Transformer Life Extention Methodlogy Thru LTC Replacement

TRANSFORMER LIFEEXTENSION METHODOLOGY

THRU LTC REPLACEMENT

PIERRE FEGHALI

NORTH AMERICAN SUBSTATION SERVICES

Independent ● Experienced ● Safety ● Quality ● Customer Focused, Not Numbers Focused

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


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


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.


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.


LTC Design ExampleVoltage Considerations


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.


LTC Design ExampleCurrent Considerations


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.


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


Auto-Transformer Configurations


Transformer Regulation Delta Winding


Transformer RegulationWYE winding


Reactive LTC Switching Operation – Coarse Fine


Reactive LTC Switching Operation – Reversing Switch


Resistive Switching LTC


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


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.


LTC Comparison

5002503005000Resistive

14005006507000Reactive Diverter Compartment

50010025025Reactive Selector Compartment

250501505Vacuum

EthyleneEthaneMethaneAcetyleneLTC Type

LTC TYPICAL GAS LEVELS


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


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


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)



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 Field Replacement Pictures(Before)


LTC Field Replacement Picture(After)


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


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


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


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


Q & A

QUESTIONS

Documents

Transformer Life Extention Methodlogy Thru LTC Replacement