The Case of a Failed TransformerCase #1 - GSU

James W. GrahamAlliant Energy

161kV GrY – 22.8kV Delta720/806.4 MVA 55/65CShell Form circa 1980Isophase Secondary BusDirect Connection to Unit13,900 gallons of oil762,200 lbs. total weight

Transformer Data

First Steps

Inspection Damage AssessmentReview Known DataSystem Impact

Initial Inspection & Damage Assessment

Field TestsWinding damage confirmedArresters OKBushings OK

Oil leak due to broken piping

Minor Tank Deformation - Upper Section

Minor Tank Deformation – Lower Section

Typical DETC Switch

AØ DETC Dislocated

Arc Damage Across Active DETC Tap - AØ

DETC Leads Disengaged

Insulation Debris on top of the phase pack

Review Known Data

No system fault prior to failurePre-fault DGA samples normalOil test data normalWinding temperature normalOil temperature normalHistory indicated some overloading

Loss of Sales RevenueCost of Replacement PowerLoss of Voltage SupportSystem Reliability ReducedScheduled Transmission Outages DeferredOther Unit Maintenance Outages Deferred

Impact On the System

Two System Connection Options

Short Term Solutions

161kV - Procure Isophase bus adapter- Install temporary transformer

345kV - Build 3-terminal bus- Procure Isophase bus adapter- Build temporary transmission line- Install temporary transformer

Locate Possible SparesSelect Option & Execute

Transformer OptionsSpare from InventorySpare from Other UtilityTransformer Broker/DealerRewind ShopsInternet Bulletin Boards

3 Possible Spares Located345-23kV 830 MVA161-20.9kV 535 MVA146-20kV 874 MVA

Locate Possible Spares

Select Option & Execute

161kV Option SelectedMinimizes construction coordination No major substation equipment requiredShortest completion scheduleLowest total cost

146.8-20kV Transformer EvaluatedOverexcitation limited < 5%Generator limited to ~96% output161kV Bus Voltage reduced 2.5%

146.8-20kV transformer purchasedTransportation ArrangedFailed Transformer RemovedTemporary Transformer InstalledSystem Operation Changes Required

GSU DETC set at +5% (154kV)Main Auxiliary transformer DETC set at –5.0%Reserve Auxiliary transformer DETC set at –2.5%345kV tie transformer DETC set at +2.5%(effectively reduces 161kV bus voltage)

Select Option & Execute

Transformer Disassembly – 4 days

One of 5 semi-truck loads of accessories

Transformer Accessories – On Site

Transformer Unit Train

Rail Car Assembly

Transformer Loading – 2 days

Staley Bridge

Temporary GSU in Service – 81 days after failure

Long Term Solutions

161kV OptionReplace temporary GSU transformer or reuseReuse existing 161kV tie line

345kV OptionBuild 345kV 3-terminal busBuild 345kV tie line back to plantReplace GSU transformerDesign new isophase bus interfaceAdd 2nd 346/161kV system tie transformer

Transformer Options

Purchase new 345-24kV transformer

Purchase new 161-24kV transformer

Repair failed 161-24kV transformer

Leave temporary transformer in place

Which Transformer Option is Best?

345kV option ruled out

Temporary transformer ruled outPerformance is better than expectedGenerator operates at less than 100%161kV System bus operating at –2.5% nominal voltageTemporary transformer retained as back-up

Purchase new 161-24kV transformer?Repair failed 161-24kV transformer ?

Prepare SpecificationsIssue RFP’s – Repair & New OptionsEvaluate Proposals

Compare Total Evaluated CostsSchedule – Critical lead times may drive a decisionManufacturer Reliability

Select Proposal

Issue Request for Proposals

Repair vs. Replacement

Advantages• Lower first cost• Shorter lead time• No physical restrictions

Disadvantages• Actual Cost Uncertain• Higher reliability risk • Limited upgrading• Fewer manufacturers• Warranty limitations

Rule of Thumb

Repairing a transformer may be viable if the repair cost is 50-75% of a comparable new transformer.The upper limit is dependent on your company’s

risk management policy and good engineering judgment.

Why Should A Repair Be Less than A New Transformer?

Repair Proposals Are EstimatesGreater Than Expected Damage Increases CostExtensive Core Damage Increases Cost Perception - Repairs Are Less ReliableScope Creep – additions & refurbishment add upTwo-way transportation costsThere is a risk the transformer is not repairable

Scope of WorkTransportation to/from plantTear Down & Failure ReportCapacity Increase/DecreaseVoltage ChangesAccessory Replacement/RefurbishmentInsulating FluidAdditional Monitoring

Repair Cost vs New CostRepair Schedule vs. New ScheduleSalvage Value of Failed Transformer

Repair Considerations

Factory Tear Down Core Removal

Top 2 Tank Sections Removed

Core & CoilsHV Side (Segment 3)

Core Removal in Progress

AØ Winding Damage Visible

AØ Winding Damage Visible – A Better View

411,000 lbs Core Steel22,000 lbs. Replaced

Factory Tear Down – Phase Pack

Phase Pack - AØ Bottom

Phase Pack - AØ Top

Low Voltage Coil Removal

High Voltage Coil Removal(Undamaged Section)

Typical Insulation Washer & Spacers

LV Coil Removal(Undamaged Section)

LV Coil Removal(Undamaged Section)

First DamagedHigh Voltage Coil

High Voltage CoilSevere Coil Deformation

Short Circuit Forces cause coils to roll over & collapse to the center core

Rift created by coil movement is 6” wide x 30” long x 10” deep

High Voltage Coil Distortion

Damaged High Voltage Coil Removal

High Voltage Conductor Burned Through

DETC Tap 3 & 4 Studs Burned - AØ

DETC Tap 3 Terminals - AØ

Spade lug

Spring Washer Missing

Evidence of Localized Heating in HV Coil(Not Failure Related)

Case #1 Failure Summary

Test Data Prior to Failure Normal Some Core Damage EvidentMinor Tank Damage due to fault pressureAΦ HV Winding Failure – one sectionHeavy Distortion in HV CoilsLV Coils – Mechanical Damage OnlyDETC Terminals DisconnectedDETC Tap 3 Terminals & Contacts BurnedDETC Leads Prone to Loosen

Case #1 Cause of Failure?

The post-fault inspection and results of the tear down indicate one or both of the active DETC leads fell open, subjecting the high voltage winding to a severe overvoltage condition. The winding failure probably started as a turn to turn or disk to disk failure.

Since the GSU was directly connected to the generator, the fault levels were extremely high and persisted for a significant period of time. This helps explain the coil distortions.

The Case of a Failed TransformerCase #2 – Main Auxiliary #102

24kV D – 7.2kV-7.2kV GrY35/39.2 MVA 55/65CCore Form circa 1979Isophase HV BusNon-segregated LV Bus3,765 gallons of oil106,050 lbs. total weight

Transformer Data

Situation Assessment

No indication of problems prior to failurePreventative maintenance recently completed Twin Main Aux. Xfmr still available for serviceTest data confirmed winding damage2nd Failure at plant in 8 months Concern - is this failure related to GSU failure?

Execute the Plan

Buy a new transformerScrap the failed transformerAssess risk to surviving Main Aux. Xfmr Coordinate installation with GSU installation

A tear down was done on site to determine the cause of failure.

Core & Coils – Segment 1

Core & Coils – Segments 2 & 4

Core & Coils – Segment 3

Melted Copper Debris - AØ

Melted Copper Debris - AØ

Tear Down - AØLV Winding

Coil Deformation

Tear Down - AØHV Winding

Heat Damage - AØ HV Winding

Heat Damage - AØ HV Winding

Conductor Damage - HV Disk #25 AØ

Conductor Damage - HV Disk #25 AØ

Key Spacer Heat Damage

Conductor Damage - HV Disk #26 AØ

Conductor Damage -HV Disk #26 AØ

Outer LV Winding Tube Damage - AØ

HV Winding Tube – Minor Carbonization

Tear Down Complete

Case #2 Failure Summary

Predictive maintenance completed within 6 mos. Test Data Prior to Failure Normal AØ HV Winding Damage primarily in 2 disksNo damage in either LV Coil of AØNo damage in BØ or CØNo core damageHeating damage indicated high currentsRelays did not detect high current flow

Case #2 Cause of Failure?

The results of the tear down indicate a turn to turn failure in the AØ high voltage winding. The heat damage and coil deflections observed indicates localized high current flow within the winding, which is consistent with this type of fault. This current was not detected until the conductor burned through and a more serious fault developed. At that point the differential relay operated followed by the sudden pressure relay.

This failure appeared to be random and not related to the earlier GSU failure.