transformer life extention methodlogy thru ltc replacement
DESCRIPTION
The Methodology to extend the operating life of power transformersTRANSCRIPT
TRANSFORMER LIFEEXTENSION METHODOLOGY
THRU LTC REPLACEMENT
PIERRE FEGHALI
NORTH AMERICAN SUBSTATION SERVICES
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WHY are we here?
Why do we use LTCs?Transformer Efficient 99%+,No moving partsLTC Maintenance is very costlyLTC is only accessory with moving partsMore than 20% of transformer failures are due to LTC failures
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LTC Operation
TRANSFORMER FAILURE CAUSE 1955 - 1988
50%
21%
11%
6%
2%
10%
0%
10%
20%
30%
40%
50%
60%
Winding Tap Changer Bushings Terminal Board Core Other
FAILURE CAUSE
% OF TOTAL
DATA FROM ANSI/IEEE C37.91, TABLE 1
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LTC Transformer DesignVoltage Considerations
V/T voltage determines transformer design factors. It is the LV phase voltage divided by the number of LV turns.For a 10% regulation, the LTC Tap winding has 10% of the LV turnsVolts per step is the voltage that the LTC goes thru to move from one tap position to the next.The above determines the LTC criteria.
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LTC Design ExampleVoltage Considerations
50 MVA 115KV to 12.47KV +/-10% LTC in LV Neutral
V/T = 12,470/1.732/160 = 45LV has 160 turns therefore the LTC tap winding has 16 turns and 1247 volts.16 turns divided by 8 taps yield 2 turns per step or 90 Volts per step.There are two Types of LTCs:
Reactive in the past the arcing in oil LTCs were used like the FPE TC25, McGraw 550 A&B, Allis Chamers TLH 20&21, Westinghouse UTTA&B etc… Today’s designers have very few choices but the Reinhausen RMV has over 90% of the Reactive LTC market.Resistive the choices are UZD, UZE or UZF.
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LTC Design ExampleVoltage Considerations
50 MVA 115KV to 12.47KV +/-10% LTC in LV Neutral
The RMV is good for up to 69KV. Volts per step of 2000 Volts. It can handle this transformer design from a voltage stand point.The resistive LTCs can handle the voltage as they are good for 34KV. Volts per step of 500 VoltsNow the current consideration.
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LTC Design ExampleCurrent Considerations
50 MVA 115KV to 12.47KV +/-10% LTC in LV Neutral
LV Current = 50 x 10^6/12470/1.732 = 2315 AmpsReactive LTC RMV is rated up to 2500 Amps
Guarantees 500,000 operations at 2500 Amps and step voltage of 2000 VoltsGuarantees 100,000 operations prior to any maintenance
Resistive LTCs are rated up to 600 Amps. A series transformer must be used to reduce the current that the LTC is switching.
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Transformer Regulation Configurations
LTC in Auto transformers In Neutral To Regulate LV – variable fluxIn Common Winding to Regulate LVIn Common Winding to Regulate HV
LTC application in Power transformersDelta WindingWYE Winding
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Auto-Transformer Configurations
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Transformer Regulation Delta Winding
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Transformer RegulationWYE winding
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Reactive LTC Switching Operation – Coarse Fine
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Reactive LTC Switching Operation – Reversing Switch
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Resistive Switching LTC
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LTC Switching Comparison
Resistive & Old Reactive – Arcing in oilArcing Selector Tap SwitchArc interacts with surrounding mediumSwitching characteristics depend on the surrounding medium.The arc voltage drop is 25-30 VoltsHigh contact wear and contamination of oilOil contamination - Particles & combustible gasses are produced (See table for Typical gas levels)Maintenance Cycle of 2-5 years
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LTC Switching Comparison
Reactive LTC – Vacuum interrupterSwitching happens in VacuumBy pass switch & Selector Tap SwitchArc is isolated from surrounding mediumSwitching characteristics do not depend on the surrounding medium.The arc voltage drop is 5-10 VoltsVery low contact wear (more than 10 times less wear) and no contamination of oilExtended maintenance cycle based on condition and not time interval.
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LTC Comparison
5002503005000Resistive
14005006507000Reactive Diverter Compartment
50010025025Reactive Selector Compartment
250501505Vacuum
EthyleneEthaneMethaneAcetyleneLTC Type
LTC TYPICAL GAS LEVELS
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Prevent the problems from re-occurring.
Fix the problem at its root. Replace the arcing in oil LTC.
Extend the life of the transformer.
LTC Replacement is the answer
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Base line testing - TTR, Resistance, Doble PF, Megger and oil sample (Day 1)
Drain LTC compartment – Dispose of oil (Day 2)
Drain Main tank oil into storage tank (Day 2)
Internally disconnect all LTC leads and Blank LTC off with Fire Blankets (Day 2)
Remove old tap changer off transformer main tank (Day 3)
Weld LTC tank transition piece (Day 4)
Weld new RMV LTC on transition piece (Day 4)
Pressure Leak Test (Overnight day 4)
LTC Replacement Process Steps
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Crimp lead extensions on all LTC leads (Day 5)
Rework/Replace reactor if applicable (Day 5)
Crimp extension leads to reactor leads as required (Day 6)
Make all internal connections to the LTC back board (Day 7 & Day 8)
Test (TTR, Resistance & Make before break) to verify new connections (Day 9)
Pressurize tank with dry air (Day 9)
LTC Replacement Process Steps
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Take a Dew Point to determine dryness of transformer (Day 10)
Rewire Controls – New Control Cabinet if necessary (Days 6 thru 12)
Perform field dry-out if necessary (Day 10 & 11)Apply vacuum and hot oil fill transformer (Day 12)Perform final testing (Day 13 & 14)Provide “as built” control, outline and nameplate drawings
LTC Replacement Process Steps
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LTC Field Replacement Pictures(Before)
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LTC Field Replacement Picture(After)
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Economic Evaluation for 50 MVA
(Based on a 10 year cycle)
1 week x 33 weeks12 weeks3 weeksOutage Length
27%20%59%100%Total
0%0%12%8%Transportation
12%10%14%10%
Installation labor (Contractor and/or
Customer)
15%10%34%82%Material
LTC maintenance -
Contact Replacement
etc…
LTC Replacement in
field
Repair of LTC failure at repair
facilityNew Transformer
Replacement
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What happens after 10 years?
Deferred Capital Expenditure by 10 yearsThe upgraded RMV LTC is portable and can be used somewhere else.RMV LTC has paid for itself by the reduction of operation and maintenance costs – Cost AvoidanceThe arcing in oil LTC has been upgraded 3 times but is still an “arcing in oil LTC”No catastrophic failure ever recorded with an RMV
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LTC Replacement Benefits
Long Term Uninterrupted availability of the transformerExtension of the maintenance intervalReduction of Maintenance outages by 90%
Life Extension as the Vacuum LTC will reduce the likelihood of LTC failure or LTC winding failure.RMV has a very Low Failure RateReduction of the transformer operating costsShort Outage only 2-3 weeksRegasket, Dry out and Degasification of transformerEconomical Solution
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Conclusion
Extends Life of transformerReasonable economics compared to failure cost or new replacement cost.NASS has experience in replacing:
FPE TC 25, TC525 and TC546Allis Chamers TLH 20 and 21McGraw 550Westinghouse UTT & URTFerranti Packard
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Q & A
QUESTIONS