the case of failed transformer
DESCRIPTION
Two cases of power transformer failures were presented and analysed on the causes of failuresTRANSCRIPT
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The Case of a Failed TransformerCase #1 - GSU
James W. GrahamAlliant Energy
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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
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First Steps
Inspection Damage AssessmentReview Known DataSystem Impact
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Initial Inspection & Damage Assessment
Field TestsWinding damage confirmedArresters OKBushings OK
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Oil leak due to broken piping
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Minor Tank Deformation - Upper Section
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Minor Tank Deformation – Lower Section
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Typical DETC Switch
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AØ DETC Dislocated
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Arc Damage Across Active DETC Tap - AØ
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DETC Leads Disengaged
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Insulation Debris on top of the phase pack
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Review Known Data
No system fault prior to failurePre-fault DGA samples normalOil test data normalWinding temperature normalOil temperature normalHistory indicated some overloading
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Loss of Sales RevenueCost of Replacement PowerLoss of Voltage SupportSystem Reliability ReducedScheduled Transmission Outages DeferredOther Unit Maintenance Outages Deferred
Impact On the System
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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
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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
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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%
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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
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Transformer Disassembly – 4 days
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One of 5 semi-truck loads of accessories
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Transformer Accessories – On Site
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Transformer Unit Train
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Rail Car Assembly
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Transformer Loading – 2 days
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Staley Bridge
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Temporary GSU in Service – 81 days after failure
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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
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Transformer Options
Purchase new 345-24kV transformer
Purchase new 161-24kV transformer
Repair failed 161-24kV transformer
Leave temporary transformer in place
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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 ?
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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
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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
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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.
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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
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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
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Factory Tear Down Core Removal
Top 2 Tank Sections Removed
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Core & CoilsHV Side (Segment 3)
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Core Removal in Progress
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AØ Winding Damage Visible
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AØ Winding Damage Visible – A Better View
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411,000 lbs Core Steel22,000 lbs. Replaced
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Factory Tear Down – Phase Pack
Phase Pack - AØ Bottom
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Phase Pack - AØ Top
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Low Voltage Coil Removal
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High Voltage Coil Removal(Undamaged Section)
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Typical Insulation Washer & Spacers
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LV Coil Removal(Undamaged Section)
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LV Coil Removal(Undamaged Section)
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First DamagedHigh Voltage Coil
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High Voltage CoilSevere Coil Deformation
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Short Circuit Forces cause coils to roll over & collapse to the center core
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Rift created by coil movement is 6” wide x 30” long x 10” deep
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High Voltage Coil Distortion
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Damaged High Voltage Coil Removal
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High Voltage Conductor Burned Through
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DETC Tap 3 & 4 Studs Burned - AØ
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DETC Tap 3 Terminals - AØ
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Spade lug
Spring Washer Missing
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Evidence of Localized Heating in HV Coil(Not Failure Related)
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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
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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.
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The Case of a Failed TransformerCase #2 – Main Auxiliary #102
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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
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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?
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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.
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Core & Coils – Segment 1
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Core & Coils – Segments 2 & 4
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Core & Coils – Segment 3
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Melted Copper Debris - AØ
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Melted Copper Debris - AØ
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Tear Down - AØLV Winding
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Coil Deformation
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Tear Down - AØHV Winding
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Heat Damage - AØ HV Winding
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Heat Damage - AØ HV Winding
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Conductor Damage - HV Disk #25 AØ
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Conductor Damage - HV Disk #25 AØ
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Key Spacer Heat Damage
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Conductor Damage - HV Disk #26 AØ
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Conductor Damage -HV Disk #26 AØ
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Outer LV Winding Tube Damage - AØ
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HV Winding Tube – Minor Carbonization
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Tear Down Complete
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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
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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.