in our experiment is shown in Fig. 2 . A 0 . 1 at the IEEE/PES 1991 Summer Meeting, San Diego, mm diameter, 1 mm long stainless steel wire was molded California, July 28 - August 1, 1991. Manuscript within the 3.2 mm thick insulation area of the model submitted January 31, 1991; made available for XLPE cable. The BD channels in the sample in which BD printing May 17, 1991. occurred at 36 kV (Emean=ll kV/mm) are shown in

Fig.3, and the behavior of simultaneously measured PD is shown in Fig. 4 . Although the magnitude of the PD was approximately 3 pC when it was measured

Impurities

0885-8977/92/$3.0001992 IEEE

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short time and finally reached 1000 pC just prior to BD. This experimental result suggests that the PD detection sensitivity used to evaluate the insulation of operating XLPE cable lines should be at least 1OpC.

2.4 PD Characteristics of a Specimen Containins Voids

Voids in the insulation layer are claimed to have an advese effect on insulation performance when ac voltage is applied for a long time [ 9 1 . For this reason, we next investigated the PD characteristics of an XLPE cable containing voids. The void-containing xLPE cables used for our experiments were manufactured in such a way that gas pressure was intentionally decreased during cross-linking. The thickness of the insulation was 6 m, and the maximum diameter of the voids was 2 mm. A cable with an effective test length of 8 m was used as a sample to measure PD.

Curve (a) in Fig. 7 shows the results of measurement at ambient temperature in the initial stage of PD. The PD inception voltage, Vi, was 17 kV (Emeat153 kV/mm), and Qmax was approximately 17 pC. Curve (b) shows PD characteristics measured after the specimen was charged at 60 kV for 10 days for the purpose of aging. This result shows that the PD finally extinguishes when voltage is applied continuously. C y v e (c) shows the results of PD measurement at 70 C. As seen in this curve (cl, when the temperature of the test sample rises, Vi drops again and PD characteristics almost the same as the original ones are observed. 2 300

4J

applied voltage (kV) Fig. 7 Voltage dependence of an XLPE cable containing

voids.

molded s ta in less steel w i r e Fig. 2 Cross-section of the model cable.

Fig. 3 BD path produced by stainless steel wire.

time passage Fig. 4 An example of PD pulses.

immediately after it started to occur, after 5.6 seconds it reached 500 pC, just before BD.

2.3 PD Characteristics of Specimens with Inserted Needles I81

The PD characteristics of XLPE cables containing inserted needles were investigated. A needle electrode with a tip radius of 5 p m was inserted into the insulation layer from outside the cable to simulate protrusion of the inner or outer semi-conductive layer. The tested cable was a 66 kV XLPE cable with an insula$ion thickness of 9 mm and a conductor size of 100 am . Fig. 5 represents a block diagram of the PD measurement circuit. Fig. 6 shows PD behavior when the needle is inserted approximately 1.0 m into the insulation layer and PD is generated at 60 kV (Emeanl6.7 kV/mm), which corresponds approximately to the normal operating stress of a 275 kV XLPE cable. The PD whose magnitude was 0.05 pC immediately after generation increased almost to 10 pC within a very

inner sed-cond.

outer s d - c o n d .

Fig. 5 Block diagram of the PD pulse measurement circuit.

0 10 20 30 40 50 time evolution of PD (min)

Fig. 6 Evolution of PD over time.

3. A NEW mTHOD OF DETECTING PARTIAL DISCHARGES IN LONG DISTANCE CABLE LINES

Based on the results of our investigation of PD behavior in the various specimens containing defects, it was demonstrated that it is desirable to detect PD of at least 10 pC in order to diagnose insulation defects in cables in actual lines. However, in existing live lines , it has been considered difficult to achieve a detection sensitivity of 10 pC because of the influence of the external noise within actual cable lines. It is for this reason that the authors have tried to develop a method o f PD measurement based on the new detection principle. This new method of measurement will be described below.

3.1 Principle for PD Detection

Fig. 8(a) shows the equivalent circuit of PD occurring in the insulation. Here the voltage vg applied to the gap Cg is given as follows:

1) vgEvt.(Cb/(Cg+Cb))=vt-(Cb/Cg) ------ (Cg>> Cb)

(a)Equivalent circuit of PD in insulation.

(b)PD detection circuit.

Fig. 8 Basic PD detection method.

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If discharges start at vg=Vg, the resultant voltage AV between the test pieces is given as follows:

N=Vg.(Cb/(Cm+Cb))%Vg.(Cb/Cm)iq/Ca --- 2)

Here, q =qr-(Cb/(Cg+Cb)), (q:apparent charge) qr=Vg -Cg

In actual PD measurement, AV generated between the capacitance, Ca, of the whole test specimen is guided to the external circuit through a coupling capacitor Ck, and the equivalent circuit is shown in Fig. 8(b). Here, the voltage, Vd, developing at the detection impedance Zd can for the most part detect AV in the case of Zck 4Zd (2ck:impedance of the coupling capacitor). This means that it is inversely proportional to Ca, i.e., the longer the test specimen, the poorer the detection sensitivity.

3.2 PD Detection

Fig. 9(a) is a connection diagram for PD detection by the new method in the IJ required for the cross-bond connection of the sheaths of long cable lines. Fig. 9(b) shows the equivalent circuit of an XLPE line centering on an IJ. For example, if PD is generated from capacitance C1 on the right side of the IJ, capacitance C2 on the left side of the IJ acts as a Ck, and PD is detected in the Zd linking the insulation cylinders. The advantages of this method are as follows:

IJ foil electrodes \ ,insulating cylinder

6 .

;able Y A L r (a) detection circuit

,high Voltage cond. .-A J.

I I metal sheath -,== Z d insulating cylinder

(b) equivalent circuit a€ detection circuit

Fig. 9 Detection of PD at an IJ.

(1)Since no special high voltage power supply or Ck is required, the work of making these connections is eliminated

(2)Because detection is performed at the IJ, where high sensitivity detection of PD is desired, attenuation of the PD pulse due to travelling in XLPE cable lines does not occur, and the PD at the IJ can be detected.

(3)It is not necessary to change the connections in the cable system tested, and it is possible to detect PD in live lines.

In order to make it possible to measure PD safely in live cable lines, metal foil electrodes should be applied to the anti-corrosion layers on both sides of the insulation cylinders of the IJ as shown in Fig.9. In this case, the sheath can be regarded as being connected via the capacitance (C3 and C4) between the metal foil electrodes and the joint casing.

3 . 3 Calibration Method for Measurement Sensitivity

(1) Indirect injection of calibration pulses from the metal sheath: Sensitivity determination in live lines is also required for quantitative PD measurement. In the present method of PD measurement, the calibration pulses are "indirectly injected"

between the high voltage conductor and the sheath of the test cable, and pulses are detected in the measurement system to determine sensitivity. A method of connecting the calibration pulse generator to the metal sheath detection area in pararell has been devised, and the corresponding circuit is shown in Fig. 10(a). Here, CO is an in-series capacitor integrated into the calibration pulse generator and ranges from approximately 10 to 5 0 pF. C1 and C2 are approximately 0.15 pF/km in the case of the cable. In the case of a long cable they are several thousands times larger than CO, i.e.,

(a) new cal.rnethod (b) conv.ca1 method Fig. 10 Comparison of equivalent circuits for the

conventional and new calibration method.

3) c1=c277co ------------------

Thus the calibration pulse generator is regarded as a constant current source.

Moreover if the impedance Zd of the detection circuit is selected to satisfy the following condition 4), the influence of Zd can be ignored.

4) Zcl=Zc2d<Zd -_---_------------

(Zcl, Zc2: impedance of C1, C2)

Fig lO(b), on the other hand, shows a circuit representing the conventional "direct injection" method. Here the influence of Zd can be ignored in the case of Zck 4 Zd (2ck:impedance of the coupling capacitor). As can be seen from a comparison between Fig. lO(a) and (b), a pulse voltage twice that in Fig. 10(b) is applied to the end of Zd in the case of the new device in Fig. 10 (a), if a constant current is supplied by a calibration pulse generator. That is, it is calibrated by a pulse voltage twice that in Fig. 10 (b) . The calibration pulses are "indirectly injected" via both metal sheaths of the IJ and are detected at the same two points with this new device. Thus, it was discovered that calibration is possible in live lines using this device, and this type of calibration is referred to as "proximate calibration" .

(ZICalibration pulse injection via metal foil electrodes: In this case PD detection is achieved by applying a second pair of metal foil electrodes to the sheath on either side of the IJ in addition to the detection electrodes, and the calibration pulses are injected via these metal foil electrodes instead. If the respective capacitances of the foil electrodes on the casing are made sufficiently larger than CO, their influence can be ignored. This then becomes the same as the case in 3 . 3 ( 1 ) in which injection is via the metal sheeth.

3.4 Indirect Injection of Calibration Pulses from the Ajacent IJ

Because of pulse attenuation, existing method of PD detection are not always sensitive enough to detect PD generated at an IJ several hundred meters away. Particularly when measurements are carried out at high frequencies, attenuation rate depends to a great extent on the frequency measured. In this case, the calibration pulses are injected via the IJ ajacent to the detection section, as shown in Fig. 9 , and this makes it possible to determine PD detection sensitivity from the cable leading to it. Calibration by injecting pulses via an ajacent IJ is referred to as " ajacent calibration".

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high-impedance Zd was connected to both sides. The calibration pulses were "directly injected" at the center of the cable and were "indirectly injected" via the two sheaths. The SA response when a calibration pulse of 2500 pC was injected is shown in Fig.13. As seen in the figure, the "indirect injection" method yielded a signal whose magnitude was twice (approximately 6 dB) that of the conventional "direct injection" method, and the results were the same as obtained when the waveform was observed on the oscilloscope. These findings demonstrate the validity of the 'indirect injection" method we have developed.

4.BASIC EXPERIMENTS INVOLVING THE NEW METHOD OF MEASUREMENT

As a result of theoretical studies of the new PD measurement method, it has been found to be an effective method of measuring PD when cable lines are long.

4.1 Sensitivity Calibration Method Experiment

(1) Calibration experiment using a low-frequency type detector: Fig.11 shows the experiment using the low-frequency method, described in section 3.3 below. The test spefimen was a 5 m long, 66 kV XLPE cable with 100 mm conductor, at whose center the metal

8 66kV XLPE cable 5m X 1 - fim

Fig. 11 Test circuit for the proposed calibration method using an oscilloscope.

sheath and semi-conductive layer were removed for a distance of 10 mm. A copper casing for a 66kV IJ was electrically connected to the exposed area. In addition, two sets of foil electrodes were attached to the outside of the PVC jacket on the casing. The capacitance between each foil electrode and the casing was approximately 1500 pF. While a calibration pulse (C0=50 pF) of 2500 pC was "directly injected" via the end in the conventional method in Fig.lO(b),in our new method in Fig.lO(a) the calibration pulse was injected via the newly devised foil electrodes. The voltages induced at the detection foil electrodes were then monitored on an oscilloscope. The results are shown in Table 1. As seen in the table, approximately twice the voltage pulse was induced in the case of the'indirect injection" via the foil electrode of. this method, and the difference between the results of the conventional method and this new method was approximately 3%.

Table 1. Results of testing the proposed calibration method using an oscilloscope.

method and sheath between two foil

PVC jacket

( 2 ) Calibration experiment using tuning-type PD detector: A calibration experiment using the tuning- type PD detector was performed. In this experiment a Spectrum Analyzer (SA) was employed as the tuning-type PD detector. The SA we used in our experiment was type of "MS2601A" of Anritsu Corp., the measurable frequency range of which was 9 kHz - 2.2 GHz. The experimental circuit is shown in Fig.12. An approximately 20 m long coaxial communication cable was used, and both ends were connected to an impedance-matching resister. At the center of the cable, the sheath electrode was disconnected and a

Fig. 12 Experimental circuit for calibration by the frequency-tuning method.

tuning freGuency (MHZ '

Fig. 13 Test results for calibration using the frequency-tuning method.

4.2 Frequency Components of the Pulses

The new PD detection method proposed here requires the measurement of frequencies up to several 10 M H z . In order to make PD detection in high frequency bands possible, the PD pulses must themselves include frequency components of a sufficiently wide range. C. A. Bailey investigated the wave forms of void discharges in polyethylene using a sampling oscilloscope and found that PD had a rise time of 0.5 ns immediately after the application of voltage and 2 ns 75 minutes later[lOl. Fig.l4(a) shows the results obtained when we investigated the frequency components of PD pulses generated when a PBPD occurred in the XLPE block. Based on the results, we concluded that the PD pulse includes frequency components up to approximately 1000 MHz. Fig.l4(b), on the other hand, shows the frequency spectrum of a commercially available calibration-pulse generator which uses a mercury switch. This also includes a frequency component which decreases to approximately 1000 MHz at a nearly uniform rate.

:urina PD measurement I

I without voltaa

' tuning frequency f(MHz) (a)partial discharge

tuning frequency f'(mz) (b)pulse calibrater

Fig. 14 An example of the frequency SpeCtrUm of PD and calibration pulses.

4.3 PD Detection with a Spectrum Analyzer

are currently available:

(1)the low-frequency method which uses a

The following three methods of measuring PD pulses

low-frequency type PD detector

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(amplified frequency:lO to 200 kHz)

PD detector (amplified frequency: several kHz to several MHz)

PD detector (amplified frequency: 100 kHz to several MHz)

It was pointed out that in a long length of cable where more than one site of PD exists, because of the "superposition error" the PD pulses could not be satisfactorily detected with tuning type PD detector [11,121. However when we use an SA with variable tuning frequency and time resolution, it is considered that the "superposition error" can be reduced, and that a high level of sensitivity could be achieved when the PD detection is performed at the measurement frequency of high S/N ratio. Fig. 15 shows the results obtained when the external noise entering the experimental cable specimen was measured with an SA. Fig. 15(a) shows the results of measurements in the

(2)the wide-band method which uses a wide-band type

(3)the tuning-type method which uses a tuning-type

m O 5 10 tuning frequency (MHz)

m 3 4 5 tuning frequency (MHz)

tuning frequency (MHz) Fig. 15 An example of noise frequency spectra.

frequency range from 0 to 10 MHz. This shows that a high level of noise exists around 4 MHz. When this area was enlarged, however, it was found that frequencies with low noise levels are present, as shown in Fig. 15(b) and (c). For example, the noise level at 3.88 MHz is approximately 35 dBm lower than the noise level at 3.82 MHz. In this case, it is preferable to inject the calibration pulse into the line by methods such as the one described in 3.3 in order to survey frequencies with calibration pulses detectable with a high level of sensitivity, i.e., pulses which have low noise levels and a high S/N ratio. It is advantageous to use an SA in which measurement frequencies can be easily varied to survey these frequencies.

4.4 Partial Discharge Detection Experiment

The IJ model shown in Fig. 11 was used, and ac voltage was applied to generate PD from the center where the outer semi-conductive layer had been removed. When this was done, PD detection was performed using the foil electrodes on both sides. Fig. 16 shows the output pulse overlapping the applied ac voltage when the ac voltage of 10.5 kV was applied. The PD pulse was detected with an SA tuned to a frequency of 4.1 MHz. The Qmax of the PD was

Fig. 16 Oscillogram of PD detected at a tuning frequency of 4.1 MHz.

approximately 10 pC, and the detection sensitivity was approximately 0.5 pC.

5. MEASUREMENT OF THE PARTIAL DISCHARGES OF A LONG DISTANCE 275 KV LIVE XLPE CABLE LINE

5.1 Line Configuration

When the new measurement method was used, the PD characteristics of The Tokyo Electric Power Co.'s (TEPCO) Minami-Ikegami Line, having a 275 kV XLPE cable 9.5 km in the total length were measured. Fig. 17 shows the line configuration. The line is routed through a tunnel 20 to 30 m underground, and PD was measured in a part of IJ section of the line.

9.5h I I

Fig. 17 Layout of the 2 1 5 kV XLPE cable line.

5.2 Measurement Circuit

The PD measurement circuit is shown in Fig.18, and the measurement setup is shown in Fig. 19. The 3-core cables are arranged in the form of an equilateral triangle, and the sheaths are cross-bonded for connection with the IJ. Since it is possible that some

ath arrester ferrite core

Fig. 18 Detection and calibration circuits of PD in a live cable line.

Fig. 19 Measurement setup at Kawadoh manhole No.239.

PD pulse signals may be by-passed or some high-frequency external noise may enter the measurement system, the connection area is provided with a split-type, high-frequency ferrite core in order to enhance the high frequency impedance of the area. Thus, the adverse effects of the cross-bonding wires are reduced.

5.3 Measurement by the Low-Frequency Method with "Proximate Calibration"

First, the detection sensitivity of the calibration pulse was measured by means of the conventional low-frequency method. Measurement was performed in an electrode configuration in which metal foil electrodes were placed on both sides of the IJ constructed at Tsurudoh No. 250 manhole. The detection electrode and calibration electrode were 200 mm wide and wrapped around the 240 mm diameter PVC jacket of the IJ. The capacitance between the joint casing and the metal foil electrode was approximately 2000 pF. The amplification frequency band was 10 to 200 kHz. Calibration pulses were "indirectly injected", and the minimum value of the calibration pulse observed on the oscilloscope was regarded as the detection sensitivity. When detection sensitivity was evaluated, it was found that the sensitivities of the respective phases of this cable line were 3000, 3200 and 2400 pC.

m -100

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5.4 PD Measurement Using an SA with "Proximate Calibration"

- 61710

_ ~ ~ _ _ f /JLn=:::h No 239 No 250 2

In the same electrode system, an SA was connected up as an amplifier, and detection sensitivity was measured by the tuning-type method. The detection impedance and pre-amprifier for the high-frequency wide-band type were used. Fig. 20 shows the frequency spectrum of the signal measured using the SA. As seen in the figure, the magnitude of the signal is greater than when noise alone was measured in the 5 to 50 MHz frequency band with 'indirect injection" of a 20 pC calibration pulse (oscilloscope output 10 pC) . Thus, as the difference between the signal measured with and without injection increases, both the S/N ratio and the detection sensitivity of the pulse increase. The output pulse of the SA at each frequency exibiting a high S/N ratio was monitored. The magnitude of PD

- v

tuning frequency (MHz)

tuning frequency (MHz) F i g . 20 Responses with and without a calibration

pulse was then measured using an oscilloscope, the SA was set at this frequency, and detection sensitivity was obtained. Fig. 21 shows the results, demonstrating that detection sensitivity was best around 10 MHz and the 1 pC value was obtained. The increase of detection sensitivity, shown in Fig 21, among the frequency band within 1-10 MBz is mainly caused by the reason that the decay of external noises become greater when a higher measurement frequency is selected. In this

pulse of 20 pC.

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No 250

1 1000 I 1 1 10 100

tuning frequency (MHz)

Fig. 21 Frequency dependence of detection sensitivity for calibration pulses injected into the same joint.

measurement no PD was detected at this high level of detection sensitivity.

5.5 Measurement usinq an SA with "Adjacent Calibration"

The calibration pulse was "indirectly injected" via the IJ constructed at Kawadoh No. 239 manhole, which is 617 m away from the Tsurudoh No. 250 manhole in Minami-Kawasaki substation, and the calibration pulse was detected at the IJ in Tsurudoh No. 250 manhole. Fig. 22 shows the results measured by the "adjacent calibration". Detection sensitivity, 15 pC, was best at 3 MHz, and no PD was detected under this condition.

$ 30001 *

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7. ACKNOWLEDGEMENTS

The a u t h o r s would l i k e t o thank P r o f e s s o r Teruya Kouno o f t h e Department of E l e c t r i c a l E n g i n e e r i n g , The U n i v e r s i t y of Tokyo, and M r . Hironobu Ohno,TEPCO, f o r g u i d i n g t h i s r e s e a r c h , and many o t h e r members of t h e s t a f f s o f TEPCO and H i t a c h i C a b l e , L t d . f o r t h e i r k ind c o o p e r a t i o n i n conduct ing t h i s r e s e a r c h .

REFERENCES

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[ Z l K . Haga, E. Sugimoto; " P a r t i a l Discharge Measuring Method i n I n s t a l l e d Cable L i n e s " , Showa Densen Denran Review, Vo1.18, No.1, pp l -6 , 1968

131R. J. Jackson e t a l ; " P a r t i a l D i s c h a r g e i n Power Cable J o i n t s : t h e i r p r o p a g a t i o n a l o n g a cross-bonded c i r c u i t and method f o r t h e i r

Nov. 1980 [4]R. B a r t n i k a s , R . M. Eichhorn e d s . " E l e c t r i c a l

P r o p e r t i e s o f S o l i d I n s u l a t i n g M a t e r i a l s " , STP 783, 88364-366, 1983 ASTM Press

[SIP. F i s h e r , K. W . Nissen, P. R6hl; " E l e k t r i s c h e F e s t l g k e i t von P o l y e t h y l e n i n Anwesenhei t F e l d v e r z e r r e n d e r E i n s c h l b s e " , Siemens F o r s c h . U. Entwickl . , Bd.10, N r . 4 , pp222-227, 1981

[61T. Endoh; " A S tudy o f Breakdown T r i g g e r s o f XLPE Cable Insulation u s i n g Pre- breakdown D i s c h a r g e D i t e c t i o n Method", Trans . IEE J a p a n , Vol.lO8-B,

17111. Suzuki , T. Endoh, Y. S e k i i ; " E x p e r i m e n t a l S tudy on t h e Causes o f E l e c t r i c a l Breakdown o f EBV XLPE Insulation u s i n g Pre-Breakdown P a r t i a l D i s c h a r g e D i t e c t i o n Method", Annual Repor t o f 1990 CEIDP

I81R. J. Dense ly , T. S. Sudarshan; "Some R e s u l t s o f Par t ia l D i s c h a r g e Measurements d u r i n g t h e Growth of E l e c t r i c a l T r e e s " , Annual R e p o r t o f 1977

[91K. W. N i s s e n , P . Rijhl; "Lgschen von T e i l e n t l a d u n g e n i n Hohlrgumen i n P o l y e t h y l e n " , Siemens F o r s c h . U. E n t w i c k l . , Bd. 1 0 , N r . 4 ,

[ lOlC. A. B a i l e y ; " A S tudy o f I n t e r n a l D i s c h a r g e i n Cable I n s u l a t i o n " , I E E E Summer Power Meet ing , No.

I 1 1 1 G . Mole; "Measurement of t h e Magnitude of I n t e r n a l Corona i n C a b l e s " , I E E E Winter Power Meet ing, No. 69 CP 88-PWR. January 1969

1 1 2 1 G . Eager , G . Bahder and D.A. S i l v e r ; "Corona D e t e c t i o n Exper ience i n Commercial P r o d u c t i o n o f Power C a b l e s w i t h Ext ruded I n s u l a t i o n " , IEEE Trans . on Power Appara tus and Systems, PAS-88, pp342-364, A p r i l , 1969

d e t e c t i o n " , I E E E P r o c . , Vol. 127 P t . C , pp420-429,

pp451-458, Oct. 1988

pp255-260, Oct. 1990

CEIDP, pp330-338, Oct .1977

pp215-221, 1981

31 PP 66-363, J u l y 1966

Ginzo Katsuta: Born i n Saitama pref . , Japan, on Aug. 30, 1955. B.S., E l e c t r i c a l Engineering Dept., Tokyo Denki University, March 1978, jo ined The Tokyo E l e c t r i c Parer Co., Inc. (TEFTO), April 1978. cur ren t ly deputy manager, t h e Underground Cable Sec t ion , Enginee r i n g Division, Pr inc ipa l ly engaged i n t h e design, construction and mintenance of underground power cables. Member of t h e IEE of Japan

Atsushi Toya: Born Dec. 1959, B.S., E l e c t r i c a l Engineering Dept., Tokyo Science University, March, 1982, jo ined t h e Tokyo E l e c t r i c Power Co., Inc. , April 1982. cur ren t ly a member of t h e Underground Cable Section, Engineering Division. Pr inc ipa l ly engaged i n t h e design, construction and mintenance of underground parer cables. Member of t h e I E E of Japan

Keiichi Muraoka: Born i n Hokkaido, Japan, Feb., 1958, B.S., E l e c t r i c a l Engineering, Ikuei Technical College, March 1978, joined t h e Tokyo E l e c t r i c Power Co., Inc. , April , 1978, cur ren t ly a member of t h e Underground Cable Section, Engineering Division. prin- c i p a l l y engaged i n t h e design and cons t ruc t ion of underground power t rans- mission l i n e . Member of I E E of Japan

Takeshi Endoh: Born J u l y 1937, B.S., Department of E l e c t r i c a l Engineering, Kitakata Commerce and Industry high school, March 1956 and t h e Department of E l e c t r i c a l Engineering, Hitachi Ibaraki I n d u s t r i a l College, joined Hitachi Ltd., April 1956, Currently Senior Researcher of Cable Research Laboratory, Hitachi Cable, Ltd., Princ- i p a l l y engaged i n m t e r i a l research on oil-impregnated paper cable insu la t ion and XLPE cable i n s u l a t i o n a s well a s

researches on p a r t i a l discharge phenomena i n power cables. t h e s i s p r i z e of t h e IEE of Japan, 1990. Member of IEE of Japan

Yasuo Seki i : Born Sept. 8 , 1936. M.S., March, 1965, PhD, 1976. The University of Tokyo, joined Hitachi Cable Ltd., April , 1965. Currently Manager of Dept. l s t , Cable Research Laboratory. Princi- pa l ly engaged i n research on insu la t ion d e t e r i o r a t i o n i n XLPE cable and t h e research and development of extra-high voltage XLPE cable and i t s accessor ies . t h e s i s pr ize of t h e I E E of Japan, 1976. Member of the IEEE, I E E of Japan and I E I C of Japan

Chuki Ikeda: Born Oct. 11, 1937. M.S., March 1962, PhD, 1976. The University of Tokyo, jo ined Hitachi Cable, Ltd., Apr i l , 1962. Currently Deputy Gerneral Manager of t h e Cable Research Labora- t o r y . Pr inc ipa l ly engaged i n t h e research and development of SF6 gas insu la ted power equipments and design/ development/research of XLPE cable and accessor ies . t h e s i s p r i z e of t h e IEE of Japan, 1975. Member of IEE of Japan

1075 Discussion

R. Bartnikas (Institut de Recherche d’Hydro-Quibec Varennes, Qu i . , CANADA): The authors have described an interesting procedure for detecting partial discharges in a 275 kV XLPE cable whilst under voltage. Their method of detection is essentially an extension of the balanced detection method often used in the past for noise mitigation with lower frequency measurements [l]. The balanced method has been already extended to very high frequencies for routine measurements on rotating machines under operating conditions [2] and now the authors have shown that the balanced method may also be applied at higher frequencies to cables under operating conditions with the proviso that isolated cable joints are employed.

In the classical balanced method used at lower detection or resonant frequencies of the RCL-type detection circuit, it is common practice to balance the capacitance of the specimen cable under test by an equivalent capacitance of a corona-free cable length [I]. The method described by the authors appears to dispose of this requirement, since either one side or both sides of the cable separated by the isolated joint may exhibit the presence of partial discharges. Furthermore, it is also interesting to note that in the test arrangement described by the authors, the cable lengths on the opposite side of the isolated joint are not equal. It would be helpful if the authors would describe in detail the procedure that they follow to establish which side of the cable is responsible for the higher intensity discharge. In addition, more information should be provided to the reader as concerns the values of C , and C, in their detection circuit; likewise the nature of the detection impedance, Z,, should be fully specified. The calibration procedure, using the metal foil electrodes requires further clarification; normally, the calibration pulse is injected via a small series capacitor in series with the specimen at the far end of the cable. What constitutes this small series capacitance in their system and what is its value? Furthermore, the rise time and voltage magnitude of the injected calibration pulse should be specified. It would be helpful if the authors would give more details on the foil application procedure and the geome- try of the foil capacitor.

The authors indicate that the detected value of apparent charge is inversely proportional to the capacitance of the specimen. However, this analysis is valid for lumped capacitance specimens only-i.e., very short lengths of cable. Normal cable test lengths behave as transmission lines and the magnitude of the detected signal is then determined by the distributed parameters of the cable. It would be helpful if the authors would provide attenuation characteristics of the system under test as this would permit the reader to interpret the full significance of the senstivity curves depicted in Figs. 21 and 22.

The authors use an experimental arrangement comprising the two cable specimens terminated in a purely low voltage resistive impedance for the calibration of their frequency tunning method; but, how does this particu- lar approach relate to the actual calibration required on the unterminated cables under test? When sensitivity is discussed, reflection effects of the unterminated speciman cables must be considered. Without the pertinent attenuation characteristics, with the cable specimen both terminated and unterminated in a high voltage termination, it is not possible to compare the pulse response characteristics of the detection method presented by the authors with those of the classical low frequency method. A complete analysis of the sensitivity question on terminated and unterminated cables has been carried out some time ago [3], in conjunction with a low frequency RCL-type detector at a resonant frequency of 30 kHz; the results are reproduced here for illustrative purposes in Fig. A, which portrays the apparent attenuation versus length characteristics obtained when a sufficiently wide calibrating square pulse of 0.1 ps rise time is injected via a 50 pF series capacitor at the far end of varying lengths of cable. Curve(a) is obtained with an unterminated cable; here, for lengths in excess of 2000 feet, the curve is simply an approximation through a scatter of apparent attenuation points due to the effect of signal reflections. Beyond 14 x IO3 feet, the attenuation slope would eventually be expected to become equal to that of curve(d). which represents the true charge attenuation. Note that with a non-ideal or lossy cable specimen, some charge attenuation does occur as a finite amount oc charge is lost due to leakage as the charge pulse propagates along the cable length. Curve(b) was obtained with the cable terminated at the far end in a high voltage termination consisting of a 005 pF corona-free capacitor (equal in value to the bloclung capacitor of the detection circuit), in series with a resistor equal in value to the characteristic impedance of the cable. The slope of curve(b) is slightly less than that of the charge attenuation curve(d) due to negative reflections arising from the practical HV termination that includes a HV capacitor in series with a resistor representing the characteristic

CABLE L E N O ( d FQT )

Fig. A: Attenuation characteristics obtained with a low frequency RCL-type detector as a function of specimen cable length: with the cable unterminated as in curve(a); with the cable terminated in an RC-termination at the far end as in curve(b); and with an RC termination at both the near and far ends as in curve(c); curve(d) represents the apparent charge attenuation curve. (After Costello and Bartnikas 131).

impedance. Curve(c) is obtained once more with the cable terminated also at the far end as in the case of curve(b) except that now a near end termination is effected by shunting the detection impedance with a resistor equal in value to the characteristic impedance of the cable. Consequently, curve(c) represents the ideal measurement arrangement with the cable fully terminated and all pulse reflection effects eliminated. However, in the routine testing of cables, specimens are generally left unterminated due to time and cost considerations; moreover, the use of terminating resistors would adversely decrease the sensitivity, notwithstanding the accompanied improvement in the measurement accuracy. The curves in Fig. A charac- terize the response of a low frequency RCL-type detector as a function of cable length and nature of the cable termination. It would be most informative for the authors to provide the reader with similar characteris- tics for their detection system for the purpose of comparison. It is anticipated that use of higher detection frequencies would lead to substan- tially higher attenuation due to losses in the conductor and insulation semiconducting shields. As a consequence, cable construction-i.e., the semiconducting shield thickness-should influence appreciably the attenu- ation values. This would adversely influence the detection sensitivity; its sole beneficial aspect would lie in the attenuation of the extraneous noise pulses.

References

R. Bartnikas and E. J . McMahon, Editors, Engineering Dielectrics, Vol. I, Corona Measurement and Interpretation, STP 669, ASTM, Philadelphia, 1979. M. Kurtz and G. C. Stone, “Partial discharge testing of generator insulation”, 1978 IEEE International Symposium on Electrical Insulation, Conf. Record 78 CH 1287, pp. 73-77, Philadelphia, June, 1978. D. A. Costello and R. Bartnikas, Discussion, IEEE Trans. on Power Apparatus and Sytems, Vol. 86, pp. 23-25, 1967.

Manuscript received August 23, 1991

S. A. Boggs, (Underground Systems, Inc., Armonk, NY.): This paper reports significant research by the authors. However, the paper lacks theoretical analysis of the reported data, and some critical parameters are not provided, such as the nature of the partial discharge (PD) detection impedance.

Wideband ( > 1 MHz) PD measurements are not always treated system- atically; however, the literature contains detailed analyses of wideband PD measurements as applied to a wide range of systems. In the case of the present paper, the literature contains everything required to support a detailed theoretical analysis. Experiments are certainly necessary, but such experiments should be seen as confirming a priori theoretical predictions and as a mechanism for determining unknown parameters which may appear therein, rather than as an end in themselves. Isolated experimental

1076

results convey very little information; experimental results supported by a well-developed theory convey an understanding of a situation which can be used to extend the range of engineering options or transferred to other conditions, and this is the goal of research.

In considering the wideband measurement of solid dielectric power cable, one must address the following facts and issues:

1. PD from electrical treeing-induced and void-induced discharges causes very short pulses which generate a spectral energy distribu- tion wtich is flat to beyond 100 MHz (typically the optimum detection bandwidth is in the range of 350 MHz) [l] .

2. Solid dielectric power cable has substantial high frequency attenua- tion. As a result, the bandwidth of the signal which reaches a PD detection circuit some distance from the PD source will have a restricted bandwidth. The theory of high frequency attenuation in solid dielectric power cable is well established [2, 31. The attenua- tion results from the high frequency displacement current passing radially through the resistive conductor and ground shield semicon- ductive layers. The attenuation vs frequency can be predicted from dielectric measurements on a small sample of the cable semicon and knowledge of the cable geometry (thickness of the various cable layers) [2, 31. As semicon specifications allow the semicon resistiv- ity to vary over several orders of magnitude, the high frequency cable attenuation can vary substantially, and the attenuation of a specific cable cannot be predicted without recourse to measurements.

3. The optimum detection bandwidth and the effect of high frequency attenuation on PD detection sensitivity in the presence of thermal noise has been addressed in the literature [I]. However, in many practical situations, electromagnetic interference (EMI) is much greater than thermal noise, and the optimum detection band and bandwidth is that which optimizes sensitivity in the presence of EMI. The EM1 can only be determined through experiment.

4. Once the (a) EM1 is determined through measurements, (b) the cable attenuation is predicted from dielectric measurements on a small sample of semicon, and (c) the distance over which PD must be detected has been determined, then the optimum detection band and bandwidth and the PD detection sensitivity can be predicted [l].

Would the authors please discuss their work in the context of the above systematic approach, and would they please provide important parameters which are missing from the paper, such as the high frequency cable attenuation characteristics and the nature of the PD detection impedance,

The authors leave much of their approach to PD detection unanalyzed in their paper. Consider, for example the authors’ “Proximate” vs “Ad- jacent” methods of calibration. The commenter got the impression that the authors consider these methods of PD calibration more or less equivalent. However, the authors show a comparison of “conventional calibration” with only their “Proximate” calibration method and never with their “Adjacent” calibration method. Figure l a shows the physical configura- tion of the authors “Adjacent” calibration, while Figure l b shows a schematic model. The important point, which is more obvious from the model than from the physical configuration, is that the calibration source is driving two transmission lines, viz, ( I ) the transmission line formed by the cable conductor and cable shield and (2) the transmission line formed by the cable shield and “earth”. One of these transmission lines has a dielectric constant of about 2.3 and a propagation velocity of about 0.66 c, where “c” is the speed of electromagnetic radiation in vacuum (about 3 x lo8 m/s). The second transmission line (ground shield to “earth”) has (predominantly) an air dielectric and therefore has a propagation velocity near “ c ” .

This system should be examined in the frequency domain, as the authors are using a spectrum analyzer for PD detection. Two parallel trasmission lines of equal physical length are connecting the signal generator and signal detector; however, these two transmission lines have different propagation velocities and, therefore, different electrical lengths. This can result in a “comb filter” effect at the detector. For a separation of 617 m, the first constructive interference will occur at about 1.45 MHz and the second at about 2.9 MHz. The degree to which such a comb filter effect occurs will depend on the relative signal magnitudes which arrive via the two transmission lines. Did the authors consider this effect, and if so, what was their conclusion?

Having looked at the authors’ “Adjacent” calibration method, we can contrast that with their “Proximate” calibration which is shown in Figure 2. This involves an essentially direct connection of the PD calibrator with the detection impedance through the two capacitors to the insulating joint. Although the calibrating pulse generator also drives the two transmission lines discussed above, the commenter would suggest that the two configu-

Z,?

PHYSICAL CONFIGURATION

Conductor

MODEL OF “ADJACENT CALIBRATION”

Figure l a (top) shows the physical configuration for “Adjacent” calibra- tion, while Figure Ib (below) shows a transmission line model. Note that the PD calibrator is driving two transmission lines, one consisting of the cable conductor to cable ground shield and the other consisting of the cable ground shield to ‘‘earth”. This provides two paths from the calibra- tor to the PD detection impedance, 2,. The paths have differing electrical lengths as a result of the differing dielectric constants of the transmission lines. This provides for the possibility of frequency domain interference between the two signal paths.

(MODEL OF #*PROXIMATE CALIBRATION” 7

~

Figure 2 shows a transmission line model of the authors’ “Proximate” calibration method. Note that the pulse generator drives the two transmis- sion lines in both directions, but is also coupled directly to the detection impedance. This configuration differs significantly from the authors’ “Ad- jacent” calibration configuration (Figure 1).

rations are sufficiently different that they cannot be considered equivalent in the sense that a comparison of data from the two methods resonably represents the affect which the intervening distance would have on a PD pulse generated within the solid dielectric of the cable. This point can be argued in two ways. First, the “Proximate” method does not represent the interference which could occur as a result of the two transmission lines which connect the calibration source and detection impedance in the “Adjacent” method. Second, in the “Adjacent” method, the calibration source is driving two sections of shield and will generate two pulses which propagate away from the calibration source. Only one of these pulses is headed toward the detection impedance; the other is headed toward the “far end” of the cable. In the “Proximate” calibration method, the pulse generator also generates two pulses which propagate away from it, but these appear in a differential mode across the detection impedance in a manner which is not equivalent to the “Adjacent” calibration method. This presumably gives rise to the roughly 2: 1 ratio between the “conven- tional” calibration method and the “Proximate” method which the au- thors chronicle in their paper (authors’ Table I). Finally, both of the “Adjacent” and “Proximate” calibration methods should be contrasted with signal generation by PD in the cable. The effect of such PD would depend on where it occurs. Consider, first, a PD which occurs in the cable dielectric near the calibration source in the “Adjacent” method (Figures 1 and 2). In this case, a signal will be generated between the conductor and cable shield which propagate away from the PD source in both directions, one directly toward the detection impedance and the other toward the far insulating joint. Part of the signal which reaches the insulating joint will reflect back toward the detection impedance, and part will refract into the ground shield-earth transmission line and likewise propagate back toward the detection impedance. The degree of reflection and refraction will depend on the impedance characteristics of the various transmission lines which join at the insulating joint. Again, we have the potential for

1077

“Adjacent” calibration method, this requires knowledge of (a) the signal magnitude coupled from the calibrator into each of the four transmission lines to which it is connected as shown in Figure 1 , (b) knowledge of the relative signals magnitudes and phases transferred from the calibrator to the PD detection impedance by the transmis- sion lines formed (i) by the cable conductor and cable ground shield and ( i i ) the cable ground shield and “earth”, and (c) knowledge of the transmission line attenuation characteristics in order to calculate (b).

2. Any interference effects which occur between signals transferred by the two transmission lines connecting the calibrator to the detection impedance in the case of “Adjacent” calibration, if significant signal is transferred via both transmission lines.

3 . Knowledge (i.e., measurements) of interfering signals as measured at the detection impedance.

With a good explanation (i.e., theory), one has verified understanding which can be applied to a range of circumstances and about which one can think quantitatively to extend the applicability and sensitivity. Will the authors please provide their perspective on this problem and the degree to which they have developed a theory which explains their data?

References

1. Boggs, S. A. and G. C. Stone. “Fundamental Limitations to the Measurement of Corona and Partial Discharge”. 1981 Annual Re- port of the Conference on Electrical Insulation and Dielectric Phenomena, reprinted in the IEEE Trans. EIK-17, April, 1982. Stone, G. C. and S. A. Boggs. “Propagation of Partial Discharge Pulses in Shielded Power Cable”. 1982 Annual Report of the Conference on Electrical Insulation and Dielectric Phenomena. National Academy of Sciences, Washington, DC. “Surge Attenuating Cable”. U.S. Patent Number 4,687,882.

2.

3 .

Manuscript received August 16, 1991

Frequency (MHz)

Figure 3 shows the ratio of PD sensitivity using the “Proximate” method to that using the “Adjacent” method (solid line) taken from the authors’ Figures 21 and 22. The dashed line shows typical solid dielectric cable attenuation, although this can vary substantially depending on the cable semicon characteristics.

interference as a result of the multiple paths to the detection impedance; however, the frequencies at which constructive and destructive interfer- ence occur will depend on the position of the PD source in the cable. Thus if a particular frequency band is chosen for detection on the basis of the EM1 spectrum, the detected signal could depend strongly on the position of the PD source in the cable, even though the attenuation of the cable is small in that frequency band. Probably the best way around this problem is to employ a sufficiently wide detection bandwidth that several construc- tive and destructive interferences fall within the detection band so that these effects average out. However, as the interferences can fall at intervals of 1.5 MHz, this would require a detection bandwidth in the range of 5 MHz, which would severely restrict the band selection to avoid EMI.

Propagation in the cable shield-earth transmission line can be eliminated on attenuated through appropriate measures. Even this propagation is eliminated, a “comb filter” effect can occur as a result of the PD signal propagating from the PD source in the cable (1) directly to the detection impedance and (2) away from the detection impedance to the insulating joint where the reflection of the signal will cause a signal to propagate back to the detection impedance. If detection is in the range of 1 to 3 MHz, the signal attenuation over 600 m is likely to be in the range of 1 to 5 dB (although it can vary depending on semicon characteristics and cable construction). Thus again, a “comb filter” characteristic can occur, and PD monitoring in a narrow frequency band could result in “blind spots”.

Finally, the commenter would note that the authors PD detection scheme appears to turn the cable sheath into a dipole antenna. For a dipole length of 1260 m (617 + 643) has a fundamental frequency of about 120 kHz with harmonics at 360 kHz, 600 kHz, etc. This may explain the very large EM1 which the authors encountered up to about 500 kHz (authors’ Figure 20).

In conclusion, the commenter would emphasize that wideband PD detection is a well developed area of high voltage engineering, and those who undertaken it should use the existing theory to analyze their methods and data. Their experimental data should be the basis for evaluating theory and determining parameters, rather than reported as significant in them- selves. At a minimum, the authors should be able to explain the ratio between the sensitivity with “Proximate” and “Adjacent” calibration, i.e., the ratio between the data in Figures 21 and 22. Figure 3 shows the ratio of these data ratio as determined from the authors’ figures, along with the typical attenuation of a solid dielectric power cable. The authors do not indicate if the data represented in Figures 21 and 22 were recorded more or less simultaneously so that the background noise spectra can be assumed identical. If this were the case, the commenter cannot explain the large difference in sensitivity at low frequencies, where attenuation of PD-induced signals would be negligible. Above 3 MHz, the sensitivities again diverge and do so more rapidly than might be expected from the typical high frequency cable attenuation, although as mentioned above, high frequency cable attenuation depends on the semicon characteristics and the cable construction. The commenter believes that an understanding of these ratios is very important, as it demonstrates clearly the ability to reconcile the expected (theoretical) and measured signal and noise levels. Such understanding requires a theory for:

1. The signals coupled from the calibrator to the detection impedance using “Proximate” and “Adjacent” calibration. In the case of the

G . KATSUTA, A. TOYA, K. MURAOKA, T . ENDOH, Y . SEKII, C. IKEDA: The a u t h o r s would l i k e t o e x p r e s s t h e i r t h a n k s t o t h e d i s c u s s e r s f o r t h e i r i n t e r e s t i n o u r p a p e r and f o r t h e i r t h o u g h t f u l comments. The p r e s e n t p a p e r d e s c r i b i n g t h e f o l l o w i n g t h r e e items is a p r e s e n t a t i o n o f r e s u l t s o f r e s e a r c h on a new PD ( P a r t i a l D i s c h a r g e ) d e t e c t i o n method which is a p p l i c a b l e t o t h e PD measurement of l o n g - d i s t a n c e XLPE c a b l e l i n e s . ( 1 ) E x p l a n a t i o n o f t h e p r i n c i p l e o f t h e PD d e t e c t i o n

method ( 2 ) D e s c r i p t i o n of t h e r e s u l t s of e x p e r i m e n t s c a r r i e d

o u t i n t h e l a b o r a t o r y ( 3 ) D e s c r i p t i o n o f t h e r e s u l t s o f PD measurements o f

a n a c t u a l l y i n s t a l l e d 9 .5 km 275 kV XLPE c a b l e l i n e .

S i n c e t h e p a p e r p r e s e n t s main ly t h e r e s u l t s o f e x p e r i m e n t a l i n v e s t i g a t i o n , it may n o t c o n t a i n s u f f i c i e n t t h e o r e t i c a l a n a l y s i s . However, t h e a u t h o r s a r e t h i n k i n g t h a t t h e p a p e r is u s e f u l f o r p r e p a r i n g a h i g h l y s e n s i t i v e PD mesur ing method which is a p p l i c a b l e t o t h e e x t r a - h i g h v o l t a g e long d i s t a n c e XLPE c a b l e l i n e s .

Answering t h e q u e s t i o n s from b o t h D r . B a r t n i k a s and Dr. Boggs a b o u t o u r PD d e t e c t i o n c i r c u i t , t h e a u t h o r s would f irst l i k e t o p r o v i d e some o f t h e a d d i t i o n a l d a t a c o n c e r n i n g o u r PD d e t e c t i o n c i r c u i t and c a l i b r a t i o n p u l s e g e n e r a t o r . The f o l l o w i n g a r e t h e d a t a c o n c e r n i n g t h e PD d e t e c t i o n c i r c u i t ;

d imens ion of m e t a l f o i l e l e c t r o d e : 240mm d i a m e t e r x 240mm width

v a l u e s of C3 and C4 : 2,000 pF v a l u e o f d e t e c t i o n impedance, Zd : 420-830 ohms

A s f o r t h e m e t a l f o i l e l e c t r o d e s , t h e a u t h o r s a p p l i e d a 0 . 4 mm t h i c k c o p p e r p l a t e w i t h t h e above d imens ion o n t o t h e o u t e r j a c k e t of t h e metal c a s i n g o f t h e IJBIlI.

1078

Concerning the detection impedance Zd, we developed and employed a special detection impedance unit which is composed of a signal transducing unit(BALUN) and a IIF probe. Fiq. 23 is the configuration of the detection impedance, Zd, employed in our calibration and in the PD detection circuit (Zd in Fig. 12 and Fig. 18). The BULAN in Pig. 23, is an element of transducing a balanced input signal into an unbalanced output signal. The probe we used, was the one which is commercially available (TYPE P6201, TEKTRONIX), the input and output impedance of which are 100 kilo ohms and 50 ohms respectively. The measured value of input impedance of the assembled unit was 420-830 ohms in the frequency ranges between 1 MHz and 10 MHz.

DETECTION - HF PROBE 7 DETECTOR

\

Fig.23 CONFIGULATION OF DETECTION IMPEDANCE ELEMENT

In the experiments, and the PD measurement of an actual cable line, we used an ordinary calibration pulse generator, the specifications of which are given in the following;

rise time of calibration pulse : 30 ns amplitude of calibration pulse : 0 - 50 volts value of series capacitor : 10 pF, 50 pF

To establish an appropriate PD detection technique, it is necessary to understand the characteristics of the PD and noise signals which we will encounter during actual PD measurements. The authors agree with Dr. Boggs's view in reference to the necessity of the study of PD pulse signals. In the course of the research, authors have made analyses of PD signals in order to develop our present PD detection technique. We have made measurements to clarify the frequency spectra of PD and noise signals.

Fiq. 24 is an oscillogram of the frequency spectrum of a noise signal, which was measured at the site of the tested joint in 275 kV XLPE cable line. The discusser will find the oscillograms of the frequency spectra of the PD and calibration pulse signals in Fig. 14 (a) and (b) in the paper.

The authors agree with both discussers' comments in reference to the necessity of the study of the attenuation characteristics of PD signals propagating in long lengths of XLPE cable. In the course of this research, authors have measured the attenuation characteristics of signals in XLPE cable [2,31. Fig. - 25 is the data of HF attenuation characteristics of our 275 kV XLPE cable, from which we confirmed that the attenuation increases rapidly with the increase of frequency in the frequency range above 1 MHz [2,41. The authors agree with both discussers' points in reference to the influence of the semi-conducting shielding layer of XLPE cable on the pulse attenuation. Fiq. 26 shows attenuation characteristics of the following XLPE cable specimens having different outer shielding constructions;

a)ordinary corrugated aluminum sheathed XLPE cable (specimen A)

b)XLPE cable with metal foil electrode applied directly onto the outer semi-conducting layer (specimen B) c)XLPE cable without metal sheath (specimen C)

(XLPE cable having semi-conducting layer only)

tuning frequency (MHz)

Fig. 2 4 FREQUENCY SPECTRUM OF EXTERNAL NOISE

10000

275kV XLPE cable with corrugated aluminum sheath

1000

h 100

m E Y \

Q v

0.01 10 100 1000 0.003 0.01 0.1 1

Frequency ( M H Z )

Fig.25 Attenuation characteristics of long length cables

d)XLPE cable with metal foil electrode applied directly onto XLPE insulation (specimen D)

From this experiment, the authors confirmed that the attenuation of specimen A, which has the semi- conducting cushion layer between the corrugated aluminum sheath and the outer semi-conducting layer of the cable, is larger than that of specimen B and D. We are considering that this is attributable to the fact that a current flows in the HF region in the semi- conducting cushion layer where the attenuation of a signal could occur. One of the authors presented a report describing the results of detailed analysis of these data at the 1991 annual convention of IEE, Japan [31.

Through the experiments to measure the frequency spectrum of PD and noise signals and to evaluate the HF attenuation of those signals, we came to the conclusion that an appropriate frequency band, which gives rise to a high level of S/N (signal to noise ratio) could presumably exist. In addition, if we make a PD measurement by using a PD detector which is tunable to such a frequency band, a highly sensitive PD measurement could be possible. This is the basic

1079

obtained simultaneously. At the time of both measurements, the levels of background noises in the low freauencv reoion were different [41, which could be the main cause of the sensitivity difference. Fiq.27 shows the data illustrating both the sensitivities obtained in both calibration methods and the HF attenuation characteristics of the PD signal (gragh 4 in Fig. 27 is calculated from graph 2 and 3 ) . Taking the HF attenuation in Fig. 27 into consideration, the difference of the sensitivities in the frequency range of 3-10 MHz is explained.

1: magnitude of injected pulses in the "proximate

2: magnitude of injected pulses in the "adjacent

3: HF attenuation of calibration pulses propagating

4: magnitude of attenuated pulses injected in the

It is helpful for the authors to have information provided by Dr. Bartnikas on the attenuation characteristics of long lengths of cable wiich have different terminating conditions, together with his very instructive comments on the reflection effect of unterminated cable systems. We understand the points of discussion in reference to the reflection effects of unterminated cable. In our future research, we would like to examine the attenuation characteristics of our systems under various terminating conditions. The reflection of pulses propagating through long lengths of cablestwould probably occur at the sites of cable joints. According to our evaluation, the reflection coefficient for the propagating PD signal at the molded type straight-through joint (NJB) is less than 5%. We are thinking as a result of this evaluation, that the influence of reflections would not be so significant.

It is very instructive and helpful for the authors to have Dr. Bogg's view on the "adjacent calibration", which is based on his detailed analyses. The authors understand the technical problems - "comb filter effect", "dipole antenna ,effect" and the interference between propagating signals etc. - which could be existing in the "adjacent calibration". As to those problems, the authors are thinking that we should make further investigations to establish the basis of this calibration method.

calibration"

calibration"

through XLPE cable

"adjacent calibration" (calculated value) 102-

2x104

& a 5Xld I I

10 102 1'03 Frequency(MHz)

Fig.26 Attenuation characteristics of XLPE cables having different outer shielding constructions

3000

1000

U a 100

x -I-, .rl

v

2 10 c, .rl m 0

c 1 4 c, 0

c, 'c1 0) 0.1

0.01 0.1 1 10 100 1000

Frequency (MHz) Fig.27 HF ATTENUATION OF CALIBRATION PULSE AND

DETECTION SENSITIVITY IN PROXIMATE AND ADJACENT CALIBRATION

philosophy of the authors' PD detection technique [1,51. In connection with the question raised by Dr. Boggs asking the difference of sensitivity in the low frequency region between the "adjacent calibration" and the "proximate calibration", the authors would like to answer the discusser that both data, shown in Figs. 21 and 22 in the paper, were not the data

References

[lIG. Katsuta et a1;Proc. of the 2nd Power E Energy Conf., Power E Energy Soc. of IEEJ, No. 350, (July, 1991)

121G. Katsuta et a1;Trans. of IEEJ 111-8, No. 11, 1223, (Nov., 1991)

[31G. Katsuta et a1;Proc. of the 1991 Annual Conv. of IEEJ, No. 1445,(Apr. 1990)

[4lG. Katsuta et a1;Report presented to the Research Meeting of Insulating Material of IEEJ, EIM-90-20, 109 (Feb., 1990)

I51G. Katsuta et a1;Proc. of the 1991 Annual Conv. of IEEJ, No. 1438, (Apr. 1990)

Manuscript r ece ived November 7 , 1991