LIFE MANAGEMENT OF POWER TRANSFORMERS ARWtr...

Dr. Dr. MarceliMarceli KaźmierskiKaźmierskiInstitute of Power EngineeringInstitute of Power Engineering, ,

Transformers DivisionTransformers DivisionŁodźŁodź, , PolandPoland

LIFE MANAGEMENT OF POWER LIFE MANAGEMENT OF POWER TRANSFORMERSTRANSFORMERS

ARWtrARWtr 20042004October October 28 28 -- 30, 2004 30, 2004

VigoVigo, , SpainSpain

Modern TransformersARWtr 2004 28 -30 October. Vigo – Spain http://webs.uvigo.es/arwtr04

IntroductionIntroduction

Managing the transformer life with improved reliability Managing the transformer life with improved reliability and reduction of scheduled outages become more and and reduction of scheduled outages become more and more important.more important.

High variety aspectsHigh variety aspects (local) of Life Management.(local) of Life Management.

Life cycle Life cycle ⇒⇒ specification, tender review, design, design specification, tender review, design, design review,review, manufacture, factory tests, operation and manufacture, factory tests, operation and maintenance + economic issues.maintenance + economic issues.

Fundamental objective of life management can be Fundamental objective of life management can be defined simply as “to get the most out of an asset”.defined simply as “to get the most out of an asset”.

Effective life management Effective life management ⇒⇒ proper condition proper condition assessment.assessment.

ARWtr 2004 Lecture: LIFE MANAGEMENT OF POWER TRANSFORMERS by Dr. M. Kazmierski



FailureFailure ModelModel

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Deterioration FactorsDeterioration FactorsThermalThermalDielectricDielectricChemicalChemicalMechanicalMechanicalEnvironmentalEnvironmental



Deterioration FactorsInsulation ageingInsulation ageing

Factors Factors -- Temperature, Mositure, OxygenTemperature, Mositure, Oxygen




Deterioration FactorsDeterioration FactorsThermal factorsThermal factors

Hot Hot -- spot temperature spot temperature ⇒⇒ Thermal Thermal deterioration of insulation deterioration of insulation ⇒⇒ DP decreasing.DP decreasing. E. g. average DP drops to about 50% in 30 years, and mechanical E. g. average DP drops to about 50% in 30 years, and mechanical strength of the insulating paper is lowered accordingly.strength of the insulating paper is lowered accordingly.

Faster deterioration at overload condition Faster deterioration at overload condition (higher temperatures). A new hazardous factor (higher temperatures). A new hazardous factor may occur may occur −− air bubbles from the insulating air bubbles from the insulating paper.paper. 180 180 °°C at 0.5 % moisture content,C at 0.5 % moisture content, 160 160 °°C at 2 % moisture content,C at 2 % moisture content, 135 135 °°C at 4 % moisture content.C at 4 % moisture content.




Deterioration FactorsDeterioration FactorsDielectric factorsDielectric factors

χχExternal influences External influences −− atmospheric (lightning), atmospheric (lightning), switching switching overvoltagesovervoltages

ιιInternal influences Internal influences −− winding resonances. winding resonances.

ιιDielectric breakdown may also be a secondary effectDielectric breakdown may also be a secondary effect

The typical failure scenario :The typical failure scenario :NondestructiveNondestructive partial discharges (PD) at operating partial discharges (PD) at operating voltage and reduction in impulse withstand strength voltage and reduction in impulse withstand strength ⇒⇒Appearance of destructive PD Appearance of destructive PD ⇒⇒ Progressing surface Progressing surface discharges occurrence discharges occurrence ⇒⇒ Creeping discharge occurrence Creeping discharge occurrence ⇒⇒ Breakdown.Breakdown.



Deterioration FactorsDeterioration FactorsMechanical factorsMechanical factors

ιιCurrent stresses such as overloads, short circuits, Current stresses such as overloads, short circuits, over over −−excitationexcitation or inrush currents which impose or inrush currents which impose electromagnetic forces on the winding structureelectromagnetic forces on the winding structure

ιιMechanical overstressing may also arise due to vibration Mechanical overstressing may also arise due to vibration caused by transport shocks or resonance phenomena.caused by transport shocks or resonance phenomena.

The typical failure scenario :The typical failure scenario :

Loose clamping Loose clamping ⇒⇒ Distortion of winding geometry Distortion of winding geometry ⇒⇒ PD PD appearance appearance ⇒⇒ Creeping discharge progressing Creeping discharge progressing ⇒⇒

Breakdown.Breakdown.

Extremely dangerous for old transformers with the insulating Extremely dangerous for old transformers with the insulating paper of DP = 450 or less. paper of DP = 450 or less.



Deterioration FactorsDeterioration FactorsEnvironmental factorsEnvironmental factors

These include rain (acid rain), sunshine These include rain (acid rain), sunshine (ultraviolet rays), corrosive gases, particles of sea (ultraviolet rays), corrosive gases, particles of sea salt, etc. Mainly the transformer tank and its salt, etc. Mainly the transformer tank and its accessories are affected by those factors.accessories are affected by those factors.

Deterioration of accessoriesDeterioration of accessoriesThe most important, and also failed more often The most important, and also failed more often than the other, are OLTC, bushings, coolers.than the other, are OLTC, bushings, coolers.



Condition ClassificationCondition Classification

Condition Definition

Normal No obvious problems, no remedial action justified.

Aged; Normalin service

Acceptable, but does not imply defect − free.

Defective No significant impact on short − term reliability, but asset life may beadversely affected in long term unless remedial action is carried out.

Faulty Can remain in service, but short − term reliability likely to be reduced.May or may not be possible to improve condition by remedial action.

Failed Cannot remain in service. Remedial action required before equipmentcan be returned to service (may not be cost effective, necessitatingreplacement).h

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Maintenance strategiesMaintenance strategies

♦ Based on defined periods of time (“Time − Based Maintenance”) − TBMTBM,

♦ As needed, depends on the transformer condition (“Condition − Based Maintenance”) − CBMCBM,

♦ Based on risk assessment („Risk − Based Maintenance”) RBMRBM,

♦ Focused on the transformer reliability (“Reliability Centered Maintenance”) − RCMRCM.



Two stage condition based Two stage condition based methodologymethodology

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Reliability Reliability CenteredCentered Maintenance Maintenance -- RCM RCM

Decision process:

Failure symptoms Failure symptoms ⇒⇒ Identification Identification ⇒⇒ Development Development forecasting forecasting ⇒⇒ Preventive maintenancePreventive maintenance

RCM implementation in practice needs:

♦Working out the failure models for the transformer subsystems and for its accessories (insulation system, OLTC etc.), in other words −What to look for?What to look for?

♦Determination a monitoring program, complementary to typical defects, in other words − What to measure?What to measure?

♦Finding a correlation between transformer characteristic parameters and its condition (normal, pre − failure, emergency, alarm etc.), in other words − Which remedy actions should be undertaken and Which remedy actions should be undertaken and when?when?


Joining different strategies (Japan)Joining different strategies (Japan)




On On −− line monitoring systemsline monitoring systemsMonitoring of transformer conditionMonitoring of transformer condition

AIMAIMDetection of failure symptoms in an early stage Detection of failure symptoms in an early stage ⇒⇒ their their development monitoring development monitoring ⇒⇒ taking appropriate remedy taking appropriate remedy actionaction

MEASUREMEASUREAcquisition, storage, processing and interpretation of large Acquisition, storage, processing and interpretation of large data volumedata volume

TASKTASK


Diagnosis editionDiagnosis edition


On On −− line monitoring systemsline monitoring systemsSystem System functionsfunctions

CurrentCurrent („(„ShortShort Term”)Term”)

Data Data acquisition acquisition (on (on −− line and offline and off −− lineline))

Data Data storage in storage in data data basesbases

Edition reports (on request, periodic, alarm)Edition reports (on request, periodic, alarm)

Warning the personnelWarning the personnel (excess of permissible limits, (excess of permissible limits, conditions likely to end with an outage)conditions likely to end with an outage)

“Prompting” remedy actions“Prompting” remedy actions

Determination of optimal and permissible loadingDetermination of optimal and permissible loading

Controlling of oil pumps, fans, OLTCControlling of oil pumps, fans, OLTC



On On −− line monitoring systemsline monitoring systemsSystem System functionsfunctions

„„LongLong Term”Term”

Long term loading forecastingLong term loading forecastingOverall diagnostic reports including expected Overall diagnostic reports including expected

life time of the transformerlife time of the transformerDetermination of dates for nearest inspections Determination of dates for nearest inspections

and repairsand repairs



On On −− line monitoring systemsline monitoring systemsSensorsSensors//SignalsSignals

Voltage of each phase, at least at the high voltage sideVoltage of each phase, at least at the high voltage sideCurrent of each phase, at least at the high and medium Current of each phase, at least at the high and medium

voltage side voltage side OvervoltagesOvervoltages coming to the transformer (through the coming to the transformer (through the

measuring tap of the capacitor bushing) measuring tap of the capacitor bushing) Oil temperature at the top and in the inlets and outlets Oil temperature at the top and in the inlets and outlets

of each cooler; ambient temperature (PT100) of each cooler; ambient temperature (PT100) State of oil pumps and fans State of oil pumps and fans Oil level, moisture in oil and dissolved gas concentrationOil level, moisture in oil and dissolved gas concentrationOperational data of OLTC and the power consumption Operational data of OLTC and the power consumption

of the motor of the motor −− drive mechanismdrive mechanismCapacitance, leakage current and tan Capacitance, leakage current and tan δδ of bushings; oil of bushings; oil

pressure of bushingpressure of bushing



On On −− line monitoring systemsline monitoring systemsGeneralized logic structureGeneralized logic structure





On On −− line monitoring systemsline monitoring systemsSoftwareSoftware



On On −− line monitoring systemsline monitoring systemsExpert systemExpert system



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On On −− line monitoring systemsline monitoring systemsExpert system tasks:Expert system tasks:

Problem identification, Diagnosis, Condition Problem identification, Diagnosis, Condition assessmentassessment

Quantitative assessment of failure development, based Quantitative assessment of failure development, based on physical mechanism knowledge.on physical mechanism knowledge.Finger Finger −− printingprinting..Trend analysisTrend analysis. .

Statistical analysisStatistical analysis. .


On On −− line monitoring systemsline monitoring systemsLocation of the system, data distributionLocation of the system, data distribution



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PPGC

RT

KA

System Administrator

Substation

SCSKS

SP1 SP2

GPRS

IEnOT

GPRS

KP

RT

PPGCCommunication

Network

Operation ServiceExploitation and Maintenance Staff

MT

DifferentAuthorised User



On On −− line monitoring systemsline monitoring systemsBenefits resulted from monitoring system Benefits resulted from monitoring system

installationinstallation♦♦Reduced risk of fault occurrence and consequent Reduced risk of fault occurrence and consequent outage with all financial implications.outage with all financial implications.♦♦Reduction to a necessarily minimum the extent and Reduction to a necessarily minimum the extent and the cost of repairs by preventing a serious fault the cost of repairs by preventing a serious fault occurrence inside the tank. occurrence inside the tank. ♦♦Planned outage at the most convenient time; outage Planned outage at the most convenient time; outage time decreased by beforehand operations.time decreased by beforehand operations.♦♦More effective, optimum More effective, optimum loadabilityloadability, resulting in , resulting in lower operation costs.lower operation costs.♦♦Optimize of strategic tasks Optimize of strategic tasks −− repair, refurbishment, repair, refurbishment, replacement.replacement.♦♦Effective policy in regard to spare transformers.Effective policy in regard to spare transformers.♦♦Possible reduction of insurance costs.Possible reduction of insurance costs.




Transformer Life TimeTransformer Life Time

♦♦Typically assumed to be 25 or 30 years, and in some cases up to Typically assumed to be 25 or 30 years, and in some cases up to 40 years. Actual life time, assuming normal loading, can be much40 years. Actual life time, assuming normal loading, can be muchlonger and many transformers still give satisfactory service aftlonger and many transformers still give satisfactory service after er more than 50 years especially if they are not moved. more than 50 years especially if they are not moved. ♦♦Life expectancy is significantly influenced in service by the Life expectancy is significantly influenced in service by the loading pattern, maintenance policy and by the steps taken to loading pattern, maintenance policy and by the steps taken to protect the transformer against externally imposed overstressingprotect the transformer against externally imposed overstressingand contamination. and contamination. ♦♦Assessment of the life expectancy should cover not only the Assessment of the life expectancy should cover not only the transformer internal components but also the tank and its transformer internal components but also the tank and its accessories (bushings and OLTC accessories (bushings and OLTC −− more attention, cooling more attention, cooling equipment and control and supervisory devices require little morequipment and control and supervisory devices require little more e than routine visual inspection and operational testing). than routine visual inspection and operational testing).


Transformer Life Time


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Transformer Life TimeTransformer Life TimeFactors that determine the remaining life can be categorised Factors that determine the remaining life can be categorised under three headings:under three headings:

strategic (ability to carry the loads, short strategic (ability to carry the loads, short −− circuit currents, circuit currents, network service voltages, network service voltages, overvoltagesovervoltages and other “normal” and other “normal” stresses applied in service),stresses applied in service),

economic (cost of losses, and maintenance costs, cost of economic (cost of losses, and maintenance costs, cost of undelivered energy),undelivered energy),

technical (only possible on a statistical base, because the technical (only possible on a statistical base, because the various initiating causes are indeterminate).various initiating causes are indeterminate).End of life may be dictated by any one factor or by any End of life may be dictated by any one factor or by any combination.combination.Avoiding of catastrophic technical failures with all implicationAvoiding of catastrophic technical failures with all implications s −− technical technical and economic is estimated as having far greater significance thaand economic is estimated as having far greater significance than any n any economic evaluationeconomic evaluation


Life Extension, Repair, RefurbishmentLife Extension, Repair, RefurbishmentWhat to do with an old transformerWhat to do with an old transformer

ARWtr 2004 Lecture: LIFE MANAGEMENT OF POWER TRANSFORMERS by Dr. M. Kazmierskihtt

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Life ExtensionLife ExtensionReasonable time horizon of the transformer life extension is Reasonable time horizon of the transformer life extension is period of period of 45 45 −− 60 years. It 60 years. It may be attained substantially by may be attained substantially by means of three different ways.means of three different ways.

Through better protection of the transformer against external Through better protection of the transformer against external stresses (operations outside the transformer).stresses (operations outside the transformer).

Through slow down the ageing process by proper controlling Through slow down the ageing process by proper controlling three main decisive factors three main decisive factors −− hot hot −− spot temperature, water spot temperature, water content in oil and in solid insulation, and oxygen content in oicontent in oil and in solid insulation, and oxygen content in oil.l.

Through proper operation inside the transformer in a course Through proper operation inside the transformer in a course of rehabilitation, refurbishment and repair. The operations are of rehabilitation, refurbishment and repair. The operations are focused on stopping or even moving back the ageing effects.focused on stopping or even moving back the ageing effects.


Repair/rehabilitation/refurbishmentRepair/rehabilitation/refurbishmentRepairRepair −− correcting known defects and restore correcting known defects and restore the transformer to its condition prior to the failure or the transformer to its condition prior to the failure or anticipated failure.anticipated failure.RehabilitationRehabilitation comprises a major overhaul to restore the comprises a major overhaul to restore the transformer to healthy condition. It involves correction of transformer to healthy condition. It involves correction of known defects, gasket replacement, control wiring known defects, gasket replacement, control wiring replacement (if necessary), overhaul or replacement of replacement (if necessary), overhaul or replacement of supervisory devices, overhaul of cooling system, repair of supervisory devices, overhaul of cooling system, repair of leaks, corrosion treatment and painting. Oil leaks, corrosion treatment and painting. Oil reconditioning or replacement will be included if reconditioning or replacement will be included if necessary. Core and winding inspection may be included necessary. Core and winding inspection may be included if a problem is suspected. Re if a problem is suspected. Re −− clamping of windings may clamping of windings may also arise. Rehabilitation does not extend the life of also arise. Rehabilitation does not extend the life of a transformer but rather enables it to meet normal life a transformer but rather enables it to meet normal life expectancy. On the other hand some extension of life will expectancy. On the other hand some extension of life will almost certainly be achieved.almost certainly be achieved.




Repair/rehabilitation/refurbishmentRepair/rehabilitation/refurbishment

Refurbishment is taken to involve rehabilitation as described above with, in addition, some degree of upgrading or uprating of the transformer, possibly including replacement of bushings, tap changers, leads and additional cooling. In some cases new windings may be envisaged.



Repair/rehabilitation/refurbishmentRepair/rehabilitation/refurbishmentEconomic assessmentEconomic assessment

The following factors should be taken into consideration:The following factors should be taken into consideration:

technical deterioration of the transformer resulting in increasetechnical deterioration of the transformer resulting in increased d risk of fault occurrence, shorter remaining life time and reducerisk of fault occurrence, shorter remaining life time and reduced d reliability of supply in a case when no action is undertaken,reliability of supply in a case when no action is undertaken,

cost of rehabilitation/repair/refurbishment and their influencecost of rehabilitation/repair/refurbishment and their influence on on the transformer specifications, remaining life time and increasithe transformer specifications, remaining life time and increasing ng safety of the transformer, personnel and the environment,safety of the transformer, personnel and the environment,

transformation cost of a new transformer, replacing the old one.transformation cost of a new transformer, replacing the old one.


Repair/rehabilitation/refurbishment




Final remarksFinal remarksProgress Progress −− more precise measurement techniques and more precise measurement techniques and

application of computer application of computer −− aided monitoring systems. aided monitoring systems. Life management covers the all transformer life periods.Life management covers the all transformer life periods.Benefits from usage of presented concept of Life Management Benefits from usage of presented concept of Life Management

of Power Transformers include recommendations on:of Power Transformers include recommendations on:

maintaining a transformer minimizing de maintaining a transformer minimizing de −− energizing,energizing,

how to operate a defective unit,how to operate a defective unit,

minimizing the remedy actions, minimizing the remedy actions,

comprehensive life extension program,comprehensive life extension program,

priority in priority in −− field repair and on field repair and on −− line processing.line processing.




Final remarksFinal remarksIt seems that the on It seems that the on −− line monitoring systems are the new line monitoring systems are the new powerful tools for effective life management.powerful tools for effective life management.

New problems arise when on New problems arise when on −− line monitoring systems are line monitoring systems are installed on transformers. From the one side the equipment installed on transformers. From the one side the equipment involved should be of the reliability not worse than the involved should be of the reliability not worse than the transformer reliability (also in expected life time category), ftransformer reliability (also in expected life time category), from rom the other the other −− the transformer design should make possible to the transformer design should make possible to install the equipment.install the equipment.

At the end it should be noted that existing on At the end it should be noted that existing on −− line monitoring line monitoring installations are useful tools only for comparatively slow (daysinstallations are useful tools only for comparatively slow (days, , minutes, seconds) changes of the transformer condition. There isminutes, seconds) changes of the transformer condition. There isno guarantee that a failure due to quick random external events no guarantee that a failure due to quick random external events will be noticed in advance.will be noticed in advance.


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