Status and trends in transformer monitoring

IEEE Transactions on Power Delivery, Vol. 11, No. 3, July 1996 1379

Status and Trends in Transformer Monitoring

CBengtsson ABB Transformers AB

Ludvh, Sweden

Abstract - A manufacturer's view on transformer monitoring is presented. Based on a review of the changes occurring on the electricity market, monitoring of transformers is discussed emphasizing its commercial aspects. Definitions of the words "monitoring" and "diagnostics" are proposed, and it is stressed that the key issues related to on-line monitoring are reliability and low cost. A survey of the most important methods for on-line monitoring and off-line diagnostics is given. The area of life assessment is discussed, and in particular, the difference between end-of-life of the insulating material and of the transformer is emphasized.

I. INTRODUCTION

Monitoring and on-site diagnostics of electrical equipment, in particular transformers, has attracted considerable attention for many years. The interest has accelerated over the last years triggered by structural changes in the electricity business. The purpose of the present paper is to discuss trends and developments in this area as seen from a manufacturer's perspective. However, in the competitive market of today, the manufacturers must follow strategies which to the largest possible extent coincide with the interests of their customers. Understanding of the on-going changes in the electricity market is in this respect essential. Research in the area of monitoring and diagnostics must be directly related to the needs of the utilities, and to reduced total operating costs.

II. ELECTRICITY MARKET

A. Market changes

The electricity market in Europe and in the US has been dominated by national or regional monopolies. During the last years, this picture has changed drastically. In the US,

Invited paper presented at the IEEE/KTH Stockholm Power Tech Conference, Stockholm, Sweden, June 18-22,1995

access to the transmission lines was opened by the Energy Policy Act of 1992. In practice, it meant that consumers can buy energy from any producer offering the most favorable conditions. In addition to the energy cost, the customer pays for the transmission of the power through the network. The effect is that electric energy has become a bulk commodity traded and sold under free competition. This has put the utilities under severe stress to reduce costs in order to be competitive.

In Europe, a similar development is seen in Norway and in Great Britain where the national monopolies have been privatized, and split up in separate companies for generation and transmission, respectively. It is expected that this with time will become reality in other European countries, e.g.,Sweden andItaly.

The trend towards free competition and privatization with the related demands on return on investments, results in a cost consciousness among utilities. In this context, monitoring and on-site diagnostics are seen as a possible way of optimizing existing assets in transformers and other electrical equipment. The main driving forces are to reduce maintenance costs, to prevent forced outages with the related consequential costs, and to work existing equipment harder and longer.

B . Consequences

The situation described above must be reflected in work made in the area of transformer monitoring. Developed systems or diagnostics techniques must have a commercial value to the utilities, i.e., it must help them to save money in their operation. A main challenge is to differentiite between what is "interesting" and what is "valuable".

The economic constraints make it useful to differentiate between "Monitoring" and "Diagnostics". "Monitoring" is here defined as on-line collection of data and includes sensor development, measurement techniques for on-line applications, and data acquisition. "Diagnostics" contains interpretation of data, but also all off-line measurements on transformers. The two areas are subjected to different WO- nomical- and technical conditions, as will be discussed in the following chapters.

Another consequence of the more competitive climate, is that many utilities are cutting down on staff and R&D, par- ticularly in areas where the equipment manufacturers are expected to have resources. Due to this, interpretation of data and the ability to make reliable recommendations, will

0885-8977/96/$05.00 0 1995 IEEE

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most likely become a key issue in transformer monitoring in for substation transformers with On Load Tap Changers the future. Therefore, diagnostic capabilities must be devel- (OLTC). It can be concluded that the main parts to be moni- oped in parallel1 to the sensor development. tored are the windings, including the main insulation, and

the OLTC. The most important parameters related to the windings and the insulation system, are gas-in-oil and partial discharges (PD). For the OLTC, several alternatives are

Monitoring equipment, which per definition is on-line possible, however, vibration monitoring seems as the strongest candidate. Monitoring of temperature and load is to be regarded as base information, and should be included in all types of monitoring applications for transformers.

III. MONITORING

and permanently mounted on the transformer, must primarily be reliable and low cost. Since failure rates of transformers in western networks are low, normally 0.2-2% per transformer-year, high cost systems for failure prevention cannot

when redundancy is available, and when the consequential costs thus are limited. In order to achieve a lower cost, some s a d i c e in functionality must be made. In most cases, it's quite sufficient to supply a reliable waming signal without on-line analysis and diagnosis, provided that diagnostic methods are available to follow up the alarm. Monitoring equipment should be designed for field installation on transformers already in operation. The main part of the transformer population is old, and it is furthermore within ehis

There are two main reasons for installing monitoring equipment on transformers. ~ k ~ ~ , by of important functions of the transformer, developing faults can be detected before they lead to a catastrophic failure. S a n d , it mtd by of (springs,

maintenance (CBM).

A . Detection of developing faults

be justified on economic terms. This is especially the case OLTC

W b i d 1% group most emerging faults may be expected. 13%

Fig.1 Typical failure distribution for substation transformers with OLTCs

2) b a d Tap Changers: The oLTc failures are do,,&

can allow for a change from periodic- to hS& shafts, drive mechanisms), follow4 by e l e c ~ c d fal& such as coking of contacts, burning of transition resistors and insulation problems.

Monitoring of the motor current of the drive mechanism gives indications of certain types of mechanical faults. Another is to meaSuTe the

the main tank [21. This is most suited for detecting coking contacts and problems of fiem~delecbc "e.

A catastrophic failure of a transformer is associated with

event is sudden and no actions for a planned outage can be taken. Here the consequential cost (e.g., loss in produced

used with some considerable costs. In the me Of a the temperature difference between the OLTC compartment and

energy? P"s penalties) be Mechanically related problems are not detectable by this mting if no is The technique alone. Recently, vibration monitoring has been also becomes more extensive; instead of repairing a lead or one winding, the entire winding set on all P ~ S m y have to be exchanged together with the oil. For each individual

be calculated before deciding on an investment. In addition,

applied as a for diagnostics of oLTcs [31. It has been &own fiat be detected, as well as wear of contacts and changes in

the also for on-line

and electrical faults

case, the payback Of a system should transition times, The method is under development, but has to become an effective

factors as environmental concerns for safe@

spills and personal motivate the instal1ation Of monitoring

fault detection at a modefate cost. The measurement technique and equipment is simple. The interpretation and its

equipment. implementation in a user fiiendly product, requires, how-

sible to monitor a large number of parameters, however at a the oil is

tions of the monitoring system and its cost and reliability, to The method is b& on analyzing the types, concentration

equipment to a moderate cost, it is necessary to focus on a faults comb&ions of gasses; for instance

parameters: with modem it is Pos- ever, deep howledge in ~ous t ics and in OLTC design, 3) Gas-in-oil: Analysis of gasses dissolved

high Cost. The challenge is balance the func- the most established diagnostic method for transformers.

the value they bring* In order to get effective monitoring and production rates of generated gases. Different types of

few Parameters. The be baed On failure arcing lea& to generation of acetylene, w h e w Statistics, and to the estimated c o ~ U e n c e s Of the respec- oxides formed by ovefieakd cellulose. Traditionally, the tive failures. Fig.1 shows a typical failure distribution [l] gasses are extracted from oil taken manually at

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when the transformer is manufactured. Two main types of sensors have been used, fibers which measure the temperature in one point [5] , and distributed fibers which measures the temperature along the length of the fiber [6]. None of these are low cost systems; in particular, the distributed fiber sensor is costly to install, and can be applied only for new transformem

6) Other methods: Other types of on-line sensors have also been investigated. Examples of such systems are on- line measurements of the moisture content of the oil, static charge in oil, and pump monitoring. Agso on-line measurements of the moisture in the cellulose by optical fiber techniques are being studied. In general, these systems do not have a strong coupling to important and frequent failure modes.

regular intervals, normally every one or two years depend- ing on the importance of the transformer. Due to the fact that the analyze technique is so well accepted and established, and due to its capability to detect a wide range of failure types, a multiple gas sensor appears to be a logical fmt choice in designing a monitoring system for transformers.

For a number of years, on-line sensors for detecting hydrogen (mainly indicative of PD, but also of arcing) have been available on the market, e.g., the Hydran sensor from Cyprotec. These sensors are most sensitive to hydrogen, but measure also other combustible gasses to a certain extent. The readings are to be regarded as warning signals, and a conventional gas-in-oil analysis should be performed after an alarm. However, the risk of missing a beginning fault due to long sampling intervals is reduced considerably.

Recently, efforts have been made to develop on-line sensors that measure individual concentrations of several gasses. Such sensors are also to be regarded as warning systems, but they will give a better indication of the type of the fault, and will give warning for heating of cellulose that the present sensors do not. Examples are the developments made by ABB (metal oxide technology), and by Micromoni- tors (metal insulator semiconductor technology). Both tech- nologies are at present in the field prototype stage waiting to be commercialized.

4 ) Partid Discharges: Glass fiber rods acting as wave guides inside the main tank has been applied to large transformers, mainly in the US. The system is expensive and complicated to install, and are therefore, in the opinion of the author, less suited for on-line monitoring. Also exter- nally fitted acoustical sensors have been tested or are under development. They are less expensive but are associated with problems related to reliability and to disturbances from the rough out-door environment in a substation. Electrical PD monitoring has several advantages, but has been diffi- cult to apply in the field due to difficulties in separating intemal from external PD sources. A recent publication, however, describes a low cost system for electrical on-line detection of PD signals [4]. The system is based on the combined use of signals from an inductive coil fitted around the base of the bushing and the signal from the capacitive tap. It discriminates between internal and external PD, and yields a warning signal of PD activity developing inside the tank. It does not indicate where the source is or how dan- gerous the defect is, and a warning signal must hence be followed by advanced PD diagnostic methods (see section Iv).

5 ) Temperature: The hot spot of the winding is the limiting factor for the load capability of the transfonner. Conventional temperature measurements are not direct; the hot spot is indirectly calculated from measurements of oil temperatures and of load current. As an alternative, fiber optic temperature sensors can be installed in the winding

B. Condition based maintenance

Few examples of condition based maintenance on transformers using monitoring systems have been reported. There is, however, a strong interest among utilities to switch to CBM in order to reduce their maintenance costs. Integration of the complete substation monitoring is needed, since CBM is to be applied on the sub-station level in order to be most effective. The main part of periodic maintenance costs on transformers is related to the LTC. No dedicated sensors for LTC-CBM are available today, only the systems described in IIIA2.

IV. DIAGNOSTICS

According to the proposed definition in section 2B, "diagnostics" contains all interpretation of data, as well as sophisticated off-line measurements, Diagnostics is normally used either for determining the actual condition of a transformer, or as a response to a received warning signal. Since it is not a permanent installation, the use of high tech, sophisticated and costly equipment combined with skilled personnel can be economically justXied.

Monitoring equipment is necessary for getting access to relevant data, but without well developed methods for interpreting this data and for giving "mendations, there is limited practical value in installing such equipment. It is thus important that diagnostic techniques are developed which are matching the sensor systems installed on the transformers. This means that key techniques are gas-boil interpretation combined with PD localization and characterization, if the proposed key parameters for monitoring are used. Besides these methods, other important diagnostic tools are available. A survey of such methods is given below grouped by failure type. In general, as the diagnostic methods become more advanced, the importance of a deep knowledge of transformer design increases. In many cases,

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it is not possible to make a reliable diagnosis without knowing the transformer design in detail.

A. Thermal related faults

1) Gas-in-oil analysis: Gas-in-oil analysis is the most important diagnostic method today for faults leading to elevated temperatures. Methods for interpretation are for example given in the JEC standards. These interpretation schemes have some deficiencies, and there is general agreement that they need to be revised. A considerable amount of work has been spent on improvements in these rules. In order to get a reliable interpretation, it is necessary to take into account not only the absolute values but also the production rates of the different gasses. Moreover, the type of transformer under investigation should be considered when making the analysis; a generator step-up unit shows a different pattern than a system transformer, and a HVDC transformer with much solid insulation differs from a fur- nace transformer. When an indication of a fault is found, detailed knowledge of transformer design and how it oper- ates in the field is needed to make the correct localization and to give the most reliable recommendation.

2) Degree of polymerization (DP): Determination of the DP value of cellulose is a standard method for quantifying the degradation of cellulose. It is a measure of the average polymer length of the cellulose molecules. When cellulose ages thermally, the molecular chains are broken, and when aging is far progressed, the paper becomes brittle and loses its mechanical stability. The method is good for a quantitative measurement of thermal aging. The main drawback is that it requires a paper sample taken from the transformer. This is an intrusive action which requires qualified service personnel and that the transformer is taken out of service. The sample cannot be taken from inside the winding, but is normally taken from one of the high current leads in the upper part of the transformer.

3) Furanic compounds analysis: When cellulose materials age as a result of thermal stress, furanic compounds are generated as a degradation product. Furanic compounds are a family of chemical substances that differ in stabiity and production rates. These compounds are dissolved in the oil and can thus be detected and studied by standard analytical methods. It has been shown that there exist relationships between DP and the concentration of furanic compounds [7], which would allow for an indirect measurement of the degree of aging of the cellulose. The situation is however complicated by the fact that different types of paper show different production rates of furanes, and that the concentration is dependent on the mass ratio between oil and cellulose. Furthermore, the question of stability of the compounds with time is still not satisfactorily shown at realistic transformer temperatures and concentrations of dissolved oxygen. The conclusion is that further studies are needed

before the value of furanic compound analysis is finally documented.

Furanic compound analysis sometimes is mentioned as a method for determining the end-of-life of a transformer. This is not the case, it is possibly a method for determining end-of-life of the insulation material.

4) Thermography: A sometimes effective mexhocl which does not require the transformer to be taken out of service is thermography. It gives a position indication of the& problems, e.g., excessive stray fields, and is a useful com- plement to gas-in-oil analysis.

B . Dielecfric related faults

Partial Discharges in the main insulation often poses a major threat to the function of the transformer. To give a correct diagnosis after receiving an alarm signal via sensors or via gas-in-oil sampling, it is necessary to localize and to characterize the PD source.

Localization of partial discharges is made acoustically using different methods for triangulation. This requires deep knowledge of wave propagation in different types of materialsjiquids (see e.g. [8]) and is a task for highly qualified experts. On the practical side, noise suppression in a substation environment poses the largest challenge.

Characterization of the type of PD, e.g., void in main insulation or metal particle, can be made by using Phase Resolved Partial Discharge Analyze, PRPDA [9]. This is a modem PD measuring system which performs both data acquisition and data processing of conventionally detected PD signals. The PD pulses are presented with respect to charge intensity, phase position and number of pulses. The obtained patterns form a "fmgerprint" which is indicative of a certain type of defect. The transformer needs to be de- energized a certain period of the investigations.

C. Mechanical related faults

Faults of mechanical origin in the active part of the transformer are often a result of short circuit forces, or of possible vibrations of supporting parts of the windings or of the core. Resulting faults are deformations of the windings or of the cleats and leads.

1) Frequency Response Analysis (FRA): By measuring the transfer function of the transformer, deformations of the windings can sometimes be detected, provided that a reference fingerprint of the unit is available [ lo]. Deformation or changes in geometrical distances of the windings, leads to changes in internal capacitances, and thereby a change in the transfer function of the transformer. In practice, an h- puke is injected on one side, and the fourier spectrum is measured of both the impulse and of the response on the other side. The transfer function is calculated by dividing the two spectra. This method requires the transformer to be

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ultimate question to be answered is how many years a unit has left before it fails. The area of life assessment is subjected to a number of misunderstandings. First, it is important to define what we mean with end-of-lie (EOL). EOL is reached when a transformer is not able to fulfill its required function also in an economical perspective. C I G d [14,15] distinguishes between technical-, strategical-, and economical EOL. The trend is to focus too much on the technical EOL. It is seldom that transformers are replaced by technical reasons only; the main resons for taking transformers out of service are cost related,-the total operating cost should be minimized. Such reasons are for instance of stra- tegic nature (changes in load patterns, changes in voltage level, etc.) Second, it is equally important to distinguish between the technical EOL of the insulation materials and the EOL of the transformer itself. For example, in an extensive Finnish study in which more than 300 transformers delivered before 1965 were analyzed, it was found that no correlation whatsoever existed between EOL and load! It demonstrates the relatively low importance thermal aging has on the EOL, at least in a Nordic climate.

The technical EOL of a transformer is determined by many facton. It is a function of both design, historical events, previous operating conditions, its present status, and of the future service conditions. Most methods proposed for estimating EOL is focusing on one of these aspects only, i.e., the present status of the insulating material. It is, however, easily recognized that not only load and temperature, but also the experienced number of short circuits and over- voltages, design weaknesses, repairs, transportation between sites, etc., will influence its ability to fill its function. To make this t y ~ e of estimations, a very deep understanding of the technical dependencies in the transformer is needed. It is however important to have access to historical information concerning the operating conditions. This can only be provided by the utilities themselves. The importance for the utilities to collect such data should be stressed.

Considerable efforts to develop evaluation schemes are being made at present. An approach taken by ABB together with a major European utility, is to develop methods for rankiig a given group of transformers with respect to EOL, taking into account the factors discussed above. The advan- tage of this approach is that it avoids the difficulty of assess- ing a quantitative value of the remaining life, and at the same time the ranking is a useful tool for the utilities to implement their replacement- and maintenance strategies. Estimations giving remaining life expressed in years, is according to the opinion of the author, neither realistic nor to be regarded as serious.

disconnected. It is one of the more frequently used techniques for this type of diagnosis, but has great uncertainties due to the fact that the result is affected by a large number of factors. A predecessor to FRA is Low Voltage Impulse measurements.

2) Leakage inductance: This is a traditional method for detecting changes in the winding geometry. As a result of a short circuit, the inner winding has a tendency to decrease in diameter whereas the outer increases. This leads to a higher leakage flux between the windings and thus a higher measured leakage reactance.

D. General degradation

1 ) Dielectric response: In dielectric response measurements, the relaxation of the insulation after excitation by either a low frequency sinusoidal or a DC source is analyzed. This family of methods is primarily intended for determining the moisture content of the materials. However, the possibility to determine aging has also been suggested. Measurements can be made either in the time domain (recovery voltage [ll]), or in the frequency domain [12], and it can be shown that these two methods reflect the same physical quantities [13]. The transformer must be disconnected from the net before performing the investigation. Systems for measuring recovery voltage are CommenAaIly available. However, the matter of interpreting the signals is still left to be solved. At the present time, it is not possible to make quantitative statements concerning the moisture content in a transformer if no reference measurement of that particular transformer is available. It is important to under- stand the transformer design, since the dielectric response not only depends on the condition of the materials, but also on the geometry of the active part and of volume ratios between solid and liquid insulation. It can also be noted that measurement of the loss angle (tan delta) is a special case of dielectric response, but at one fixed frequency (50 or 60 W.

2) Oil analysis: By determining the dielectric strength, moisture content, loss angle, resistivity, etc., an over-all assessment of the status of the insulating oil can be made.

3) Furanic Compounds analysis: This method (see Section IV, A3) is also applicable as a tool for giving an indication of the general degradation of the cellulose insulation in the transformer.

V. LJFE ASSESSMENT

The assessment of remaining life of transformers is one of the most important issues related to monitoring and diagnostics today. It is intimately related to the utilities’ needs to schedule replacements in a way that they maximize the use of their equipment while still minimizing failure risks and keeping their commitments to their customers. The

The resmcturing of the elmtricity market will c;ontinue, and there will be strong incentives in the future for the

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utilities to optimize the use of their assets, e.g., by using diagnostics and monitoring systems. Technically, the following trends are anticipated: * A shift from sensor development towards interpretation and diagnostics. The use of information technology and artificial intelligence will gradually increase as the interpretation skills grow. The role of universities will be to focus on the basic physics needed for interpretation, preferably in collaboration with manufacturers and utilities. * Development of diagnostic methods which can be applied without taking the transformer out of sehvice The utilization of transformers is expected to increase, and thus will new types of diagnostic methods become necessary. In particular, methods for determining the mechanical status of windings are needed. * System aspects will become more important. The transformer will not be looked upon as a sepmte apparatus, but rather as a part in a system. This will put demands both on the monitoring systems as well as on the relative importance of different diagnostic methods. * A shift from off-line diagnostics to on-line diagnostics. The distinction proposed in this paper between on-line monitoring and off-line diagnostics is useful today, but as technology and interpretation skills develop, more on-line diagnostics will be made. The rapid development of com- puters and information technology will facilitate this pmc- e S S .

W. ACKNOWLEDGMENTS

The Author gratefully acknowledge the contributions from the ABB Diagnostics team; R.Baehr, T.Bengtsson, P.Boss, C.Claibome, T.Eriksson, N.L.Fantana, G.Frimpong, J.Fuhr, M.Gorman, T.Grestad, U . m e r t , R.Johansson H.Kols, Mkijon, L.Martinsson, T.V.Oommen, L.Pettersson, and EPetrie.

[l] CIGRI? Working Group 05, "An International Survey on Failures in Large Power Transformers in Service", Electra, N0.88, 1983.

[2] J. Reason (Ed.), "Oil Temperatures Pinpoint Load-Tap- Changer Wear", Electrical World, Sept. 1993

[3] T.Bengtsson et al., "Acoustic Diagnosis of Tap Changers", Submitted to CIG& 1996.

[4] TJ3riksson. Mki jon and C.Bengtsson, "PD On-Line Monitoring of Power Transformers", Accepted for Publication at IEEE Stockholm Power Tech., 1995.

[5] W.Lampe et al., "Hot-Spot Measurements in Power Transformers", CIGRE, Paper 12-02, Aug.-Sept., 1984.

[6]P.Boss and H.Briindle, "Measurement of the Temperature Profile in Transformer Windings", Trafofech 1994, P a p V2, Bangalore 1994.

[7] T.V.Oommen et al., "Analysis of Furanic Compounds from Cellulose Aging by GC-MS, and Attempts to Correlate with Degree of Polymerization", CZGd Berlin Symposium, Paper 110-2, April 1993.

[8] T.Bengtsson et al., "Identification of PD sources in SOL ids", 6:th DMMA Conf., IEE Conf.Pub1.363, 1992, p 29.

[91 J.Fuhr et al. "On-Site PD Measurements on Power Transformer" (report in German), ETG Tagung, Stuttgart, May 1995

[ 101 E.Hanique, H.Reijnders and P.Vaessen, "Frequency Response Analysis as a Diagnostic Tool", Elektrotechniek, Vol. 68, 1990, p549.

[ 111 A.Bognar et al., "Diagnostic Tests of High Voltage Oil- Paper Insulating Systems using DC Dielectromerics", CIG& paper 15f33-08,1990 .

[ 121 U.G&ert and B. Nettelblad, "Measurement Tech- niques for Dielectric Response Characterization at Low Frequencies, NORD-IS Conference, Paper 7.1, 1990.

[ 131 E.Ildstad, U.Giifvert and P.l"ing, "Relation between Return Voltage and Other Methods for Measurement of Dielectric Response", ZEEE Znterna- tional Symposium on Electrical Insulation, June 1994

[ 141 L.Pettersson, "Estimation of the Remaining Life of Power Transformers and Their Insulation", Electra NO. 133, Dec.1990, pp 65-71

[15] CIGR6 Working Group 12-09 on Thermal Aspects, Lifetime Evaluation of transformers, Electra, No. 150, 03.1993, pp 39-52.

Clam Bengtsson was bom m Malm6, Sweden on July 10, 1956. He received a USE. A p e m Applied Physia f" the Uppsala Institute of Technology m 1979, whe= he also received his Dr.Sc. degree on stress effeds m grain oriented SiFe m

ABB Transformers Development Laboratory, and i now leading the Monitoring and Diagnostics activities within ihe ABB Busmess Area Power Transformers.

Documents

Status and trends in transformer monitoring