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Euro TechCon 2005 43

UNDERSTANDING FAILURE MODES OF TRANSFORMERS

Victor SokolovZTZ-Service, Ukraine

Abstract

The paper discusses factors that impact the reliability of large power transformers and theirtypical failure modes primarily when in service for many years.

Introduction

A failure is usually a "tuning fork" of Life Management procedures. Failure analysis delivers keyinformation providing insight for determining “what happened?” and “what to do?” in terms ofmanaging network reliability , assessing risk, optimizing maintenance, and estimating end of life.Ultimately, the information aids in improving design and manufacturing of equipment. Failuremodes and causes may differ markedly depending on user specifications, transformerapplication, design features, and, in particular, on the susceptibility to service deterioration andexternal exposure. In order to understand the cause of failure properly all factors such as designanamolies, operating conditions and the mechanisms which reduce safety margins should beconsidered. This paper attempts to examine large power transformer reliability based on ZTZ-Service database statistics. Typical failure-modes and failure causes are discussed, using designreview as a main instrument of investigation.

Failure Statistics

Updated Failure Statistics

Many experts describe failure occurrences in terms of the “bathtub curve” where it is predictedthat transformer failures increase through time. However, available statistics have not yetrevealed a correlation between the number of failures and advancing years in service. In fact, thestatistics show peak failures occurring around 19-21 years after the transformer has been inservice 1,2 ,3.

In spite of the fact that a huge transformer population has already been in service for 25-40 ormore years there is still little information available about the units that have failed primarily dueto thermal degradation of insulation material.

The ZTZ-Service database covers failure events since 1959-60. Analyzed equipment includeslarge power transformers of different applications including over 5,000 units rated 100 MVA andabove and shunt reactors in the 400-750 kV range primarily from CIS countries. Since 1994database has been supplemented with information obtained from worldwide failure events.

In order to gain some insight into failure statistics over a wide time range including recentperiods, four large groups of transformers of similiar type installed in CIS countries have beenanalyzed: autotransformers 125-200 MVA, 220/110 kV (observation period 1964-2005, 27,505transformer-years), 125-200 MVA, 330/110 kV (1963-2005, 9,477 transformer-years; 167 MVA500/220 kV (1965-2005, 13,749 transformer-years), and generator transformers 400 MVA, 330kV (1969-2005, 1,600 transformer-years).


Failure rates have been determined as the ratio of the number of failures of a given populationover a given period of time to the number of accumulated service years for all transformers inthat period of time. Hence Failure rate (%pa) = Failures /Transformers x service yearsFig.1 and 2 represent statistical distributions of the failure rates over a wide range of time.It was found that failures diagrams are only partly predicted by the classical ‘Bathtub’ curve.One can suggest that failure profiles in the time range until 35-40 so far consists of wear-in incompany with random failures.

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Figure 1Bar graphs of failure rates of autotransformers 220/110 kV (left) and

autotransformers 330/110 kV (right)

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Figure 2Bar graphs of failure rates of autotransformers 500/220 kV (left) and

GSU transformers 400 MVA, 330 kV (right)

It’s remarkable that we have not observed symptoms of increasing failure rates for GSUtransformers with time. This finding also relates to very large GSU transformers rated from 600-1000 MVA which are installed in CIS countries. One may suggest several reasons for thisphenomenon:

Specification of moderate ratio of generator-transformer rated powers equalapproximately to 0.8.

Application basically OFWF cooling system for the units rated above 600 MVA Specification of large generator transformers without LTC and NLTC Application only high quality Naphthenic based inhibited oil, membrane sealed

conservator and permanent regenerative filters to absorb initial by-products


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It is clear that failure profiles cannot be determined by a single function due to the presence ofseveral mechanisms of degradation. However, each common failure-mode can be described by aparticular function. Advanced wear in transformers during early years of service (first threeyears) is associated basically with insulation and LTC failures. We then observe a rise in failurerates in the time range of 7-15 years which is mainly due to the weak design of bushings andmechanical movement of windings. These failure histograms closely mirror the total failureshistogram. One can observe wear in form of OLTC failures, though failure rate due to contactdeterioration can be approximated with lognormal distributions. There is no obvious evidence offailures due to paper insulation aging. However, a clear trend of increasing insulation dielectricfailures with time was found.

Total failures Bushings failures

Mechanical winding failures LTC failures

Dielectric winding insulation failuresFigure 3

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Failure-modes of Power Transformers Versus Application

Failure analysis was performed on the basis of worldwide information available.During the period 2000-2005, 108 major failures of large power transformers manufacturedby nine different companies were observed.

Table 1 compares power transformer failures reported from 1996-98 by Doble clients (52failures) to ZTZ-Service. The data show that the average age of failed transformers is stillbetween 20-22 years. However, the percentage of failures from units “older than 25” isbecoming more significant. Note the meaningful number of “early failures” occurring in aperiod less than 5 years.

ZTZ-Service 2000-2005, %Doble clients1996-98, % GSU Transmission

Total number of failures 52-100% 45-100% 63-100%

Average age, years 22.4 21 20.5Over 25 years % 43 44.1 32Less than 5 years, % 7.5 2.94 9.4

Table 1Major failures of power transformers rated 100 MVA and above

Table 2 (shown on the following page) represents failure-modes separately fortransmission autotransformers, generator transformers, and couplingtransformers. It was found that transformer application correlates with particularfailure modes. The most frequent failure modes that are inherent in specificgroups are:

Transmission transformers (autotransformers)

Dielectric mode windings insulation (31.6%), basically HV and TW windingsinvolved

Bushings insulation (38%) Winding movement (9.5%), basically common and tertiary windings involved

Generator transformers

Dielectric mode windings (49%), predominantly HV windings involved insulation Thermal-mode failures (13%), basically attribute to the units that operate ratio of

generator-transformer rating 0.9, LV windings involved Leads and connections overheating (13%).


Auxiliary Power Plant Transformers (coupling transformers)

OLTC insulation failures & contacts heating (43%) Leads and connections (14%) Winding movement (14%) , basically LV and sometimes TW involve

Transformer applicationFailure -mode ComponentCouplingtransformers

GSU Transmission

Winding minor (turn,coils) insulation

28.5 37.8 14.3Dielectric

Major insulation - 11.2 17.3

Thermal Conductor insulation - 13.3 4.8Mechanical Winding distortion 14.3 4.4 9.5Magneticcircuit

Core/magnetic shields* - 4.4 4.8

Currentcarrying

Leads, connection 14.3 13.3 3.2

Bushing 13.3 38

OLTC** 42.8 4.4 7.9

Accessories

DETC 2.1 -Totalfailures,%

100 100 100

Table 2Failure-modes (major failures) of power transformers for different applications

Dielectric-related Failure Modes

Failure Causes

Statistics show that about 30-50% of the total number of dielectric failures have beenassociated with damage to the windings.

A failure occurs when the dielectric withstand strength of the insulation space is exceeded byoperating stresses. Basically, three main causes of failure may be considered:

Insufficient safety margin due to the underestimation of operational stresses Operational stresses exceeding specified levels


Critical deterioration of the safety margin

With time one can apparently expect a proportionally greater number of failures resultingfrom deterioration of the safety margin which automatically increases the effects of initialdesign margins as well as historical operational stresses. All three causes should beconsidered in determining transformer integrity.

Visual damage is seen predominantly on HV windings and Tap Windings due to inherentcomparatively elevated dielectric stresses. Typically, the following failure modes areinvolved:

Breakdown of the space “ bushing shield –turret or tank wall” Creeping discharges across insulation between phases or space “winding-core” Overlapping the winding (HV) from line coils to neutral ones Breakdown between tap leads or from lead to earth Short-circuit between adjacent coils or group of coils Short –circuit between turns

In most cases the main reason for failure is associated with critical deterioration of the safetymargin. Failures have occurred with under-rated voltages or in combination with transients.Degradation occurred mainly because of these critical factors:

Entrance of free water through poor sealing of bushing lead or explosion vent; Conductive particles from the outside: metal particles from worn out pump bearings

or carbon form LTC diverter switch compartment Conductive particles from the within: conductive by-products from oil oxidation,

carbon from the site of localized oil overheating, and formation and sediment ofcopper sulphide

Contamination with “natural” particles, namely cellulose fibres in combination withwater

Failure Mechanisms Involving Insulation

Both experiments and experiences have shown that the dielectric strength of transformerinsulation is determined by the dielectric strength of oil. The following failure mechanismshave been typically involved: breakdown of large oil gap, breakdown of oil duct betweencoils, surface discharge, creeping discharge, and occurrence of destructive PD in oil layersbetween conductors (turns).

Breakdown of Large Oil Gaps. Large oil gaps, particularly those that are not divided bybarriers are very sensitive to oil contamination from particles as well as to the distortion ofthe electrical field on electrodes surfaces.

Dielectrophoretic forces attract particles to HV electrodes from a certain distance dependingon the electric field distribution at this electrode, on the particle shape and moisture


concentration. The stirring of oil (e.g. from starting cooling pumps resulting in particlemigration) or the lowering of the oil temperature causing an increase in the relative saturationand accordingly an increase of particle conductivity could result in breakdown even underoperating voltage. Poor electrode quality enhances the likelihood of failure.

Several failures have occurred due to the poor performance of 500 kV bushing shields;namely the hidden defect metallic mesh with sharp edges. Figure 1 shows partial dischargeactivity under the effect of switching surge and power arc to the turret .

Figure 1Hidden defect on metallic mesh of resin covered shield from a 500 kV bushing

Breakdown of Oil Duct Due to Free Water. Poor or deteriorated seals of lead bushingsas well as the poor sealing of explosion vents allow the entry of rain water into transformerswhich result in a sudden breakdown of the oil duct between coils of HV or TW windings .

Figure 2Short-circuit between coils due to introducing free water

Left: damage of 400 kV winding due to penetration of water through poor bushing topsealing

Right: Short circuit between coils of regulating winding disposed under explosion vent

Special studies of the impact moisture on winding insulation 4 shown that coil type windingsare mostly susceptible to moisture contamination. The presence of moisture within the oilduct can reduce strength to an operating voltage of 6-10 kV, which is typical operating


voltages between coils for HV and Tap windings. On the other hand wet paper results inreduction of the dielectric withstand strength by only 20%.

Fig. 3Impact of moisture on dielectric withstand strength of winding insulation1. Disc winding with radial cooling duct (sensitive construction)1a-Dry insulation and oil; 1b-Wet (not dried) insulation (strength reduction by 20%);1c-high moisture in oil and on surface (strength reduction by 5 times)2. Disc winding without paper insulation, high moisture in oil3-Helical type winding without cooling ducts, high moisture in oil and on surface.

Surface discharge. The occurrence of surface discharge is basically associated withtransients. Two failure mechanisms are proposed: 1) Oil breakdown progressing intoinsulation destruction and 2) Surface discharge as self-firing phenomenon.The magnitude of the electric field tangential component that can result in PD and forcing oilout of the pressboard could be the criterion for the dielectric strength across the insulationsurface. Using non-aged, dry and clean insulation it has shown that surface discharge canoccur under an electric field stress of 6.5-12.5 kV/mm on condition if the ratio of average andmaximum field intensity in the oil gap is 0.4-0.5 or less (e.g. sharp electrode). Apparently, thecontamination of surfaces with conductive particles reduces the value of critical fieldintensity.

One particular “aging problem” is the accumulation of conductive and polar particles in oildeposited on surfaces. Insulation surface contamination has been observed in the form ofadsorbed oil-aging products with cellulose or deposit of conducting particles and insolubleaging products in areas of high electrical stresses. Contamination results in the distortion ofelectrical fields and the reduction of surface discharge voltages. Studies on windinginsulation show that the deposit of sludge and a high contamination level can reduce thedielectric withstand strength of impregnated insulation system under the effect of switchingimpulses by 18-24% 5.


The failures occur when there is a breakdown between coils and the HV winding under theeffect of switching surges and lighting impulses. In several cases the operation of SF6 circuit-breakers triggered these failures.

Figure 4

Damage of insulation contaminated with conductive particlesLeft: Failure of 330 kV winding contaminated with oil by-products under lighting surge

Right: Surface discharge across the barrier under 500 kV winding. Surface contaminated withmetal particles-products of wear out of oil pump bearing

Creeping discharge. This is, likely, the most dangerous failure mode that typicallyresults in catastrophic failures under normal operating conditions. The phenomenon occurs inthe composite oil-barrier insulation and progresses in several steps:

1) Partial breakdown of oil gap.

2) Surface discharge in oil across a barrier (an appearance of black carbonized marks on thebarrier).3) Microscopic sparking within the pressboard, resulting in traces of carbon in thepressboard.The presence of some excessive moisture stimulates vapor bubble formation and thedegradation of material. The creeping process can continue for minutes to months or even


years, until the treeing conductive path causes shunting of an essential part of the transformerinsulation resulting in a powerful arc.

.Three critical factors are recommended in evaluating the likelihood of creeping dischargeoccurrence:1) Specific insulation design configuration (e.g. presence of creeping path across pressboardbetween electrodes; winding disk-to-disk transposition touch to adjacent barrier; touch ofbarrier to bushing or grounded details). Voltage class prone to insulation damage is 220 kVand above.

2) High enough dielectric stresses: magnitude of tangential component of electric field stress(1,0 kV/mm ).

3) Presence of source of initial critical ionization of high energy causing carbonized markson barriers: gas (air) bubbles (pumps cavitations, residual air after refilling with oil, andintense local oil heating) penetration of free water, metal particles contamination, and staticelectrification.

Figure 5Creeping discharge progressing across the barrier of insulation between

phases of 330 kV autotransformer

Destructive PD Occurrence Between Turns. The process of turn-to-turn failures startsfrom the occurrence of PD over 400 pC within oil layer between conductors with graduatedincreasing PD intensity up to the paper destroying level of 100,000 pC and above. Tests onwinding models have shown that the total duration of failure progression is in the range of 5-40 hours. The destructive stage could last up to 10 Hours . The period of time elapsing beforefailure is too short to expect any preventive detection by means of DGA. Presumably only PDon-line monitoring could detect a faulty state.

PD incipient dielectric stress for turn insulation under AC voltage is typically very high (over20-30 kV/mm). In order to cause PD a combination of factors would be required: substantialvoltage between conductors, significant deterioration of dielectric properties and additionalstrength reduction factors (e.g. reduction of PD incipient voltage at high temperature). It isexpected that stressed interleaving disc windings would largely be impacted.


Recently, a number of transformer and shunt reactor failures occurred as turn-to-turn short-circuits during normal operation due to dramatic contamination of insulation with coppersulphide 6-10. It originated basically from non- inhibited oil containing presumably non-corrosive sulphur components such as natural inhibitors. The impact of temperature electricalstresses and time resulted in the transformation of non-corrosive sulfur to corrosivemolecules. Further reactions produced copper sulphide and deposit on paper.

Similar failures occurred also with HVDC transformers particularly with valve windings,which are subjected to frequent and intensive transients, and DC fields that promote thedeposition of conductive particles on surfaces.Tests show that copper sulphide sediment can result in an increasing dielectric loss factor upto 38% at 100C 6, reduction of contaminated paper breakdown voltage from 20 kV/mm toless that 1 kV/mm 9, and reduction of the PD initiation voltage between conductors from 20-30 kV to 2 kV or less (5% probability) 10.The damaged area of failed winding was typically confined within several coils.The appearance of windings contaminated with copper sulphide and those contaminated withoil sludge particles is very similar (fig. 6). In order to identify sulphur contaminationseparation of the paper layer from conductor down to the copper would be required.

Figure 6

Appearance of winding with conductive depositLeft: Discoloration due to copper sulphide deposit

Right: Discoloration due to oil sludge sediment. Winding construction involved thearrangement of radial spacers to provide directed oil flow through the coils.

Thermal Failures

Analysis has shown the following failure causes for thermal failures:


Overheating of tap leads located between regulating coils of HV windings connectedto the no-load tap changer.

Overheating of the coils of windings blocked with insulating collars preventing oilflow and proper cooling.

Underestimation of winding temperature, especially of LV windings in largegenerator transformers with OFAF cooling system including insufficient cooling oilthrough the windings.

The main reason of failures was not normal aging but design deficiency.

Figure 7Failure of 700 MVA generator transformer after 25 years due to overheating the

two top coils of LV winding (CTC wire) resulting in short circuit betweenparallels and then between turns

There has been a rather common opinion e.g. 11 declaring:“If the respective degree of polymerization of the insulating paper falls down intointerval DP = 400 … 300, the operation of the transformer must be ended definitively”.Experience has shown that aging profile of large transformer is typically greatly nonuniform.There have been numerous transformers particularly large generator transformers where DPlevels of some top winding components could be expected to be less than 300-250.Accepting the aforementioned statement would necessarily require the removal of asignificant number of transformers, many of which exhibit in some locations DP numbers ofless than 250 but that operate quite satisfactory.

The question becomes: Maybe mechanical weaknesses of the conductor insulation are not sodangerous as it was traditionally suggested? This question makes sense. Conductor insulationis subjected basically to compressive stress. The reduction of the DP below 200-250 wouldbe likely not so critical for continuous disc windings and particularly for layer windings.The exception would be for CTC (continuously transposed conductor) wire, which could besubjected to very high compressive stresses. It’s remarkable that for the last few years therehave been a number of transformer failures associated with short-circuits insulation betweenCTC wire strands because of overheating and critical decomposition of insulation.


More sensitive to aging deterioration could be also winding construction allowing conductorstitling and bending under short-circuit stresses, and having sensitive spots affected bymechanical stresses e.g. unsuccessful transpositions.

Figure 8Construction of transposition allowing damage of insulating

under the effect of axial and twisting stresses

Damage to Leads and Connections

Over 13% of failures of highly loaded generator transformers are associated with overheatingleads and connections.Basically three failure modes have been observed: Overheating the insulation of winding exitleads; Overheating soldered connections; Overheating bolted connection to bushings

Overheated Lead Insulation

There have been several cases associated with the overheating of winding leads thatcontained the same wire as the windings, which is typical when winding a transformer fromCTC wire


Figure 9Overheating and burning out leads insulation in 700 MVA GSU transformer

Left: Burning out internal layers of HV leadMiddle: Overheating and short-circuit between parallels of LV lead exitRight: Overheating of HV winding lead termination

Design review and relevant calculations have shown that the performance of leads withwinding wire without increasing cross-sections can be a subject of special concern especiallywhen a thick lead insulation is used. One should emphasize that design review is likely theonly effective tool to identify the problem. Considering a limited amount of overheatedinsulation DGA and Furans analysis show clear symptoms of fault only at the stage whenshort-circuit between strands and insulating burning occur.

Mechanical Failures

About 10% of transformers fail due to movement of winding under the effect of short-circuitstress. Most failures (70% ) occur after 28-42 years of service and others in mid-age(14-16 years).

Up to 80% of failures occur due to radial buckling of the common windings ofautotransformers and LV windings of step-down transformers. One generator transformerfailed under effect of short-circuit on the LV side. Tilting deformation and significantlyloosed winding clamping was revealed on the LV winding of step-down transformers that


experienced three phase short-circuits with limited current but for long duration (during 1;530sec). Tilting of conductors on HV windings was found also in GSU transformers as a result offrequent short-circuit events on the HV side. Design review of the failed transformer revealedthat the 300 kV winding was performed as helical type and the safety margin to axial stresseswas only 0.84.

Design review using modern methods has shown that in most cases dynamic stability is notsufficient to stand specified stresses. We used the method, which was developed by Dr.Lazarev (Zaporozhye). The method allows pinpointing not only the likely damaged windingbut also the form of loss stability (Fig. 10). In most cases when wire from annealed copperwith conductor yield strength of less than 100 MPA, a radial form of loss stability could beanticipated.

Figure 10 Forms of loss radial stabilityLeft : Half-shifted form of loss stability. No radial support, insufficient compressive forceRight: Shifted form of loss stability. No radial support, sufficient compression force

For example, a step-down 80/33 kV transformer failed due to the dramatic distortion of LVwindings (Fig 11). It was found that the transformer, which was manufactured in 1974 hasvery low radial stability (Tabl. 3). Taking into account a long service life and inevitablyloosening clamps, a half-shifted form of deformation is expected.

Table 3Safety marginWinding, tap position

Radial AxialRW, max 6.4 6.4LV, max 0.54 0.94HV, max - 3.63


Figure 11Radial buckling of LV winding after 3 phases short-circuit on LV side

Half-shifted form of deformation revealed.

Failures Associated with Magnetic Circuits

There have been a few cases of major failures associated with faults in magnetic circuitsystems, however, a number of cases occurred which caused intensive gas generation andunwanted scheduled outages. Probable defects can be grouped under two general headings:1) Defects associated with main magnetic flux, and 2) Those associated with stray flux.

Defects Associated with Main Magnetic Flux form loops from circulating currentslinked with main flux. In fact this group makes up about 20% of magnetic circuit failures andit results in the dissipation of high energy and intensive gas generation with the activation ofBuchholz relay. The cases observed were basically attributed to loosening winding pressbolts and short-circuit to metallic press rings or to core yoke.

Defects Associated with Stray Magnetic Flux present the main cause of localized oiloverheating and gas generation, and DGA concern. They can be classified into two groups:

1) Overheating under effect of eddy current induced by intensive stray flux

2) Overheating and (or) sparking in a loop for circulating current, linked with stray flux.In the first group typical defects are overheating of the core frame due to absence or improperdisposition of magnetic shields on the frame (Fig.12 left), overheating of pressing of thepressure bolt that situated just under the core yoke (Fig.12 right), overheating a part of thetank wall due to improper shielding.


Figure 12

Local overheating due to eddy current induced by stray fluxLeft: overheating the bottom frame and adjusted insulation in 730 MVAgenerator transformer due to improper disposition of magnetic shieldsRight: Overheating the pressing jack in 417 MVA generator transformer

Loop currents depend on the electromotive force induced by the magnetic flux F , resistanceof the members that form a loop Z cir , and contact resistance Rtr

trcir

circir Rz

EI

; (1)

22 m

cirFfE (2)

Loop resistance is of an order Zcir10-3 Ohm and inducing electromotive force even of 1VResults in current up to 1000 Amps.

Two mechanisms of overheating of members that form circulating current loops have beenobserved:

1) Loose contact in circulating loop provided with construction (Fig. 13)

2) Shorting between core members forming the loop: Shorting magnetic shunts to core and the tank Shorting bottom frame to tank (Fig. 14) Shorting top frame to tank

The latter forms loops of large dimensions, allowing induced voltage up to 10 V and resultingin heating and arcing.


Figure 13Overheating in the loosened areas between members formingcirculating current loops in 700 MVA transformer

Figure 14Traces of overheating in location of contacts

Core frames with tank bottom


Bushing Failures

HV bushing remains one of the weakest transformer components responsible sometimes formore than 30% of transformer failures. Recently it was reported10 that sixty-three failures ofbushings on large power transformer have occurred since 1995 from one manufacturer. It isremarkable that the age of failed bushings was only between 2 and 15 years. Fifteen failureswere accompanied with an explosion and likely destruction of the transformers. Forty-ninebushings were removed from service due to signs of PD gases. Twelve failures wereassociated with overheating paper of the core after just 6-8 years of service.

A recent failure survey in Australia and New Zealand 3 shows that bushing explosions are themain reason for oil fires. Survey data associated with fires events from 2002-04include eleven transformer failures and ten fires were caused by oil-paper bushing and cablebox failures. It was found that Risk Transformers causing oil fire = 0.09 % or~ 1 / 1000 Transformer years.

Failure Modes. Experience has uncovered the following failure modes:

Internal discharges leading to internal gas and pressure build up and ultimately anelectrical breakdown between the central conducting tube and the bushing flange,which could be caused by the paper not being properly impregnated with oil. Designreview of some core construction revealed overstressing of some condenser layers andpossible mechanical sliding (displacement) across the central tube

Mechanical failure of the central support tube allowing loss of oil within the bushing. High temperature of the central tube and adjusted paper, during overloading. Deterioration of copper grounding layer in contact with aluminum foil. Vacuum formation in oil-gas separation system due to underestimation of volume of

nitrogen cushion, followed with water enter

CIGRE WG A 25 “Bushings Reliability” was set up in 2004 with the main aim to improvebushing reliability or at least to prevent the decrease of bushing performance (trend due toeconomic pressure), the long term impact of which can be catastrophic for transformerreliability.

Effect of Conductive Residue on Porcelain. Special attention should be made towardsfailure modes associated with the degradation of the dielectric withstands strength of oil andacross the core and porcelain surfaces that progresses in flashover along the surface. Thesephenomena are typically originated from critical aging the oil, formation of semi-conductiveresidue on the lower porcelain;

Discharges across the inner part of the transformer end porcelain are an outcome of atypical aging-mode phenomena in the bushing. The failure process is initiated and developingwithin the oil channel between the core and lower porcelain. Another option is formationconductive residue on the external porcelain surface by means of attracting conductive by-products from transformer oil.


Figure 15Formation of semi-conductive residue on lower porcelain

Left: Internal staining with aged oil by-products containing metal colloidsRight: Deposit of by-products of external surface attracting by electrical field from

aged oil in 400 kV generator transformer. Concentration residue on side facing tank wall

Electric field intensity in the oil channel and across the surfaces of core-end components andinner porcelain is established both by the bushing insulation construction and by dispositionof the bushing end relative to the grounded parts and the winding.

Impact of Transformer on Bushing State. The transformer in many instancessignificantly affects the oil temperature within the bushing. Hot transformer oil is one of themain sources of the bushing heating. Another two sources are dielectric losses in the core andresistance losses in the central conductor. Heat radiated from the tank top cover is a source ofelevating temperature of the cooling medium (air around the bushing). Current densitythrough the central conductor and actual transformer/bushing current ratio includingpermissible transformer overloading determines hot spot temperature within the bushingaffecting paper temperature.

The transformer distorts the electrical field within and around the bushing. Strengthening theelectrical field within the bushing, specifically, in the oil between the core and lowerporcelain due to the approach of conductive layers to the grounded components andtransformer winding should be considered. Accordingly, the contamination of transformeroil with conductive particles may result in those particles being attracted by the bushing’selectrical field and depositing on the surface (porcelain) and dramatically deteriorating thedielectric strength


Figure 16Electrical field of the bushing porcelain surface

The electrical field intensity across the porcelain surface facing the tankwall may be as much as six times greater than that on the opposite side

Failure of OLTC’s

The failure rate of power transformers associated with OLTC problems varies in the range of5-20 %. Failure analysis incorporating design review and considering both OLTC and thetransformer itself highlights some factors that impact on reliability issues:

In many instances mechanical and dielectric performance of the transformer isdetermined by the state of tap winding and leads.

Choice of OLTC with a low ratio of maximum rated through fault current for LTCandthe maximum current for the transformer. According to IEC 60542 this ratio shall beat least 120%, and the temperature rise of LTC contacts above the oil shall be notmore than 20C. Experience has shown that for rarely moved contacts the temperaturerise shall be less that 15C.

Underestimation of the impulse transfer function allowing in some cases voltagevalues between steps or with respect to ground above the test voltage of the OLTC.

Many failures initiated with short-circuit between steps in diverter or selectorswitches were accompanied with distortion of the tap winding. In fact, it is rare thattransformers are designed to withstand a short-circuit between LTC taps.Accordingly, a comparatively minor failure in OLTC (e.g. burning out resistance) hasresulted in major transformer failures and long-term unit non-availability.


Experience highlights the necessity to pay more attention to the diverter switch reliability.In particular, the following factors should be considered:

Aging deterioration of the oil due to the effect of a high resistors temperature By-products sediment on insulating surfaces affect on degradation of dielectric

strength Mixture of carbon, water and polymerized by-products are difficult to filter out

properly Temperature rise of shunt contacts can sometimes exceed temperature rise of selector Reversing contacts are a weak spot that requires special attention Contact overheating can result if flashover between the phases followed with

explosion and fire.

Figure 17

Dielectric-mode failure in OLTC diverter switchLeft: Flashover between the contacts cause by cooking and oil contamination

Middle: Deformation of Tap winding under short-circuit stressesRight: Insulation contamination and PD activity in diverter switch


References1. Stig Nilsson,Stan Lindgren “Review of Generator Step-up Transformer Failure data”,EPRI Substation Equipment Diagnostics Conference V!, Feb 16-18,1998.

2. William H. Bartley “Analysis of transformer failures-a twenty year trend”Proceedings of the 2000 International Conference of Doble Clients - Sec 8-5.

3.A Petersen, P L Austin “Impact of recent transformer failures and fires. Australian andNew Zealand Experiences” 2005 CIGRE A2 (Transformers) Colloquium, Moscow.

4. Ryzhenko, V. and Sokolov, V., "Effect of Moisture on Dielectric Withstand Strength ofWinding Insulation in Power Transformers," Electrical Stations [Electric Power Plants],1981, No. 9 [in Russian].

5. L.Lungaard, D.Linhjell, J.Sund, G.Jorendal “Influence simultaneous AC stresses onImpulse Breakdown in oil-paper insulation system” Eight International Symposium on HighVoltage Engineering , 1993.

6. T. V. Oommen ,C. C. Claiborne R. S. Girgis Wayne Ball“Sulfur Corrosion Tests andCorrosion Effects in Transformers”TechCon NA Proceedings, 2003.

7. Corrosive Sulphur its origin detection and prevention” Siemens presentation at Cigre SC-A2 Colloquium “Transformer Reliability and Transients”,20-24 June, 2005, Moscow, Russia.

8. Areva Presentation at ABINEE, Workshop on Corrosive Sulphur in Oil, June 2-3 2005,Sao Paulo, Brasil.

9. Toshiba Presentation at ABINEE, Workshop on Corrosive Sulphur in Oil, June 2-3 2005,Sao Paulo, Brasil.

10 Quality of oil makes the difference. ABB discovers the solution to transformerbreakdownsABB Review 3/2004.

11. J.Needly, G. Newesely “Evaluation of the extent of ageing of paper in oil immersedpower transformers”, CIGRE paper D1-302, CIGRE session 2004

12. Reiner Krump Discussion on Marshall F. Turley paper “Recent Failure Experience withHSP Bushings”, Proceedings of the International Conference of Doble Clients, 2004, PaperBIIT-3-A

13. V.Sokolov“Detection and Identification Typical Defects and Failure Modes in HVBushings”,Colloquium SIGRE A2 in Merida, Mexico, 2003

14. V.Sokolov “Detection and Identification Typical Defects and Failure Modes in LTC”,Colloquium SIGRE A2 in Merida, Mexico, 2003


Biography

Victor V. Sokolov received his degree in electrical engineering from the KharkovPolytechnical University in Ukraine in 1962. In 1964 he completed a postgraduate program atthe National Polytechnic Institute in Moscow with a major in Physics of dielectrics. His PhD,received in 1982 from Kiev Polytechnic University, is in the area of EHV transformerdiagnostics.

He started his professional career at the Transformer Research Center in Zaporozhye. Until1990 Dr. Sokolov worked in the Installation and Maintenance Department at theZaporozhtransformer Corporation in the area of reliability.

Since 1990 he is a technical director of the Scientific and Engineering Center ZTZ-Service inZaporozhye, Ukraine.

Dr. Sokolov is a member of CIGRE (SC A2, Transformers). He was convener of the WG A218 “Transformer Life Management”, and Special Reporter on preferential subject “On-SiteOperation” at the CIGRE Transformers Session in 2004.He has published over 100 technical papers including 13 papers for TechCon’s Conferences

understanding failure modes of transformersztz-service.com.ua/sokolov/articles/understanding failure...

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