TRANSFORMER INTERNAL OVER-VOLTAGES-

7/30/2019 TRANSFORMER INTERNAL OVER-VOLTAGES-

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TRANSFORMER INTERNAL OVER-VOLTAGESCAUSED BY REMOTE ENERGISATION

J. A. Lapworth, P. N. Jarman and T. Breckenridge

Doble Powertest Ltd., National Grid and SP Power Systems Ltd.United Kingdom

SUMMARY

There has been continued interest over recent years in transformer problems arising from

interaction with the system, particularly the effects of fast transients associated with SF6 orvacuum breaker switching. This paper discusses several unexplained dielectric faults andfailures that have occurred over the last 10 years in a large population of otherwise very

reliable UK power transformers. No problems associated with fast transients wereexperienced but there appeared to be evidence of problems arising from interactions with the

system of a more mundane form. All the problems could be attributed to a common failuremode: internal over-voltages arising from part-winding resonance initiated by remoteenergisation, either manually or by the action of delayed automatic re-closure schemes. A

range of typical symptoms are described. Although the possibility of producing transformerover-voltages by remote energisation has been previously reported, it appears that the

importance of the problem in causing internal faults has been underestimated and is notwidely recognised. Possible mitigation measures are discussed but cannot be fully effectiveuntil this comparatively rare but nevertheless important phenomenon is understood better.

KEYWORDS

Transformer - Dielectric Failure - Internal Over-voltage - Part-winding Resonance - RemoteEnergisation - Transformer Feeder

21, rue dArtois, F-75008 PARIS A2-305 CIGRE 2006http : //www.cigre.org


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1 INTRODUCTION

Power transformers are critical elements in electric power networks, being by far the most

expensive asset in a substation and with long manufacturing times. Fortunately, modernpower transformers are usually very reliable. In the UK, failure rates of large power

transformers (where they have to be replaced or repaired at works) are very low, less than0.5% p.a. on average. This may be attributed to several factors:

Good specification Experienced manufacturers Robust dielectric testing in the factory before acceptance A mature and well planned and operated network

UK power transformers undergo rigorous dielectric testing in the factory before acceptance.Every transformer on the 400 kV transmission system has undergone lightning impulse tests

at 1,425 kV peak, including chopped waves to simulate the flashover of coordinating gaps. Ashort duration induced over-potential test at 2.7 p.u. rated voltage, including partial discharge

measurements at 1.6 p.u. before and after the peak over-potential, is also a routine test onevery unit. On every new design, switching impulse type tests are also carried out. Inservice, UK transformers have traditionally been protected by screened coordinating gaps, but

it has been normal practice for some years to fit surge arrestors to all new installations, andretro-fit these at critical locations where switching over-voltages are expected. In the UK ithas been the practice to use graded insulation for HV voltages of 132 kV and above, with

neutrals solidly earthed.

Failures in the early years of service due to manufacturing faults have been virtuallyeliminated and up to 50 years of service experience to date does not show a correlationbetween failure rate and age. In other words the 'bathtub curve' is flat, with a failure rate

which is low by international norms: less than 0.5% p.a. for the UK population of over 800large transmission units. Nevertheless a few faults and failures still occur, often initiated by

unusual external events such as short circuits and lightning strikes, since these are by far themost severe stresses that transformers experience in service. As far as possible, transformersare designed and tested to ensure that they withstand such extreme events, but factory tests

may not fully simulate all possible system events. It is also likely that the spare margins ofdielectric and mechanical strength over expected operational stress built in by specification

and design is degraded over time as a result of minor faults and ageing processes.

A sophisticated system of monitoring transformers has been evolved to provide an early

warning of problems. Most faults in transformers can be detected at an early stage bydissolved gas analysis (DGA), and this has been developed to a fine art in the UK, with

automated mercury-free vacuum extraction systems allowing accurate analysis down to verylow levels of acetylene, providing reliable trend analysis. Sophisticated off-line diagnostictests have been developed to supplement routine DGA monitoring, e.g. Frequency Response

Analysis (FRA) which can detect winding movement caused by short circuits before finalfailure. And last but not least, an increasing variety of on-line (continuous) monitoring

systems are becoming available to allow closer monitoring where this is required and can bejustified.


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2 UNEXPLAINED UK DIELECTRIC FAULTS AND FAILURES

Despite advances in manufacturing and monitoring technologies, some unexpected or'random' faults and failures still occur. These are usually dielectric in nature, and often

without any obvious cause.

With sophisticated DGA analysis techniques it is possible to detect dielectric problems at a

very early stage in their development. Acetylene is the key diagnostic gas. Most UKtransformers operate with no detectable level (< 0.2 ppm) of acetylene in their main tanks.

Practically any discharge event can be detected from the trace of dissolved acetylene that isleft in the oil. Sometimes this can be due to some relatively innocuous sparking activity, e.g.metallic components at a floating potential, the most common examples being loose winding

clamping screws or floating stress shields in tap-changers. If the discharge is of a sufficientmagnitude and active, Radio Frequency Interference (RFI) techniques can be used to detect it

and determine under which circumstances it occurs, e.g. which tap-position, while acoustic

emission techniques may be able to provide a location.

Many dielectric faults are detected and subsequently diagnosed, and sometimes repaired.However, some discharges seem to be one-off events, detected by a step change in acetylene

in the next routine oil sample, with no subsequent sign of further activity and no prospects forfurther investigation. In some cases the discharge event must have been substantial, judgingby the level of acetylene.

Some of these discharge events are of immediate concern since they cause protectionoperations, which usually remove the transformer from service, and yet often no fault is found

and the transformer is successfully returned to service.

Most discharge events are probably triggered by system over-voltages arising from switchingor lightning strikes, but it is not always possible to correlate with certainty evidence ofdischarge activity with a particular system event.

In the following, several cases of dielectric faults and failures are described, with a range of

consequences, from just DGA step changes to a catastrophic dielectric failure. Many areunexplained. However, for almost all of these a common cause can be suggested, althoughnot proved: energisation of the transformer from a remote location via a significant length of

overhead line. The remote energisation may arise from either manual switching or automaticre-energisation of the overhead line following a circuit trip initiated by an automatic Delayed

Auto Re-close (DAR) scheme since it is common practice in UK mesh substationarrangements for transformers to be connected to incoming lines without an interveningcircuit breaker.

Similar symptoms were exhibited in most of the cases described, including some but not

necessarily all of the following:

Step changes in DGA, particularly acetylene Buchholz gas alarms and/or oil surge trips Operation of electrical protection, particularly differential protection Winding over-voltage damage

Where winding damage was caused, this invariably involved the HV or series winding.Significantly, the damage was not at the HV line end, but elsewhere, usually at some


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discontinuity between parts of the HV winding, such as the junction between the main and tapwindings.

3 CASE EXAMPLES

3.1 Case 1: Small step change in DGA

This case involves a 30 year old generator transformer: one phase of a 423 kV 783 MVA

single phase bank. In late June 2000, routine monthly oil sampling revealed a step change inmain tank acetylene to 20 ppm (Figure 1). The tap-changer could be ruled out as the cause of

the problem because, as per usual UK practice, it was installed in a separate oil compartment,and gas levels were much lower there. Previously there had been no detectable acetylene inthe main tank, as expected for this particular design of transformer which has no known

generic dielectric faults. None of the other 14 single phase units of this design have exhibitedany such symptoms of a dielectric problem.

Small step change in main tank acetylene for generator transformer

Figure 1

Subsequently, the DGA history has shown a downward trend in gas levels, with no significant

recurrence of the discharge activity. At the moment there is no definite explanation for whatappears to have been a one-off event, but earlier that month the long overhead lines to the

station tripped several times during summer storms and were reclosed by DAR, with dead linecharge being applied from the remote end, so some form of internal over-voltage event arisingfrom this is suspected as the most likely explanation. It may be relevant to note that an

unexpected transient over-voltage was recently observed when one of the transformers at thestation was de-loaded, presumably caused by some switching transient phenomenon.

3.2 Case 2: Large step change in DGA

This is an example of a much more substantial step change in DGA. In November 2000 theanalysis of a routine yearly oil sample from a 40 year old 275/132 kV 120 MVA auto-transformer showed what can only be described as a huge jump, to about 750 ppm acetylene

(Figure 2). Since the sample had been taken some time before and the transformer was still inservice it did not seem likely that this was part of a rising trend before failure, since it would

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most likely already have failed. Moreover, the hydrogen concentration was much lower thanthe acetylene, suggesting an old discharge event. RFI checks failed to detect any dischargeand further oil samples showed a downward trend, i.e. no evidence of an active fault. As soon

as possible the transformer was switched out of service for off-line diagnostic tests, whichfailed to find any indication of a dielectric fault.

Large step change in main tank acetylene for transmission transformerFigure 2

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Since the design, and this transformer, had a history of sparking activity at loose winding

clamping screws, this was the obvious explanation for the step change in dissolved gases.However, analysis of oil samples taken from a sister transformer at the same site had shown asimilar step change in the same period, but of a lesser extent. The RFI survey at site haddetected intermittent discharge coming from this transformer, which was located to the

bottom of the windings by acoustic emission techniques, i.e. apparently a winding dielectricfault rather than sparking at a loose winding clamp. A decision was made to accelerate the

planned replacement of both transformers, with the second one being the priority.

It seems too much of a coincidence that two transformers at the same substation should

exhibit similar substantial step changes in DGA in the same time period, both due to internaldefects. A more likely explanation for the apparently coincident one-off discharge events is

considered to be over-voltages arising after tripping of the lines to the substation, which werere-closed with dead line charge being applied from the remote ends.

3.3 Case 3: Spurious Protection Operation

In the summer of 2001 a relatively new 275/132 kV 240 MVA auto-transformer installed at a

remote part of the UK grid system was tripped out of service by differential protection andBuchholz oil surge relays immediately after being energised from a remote location. Oil

samples indicated that a discharge had taken place in the main tank. Off-line diagnostic testsfailed to find any dielectric fault, so the transformer was re-energised off the system from thetertiary to working volts for several hours using a diesel generator set without any problem

being found. Because of lingering concerns, an internal inspection was also carried out but


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again no fault was found. Eventually the transformer was switched back into service and hasoperated without any further problem since.

Because ferro-resonance alarms were activated during the initial incident, a one-off over-voltage incident caused by this phenomenon was originally suspected. However, ferro-

resonance would normally only produce core overheating and possibly a Buchholz alarm, sothe possibility of an internal flashover caused by a switching transient must also beconsidered. Presumably the internal discharge was not at a critical location since there has

apparently been no subsequent deterioration.

3.4 Case 4: Dielectric Faults of Auto-Transformers of a Particular Design

In 2001 a 400/275 kV 1,000 MVA auto-transformer at a substation that was teed off a long

400 kV line started to show signs of dielectric distress after being energised from the otherend of the 400 kV line instead of the normal practice of being energised locally from the 275

kV side. Off-line diagnostic tests suggested the possibility of a dielectric fault that had been

observed previously in other transformers of the same design, so an internal inspection wascarried out, which failed to find any definite evidence of damage. A decision was made to

take the transformer to a manufacturers works for detailed examination. No conclusiveevidence of a problem was found. Some insulation external to the main windings was

replaced and dielectric testing was carried out before the transformer was returned to serviceat another substation. Unfortunately, after a short time further evidence of a dielectric faultappeared, so the transformer was removed from service. The transformer was subsequently

the subject of a comprehensive discharge detection experiment in which the transformer wasenergised from the tertiary using a diesel generator set at various voltages up to 1.2 p.u.Discharge was detected, which appeared to be coming from within the winding assembly, but

a precise and unambiguous location for the fault could not be agreed.

In early 1998 another transformer of the same 30 year old design had failed catastrophically atanother site as a result of a tracking fault along an inter-phase barrier board, between themiddle (400 kV) and bottom (275 kV) of the series winding (Figure 3). With this particular

design there are no external wraps on the winding assemblies, so the closest barrier board hadbeen able to come into contact with the paper conductor insulation of the series windings after

becoming warped with age.

Figure 3


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DGA history in years before failureFigure 4

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The transformer was being monitored by monthly oil samples because the DGA signature was

considered to be of concern, the last sample having been taken 14 days before the failure.There was no obvious evidence from the DGA history (Figure 4) of any imminent failure: in

fact all dissolved gases had been showing a consistently falling trend up to and including thelast sample. There had been a step change in the DGA signature 6 years prior to the failure,presumably due to some over-voltage event, most likely a lightning strike on the attached line.

Some years previously another transformer of the same design at another substation had beenswitched out of service after developing a gassing fault that resulted in persistent Buchholzalarms. The subsequent investigation revealed the same inter phase barrier board trackingfault. In addition, dielectric damage was observed in the major insulation between common

and tertiary windings, so it would appear that a significant over-voltage event had beenexperienced. There was no evidence from the previous routine oil sample, taken some time

before, of any dielectric fault at that stage, so this particular fault had apparently developed tofailure within a year. It was probably relevant that this transformer was attached to long linesover a range of mountains which are known to suffer from a higher than average incidence of

lightning activity.

Obviously, particular features of this design allow this type of tracking failure to occur andageing processes obviously also contributed, but it is believed that an over-voltage event is anecessary pre-requisite to initiate the generic failure mode. The particular winding design

used (simple disc) may also be a factor in allowing a higher than expected internal over-voltage at a particular point, possibly at the bottom of the series winding (junction of commonand series windings).

3.5 Case 5: Dielectric failure after remote switching

A new protection system was being commissioned on a circuit comprising a 92 km overheadline route incorporating a 2 km long cable 9 km from the substation. At the substation a 34

year old 400/132 kV 240 MVA autotransformer was connected directly (without a circuitbreaker) to the line. As part of the protection commissioning the transformer was energised 6


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times from the remote end of the line. On the sixth energisation there was a violent explosionwithin the tank that operated the Buchholz oil surge trip and caused a minor rupture in a tankweld.

Subsequent inspection revealed that one phase of the centre-entry series winding had flashed

over from about half way down the lower half of the winding, along an insulating wrap to thestress ring at the bottom 132 kV end of the winding. The stress ring then broke down to thecore taking the full 132 kV to earth fault current, thereby causing an explosion. The initial

impulse failure left a small mark on the outer copper conductor and a puncture through theconductor insulation and the first winding wrap.

There was no previous history of problems or bad DGA results on this transformer, which hadbeen protected with screened co-ordinating gaps.

3.6 Case 6: Dielectric failure after remote switching

This case involved a 40 year old 132/33 kV 60 MVA distribution transformer that wasnormally energised from another substation 30 km away via a bulk oil breaker. On the

occasion in question, because of maintenance work, the transformer had to be energised fromanother substation 70 km away via an SF6 breaker. Almost immediately the transformer was

tripped out of service by protection and the pressure relief device operated. A flashover onthe main tank side of the tap-changer barrier board had taken place between tap leads at thetop of the tap winding (closest to the main winding) (Figure 5). Note that this transformer

had surge arrestors fitted to the HV side.

Flashover damage between tap leads inside main tankFigure 5

3.7 Case 7: Dielectric failure after lightning strike

In late 2003 a 30 year old 275/33 kV 100 MVA transformer tripped out of service after alightning strike at the other end of the attached 275 kV overhead line. Oil samples confirmed


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a dielectric fault in the main tank. There had been no evidence of any prior dielectric problemwith this transformer or any others of the same design.

Off-line diagnostic tests suggested a dielectric fault affecting the HV winding, and this waslocated to the B phase after the star connection of the HV winding was broken in the tap-

changer to separate the phases. However, the lightning strike was not to this phase.Therefore a decision was made to re-energise the transformer from the LV side using a dieselgenerator set to confirm and locate any discharge fault. A discharge fault was detected at the

top of the B phase winding assembly.

The design of the transformer in question features a split double concentric HV winding andthe subsequent investigation showed that the dielectric fault was a flashover to the earthedcore from the lead connecting the two halves of the HV winding, which passed over the top of

the winding assembly (Figure 6). It would appear that this failure was due to an internal over-voltage of significant magnitude that arose not as a direct consequence of the lightning strike,

but when the overhead line was re-energised by DAR after the lightning strike.

Flashover damage on lead linking two HV windings

Figure 6

4 POSSIBLE MECHANISMS

The faults and failures described above were all apparently due to internal over-voltages, mostlikely arising as a result of resonance and initiated by switching transients. Two well knownbut relatively rare resonance phenomena affecting transformers are ferro-resonance and part

winding resonance. Could these have been responsible for the faults described ?

Ferro-resonance is an oscillatory phenomenon caused by the interaction of system capacitancewith the non-linear inductance of a transformer, which could be a power transformer or awound VT. The resonance is driven by capacitive coupling from an energised parallel circuit,

or possibly through the grading capacitors of a circuit breaker and can only occur when theferro-resonating circuit is switched out but not earthed. The voltages involved depend on the


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resonant frequency and the saturation limit of the transformer, they are usually less than 1 p.u.since the resonance is usually sub-harmonic. However, system frequency resonance can occurin which case voltages up to about 1.3 p.u. could be experienced. Because of the low voltages

involved, ferro-resonance is unlikely to be a direct cause of dielectric failure, but it couldleave the core in an unusual state of residual magnetism, or perhaps cause enhanced transient

voltages if re-energisation occurs during ferro-resonance. Any direct damage from thephenomenon is expected to be due to core saturation and unexpected stray flux. Powertransformers are not tested to withstand ferro-resonance: it is considered to be more a

nuisance than a serious threat.

Power transformers have more than one physically separate winding, each of which may havea different and possibly variable surge impedance due to the distributed nature of windingcapacitances and inductances, allowing the possibility of internal resonances when excited by

appropriate impulses the part winding resonance phenomenon. The junction between themain and tap windings of an HV winding is a recognised discontinuity. Part winding

resonance is the recognised explanation for how damaging over-voltages can arise within a

winding while not at the terminals and it is known that oscillatory switching transients arisingfrom repetitive re-ignitions after breaker operations or reflections in the attached network can

provide the appropriate harmonic content (kHz) to excite internal winding resonances [1].

In recent reviews of transformer-system interactions [2, 3] remote energisation was notconsidered a major problem, but earlier work [4] provided documented examples ofenergising over-voltages on transformer feeders and explained how reflections of the initially

impressed voltage step within the attached line can provide the necessary harmonic excitationat the transformer. That work was concerned mainly with over-voltages on the secondaryside of the transformer and did not consider the possibility of internal resonances. The

experience described here suggests the seriousness of this particular form of switching in

terms of producing internal transformer damage has been underestimated.

In view of the evidence that remote switching can generate damaging internal over-voltages,it is worth questioning why such problems have not been identified by factory Recurrent

Surge Oscillograph (RSO) measurements on transformer tap windings during switchingimpulse tests, or as a result of routine chopped lightning impulses which would be expected to

excite any resonant frequencies and so adequately test the winding. Presumably such tests donot adequately simulate the ability of the system to resonate with the transformer.

5 MITIGATION MEASURES

Even if the causes of the faults and failures described are not fully understood, one could stillconsider possible mitigation measures, to avoid or minimise the likelihood and consequences

of similar events.

Remote energisation appears to be a common factor in the observed faults, so the firstconsideration should be whether this can be avoided. Whereas it may be possible in somecircumstances, particularly when the possible consequences are realised, to avoid this

possibility when carrying out manual switching of the network, it does not seem likely to beable to avoid the remote energisations that occur when lines are automatically switched back

in after lightning strikes, since many lines have transformers at both ends.


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The installation of surge arrestors to limit over-voltages is another obvious measure, but it isnot clear from experience so far that these will be effective for this particular phenomenon,particularly if the over-voltages at the terminals are not high enough to trigger the surge

arrestors.

It is suspected that damaging over-voltages are not produced every time a remote energisationoccurs, so it is probably the case that some aspect of the way in which the switching occurs,e.g. the precise point on wave of switching or the degree of phase imbalance, is critical.

Therefore it may be possible to avoid the problem by controlled switching.

Since another key aspect of the problem is the interaction of system capacitance with thetransformer, one should consider if helpful changes to system or terminal capacitances can bemade.

Lastly, the transformer design itself, particularly the resonant frequency characteristics of the

windings, will probably have an important influence on whether damaging internal over-

voltages are produced. Two of the cases discussed involved transformers with simple discwinding arrangements which are known to have less smooth frequency responses from FRA

measurements. If the relevant parameters were understood it would be an option to relocatesusceptible designs away from critical locations and avoid such designs in the future by

specification and test.

Unfortunately, in the absence of a sufficient understanding of the problem, it is not possible to

make much progress in implementing mitigation measures.

6 CONCLUSIONS

It would appear that several unexplained dielectric faults and failures on UK transformershave been initiated by a common event: energising from a remote location, eitherintentionally or by the operation of an auto-reclose scheme following a line trip.

It would appear that remote energisation can result in damaging internal over-voltages in

transformers. These appear to occur particularly at discontinuities in the HV winding, e.g. thejunction of the main and tap winding. It is likely that the winding design of the transformer isan important aspect in determining its susceptibility to damage from such events.

Part winding resonance is suspected as providing the mechanism for generating damaging

internal over-voltages, and is known to have caused internal damage in other switchingsituations. Previous workers have shown that energising a line terminated by a transformercan generate significant resonant over-voltages, but the risk of causing internal damage

appears to have been underestimated.

Since there is increasing evidence that serious internal dielectric damage can be caused totransformers by such events, further consideration should be given to avoiding thesesituations whenever possible, even though they occur relatively rarely. A better

understanding of the phenomenon is required before practical mitigation measures can berecommended. In particular there is a need to record examples of transient terminal

waveforms so as to determine the voltage amplitudes and critical frequencies involved.


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7 ACKNOWLEDGEMENTS

The authors would like to acknowledge the contributions of several colleagues from various

UK utilities in providing information for this paper and agreeing to publication, particularlySimon White of British Energy and Duncan Shepherd of Scottish & Southern Energy.

BIBLIOGRAPHY

[1] G. Preininger et. al., Resonance Behaviour of High-Voltage Transformers CIGRE 1984Session paper 12-14 presented by Working Group 12.07

[2] A Guide to Describe the Occurrence and Mitigation of Switching Transients Induced byTransformer and Breaker Interaction, IEEE Draft Standard PC57.142/D1.6, April 2004.

[3] M. Glinkowski et. al., Electrical Environment of Transformers Impact of Fast Transients,Summary paper of CIGRE JWG 12/13/23.21, 2005

[4] L. Csuros, K. F. Foreman and H. Glavitsch, CIGRE Study Committee 33 paper EnergisingOvervoltages on Transformer Feeders (Electra No. 18, July 1971, pages 83-104)

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TRANSFORMER INTERNAL OVER-VOLTAGES-