Power Transformer Power Transformer Thermal RatingsThermal Ratings

Tim RaymondTim Raymond2003 IEEE Transmission & Distribution 2003 IEEE Transmission & Distribution ConferenceConferenceDallas, Texas – PN06 – Tuesday, 9-Sep-2003Dallas, Texas – PN06 – Tuesday, 9-Sep-2003

Transformer Loading Transformer Loading LimitLimit In general, transformer capacity In general, transformer capacity

limited by equipment (winding & limited by equipment (winding & oil) temperatures.oil) temperatures.

Design limits (110C hot spot) not Design limits (110C hot spot) not necessarily operational limitsnecessarily operational limits

Nameplate RatingNameplate Rating

Nameplate rating is not Nameplate rating is not necessarily operational limitnecessarily operational limit

Based on 30C (40C Max.) Based on 30C (40C Max.) ambientambient

Continuous load at rated Continuous load at rated currentcurrent

Conservative hot spot temp Conservative hot spot temp of 110Cof 110C

Results in overly Results in overly conservative ratingsconservative ratings

Transformer CoolingTransformer Cooling

Insulating oil acts as cooling mediumInsulating oil acts as cooling medium Oil flow is either natural Oil flow is either natural

thermosiphon flow (no pumps) or thermosiphon flow (no pumps) or forced flow (pumps).forced flow (pumps).

Heat exchangers or radiators transfer Heat exchangers or radiators transfer heat from oil to environmentheat from oil to environment

Fans may be used to increase forced Fans may be used to increase forced convection over radiator finsconvection over radiator fins

Oil Flow Within Oil Flow Within WindingWinding

Core & CoilsCore & Coils

Transformer CoolingTransformer Cooling

Thermal ModelThermal Model

Simple lumped parameter modelSimple lumped parameter model

Oil temperature rise function of total lossOil temperature rise function of total loss Winding temperature rise function of currentWinding temperature rise function of current

miRHSiHS

n

iRTOiTO

K

R

RK

2,,

2

,, 1

1

Steady-state Temperatures:

2,2,2,2, HSTOAHS

Thermal ModelThermal Model

Thermal ModelThermal Model

Oil thermal time constant:Oil thermal time constant:For OA and FA (ONAN and OFAF) the thermal For OA and FA (ONAN and OFAF) the thermal

capacity, C, equals: capacity, C, equals:

C = 0.0272 * WC = 0.0272 * WCCCC + 0.01814 * W + 0.01814 * WTankTank + 5.034 * + 5.034 * VVFluidFluid

For DFOA and NDFOA (ODAF and OFAF) the For DFOA and NDFOA (ODAF and OFAF) the thermal capacity, C, equals:thermal capacity, C, equals:

C = 0.0272 * WC = 0.0272 * WCCCC + 0.0272 * W + 0.0272 * WTankTank + 7.305 * + 7.305 * VVFluidFluid

The thermal time constant of the bulk oil at rated The thermal time constant of the bulk oil at rated temperature equals:temperature equals:

O,RO,R = C * = C * TO,RTO,R / P / PT,RT,R

Thermal ModelThermal Model

n

URTOUTO R

RK

1

12

,, 1,1,,2, 1 TO

t

TOUTOTOOe

mURHSUHS K 2

,, 1,1,,2, 1 HS

t

HSUHSHSWe

2,2,2,2, HSTOAHS

Top oil temperature rise at time t2 = t1+t

Hot spot temperature rise at time t2 = t1+t

Hot spot temperature at time t2 = t1+t

RiskRisk

Any energized transformer has Any energized transformer has finite risk of failurefinite risk of failure

Risk increases with operating Risk increases with operating temperaturestemperatures

““Overload” is possible, if risks are Overload” is possible, if risks are considered and mitigated to considered and mitigated to acceptable levelsacceptable levels

RisksRisks

Three areas of risk:Three areas of risk:– Long-term (insulation aging)Long-term (insulation aging)– Short-term (bubbles, oil expansion)Short-term (bubbles, oil expansion)– Ancillary components (bushings, Ancillary components (bushings,

LTCs, CTs)LTCs, CTs)

Long-term RiskLong-term Risk

Insulation ages at any Insulation ages at any temperaturetemperature

Rate of aging increases with Rate of aging increases with temperature (roughly doubling for temperature (roughly doubling for every 6C rise)every 6C rise)

Effects mechanical strength of Effects mechanical strength of paperpaper

Insulation AgingInsulation Aging

Oil and paper adversely affected Oil and paper adversely affected by heatby heat

Oil is replaceable, paper is notOil is replaceable, paper is not Paper ages at any temperaturePaper ages at any temperature Pyrolysis, hydrolysis, oxidationPyrolysis, hydrolysis, oxidation Break bonds in cellulose chain, Break bonds in cellulose chain,

decreases mechanical strength decreases mechanical strength

Insulation AgingInsulation Aging

Arrhenius Reaction RateArrhenius Reaction Rate

A is reference hot spotA is reference hot spot– A=95C for 55C insulationA=95C for 55C insulation– A=110C for 65C insulationA=110C for 65C insulation

Integrate FIntegrate FAAAA over time to get equiv. over time to get equiv. agingaging

Life “end point” difficult to determineLife “end point” difficult to determine Based upon moisture content of 0.5% Based upon moisture content of 0.5%

and low O2 and low O2

273273, HSRHS

AA

AA eF

Insulation AgingInsulation Aging

Source: IEEE C57.91

Short-term RisksShort-term Risks

Reduction in dielectric strength Reduction in dielectric strength due to “bubble” formation due to “bubble” formation (>140C)(>140C)

Function of moisture content, gas Function of moisture content, gas content, pressure & temperaturecontent, pressure & temperature

Bubble FormationBubble Formation

Source: EPRI Report RP 1289-1

Bubble PredictionBubble Prediction

2 models available2 models available– Complex model by Rouse et al., sum Complex model by Rouse et al., sum

of instantaneous partial pressure of of instantaneous partial pressure of gas and water vapor vs. static gas and water vapor vs. static pressurepressure

– Empirical model by T. V. Oommen Empirical model by T. V. Oommen relates moisture content, gas relates moisture content, gas pressure and static pressure to pressure and static pressure to onset temperatureonset temperature

Bubble Prediction Bubble Prediction (Oommen)(Oommen)

273 - 30

V

P - W 1.4495 + 22.454

6996.7 =

1.585g) W 0.473 (

presWPbubble

WP

EXP

lnln

Where:

bubble is bubble onset temperature

WWP is moisture content of paper (% by wgt)

Ppres is static pressure

Vg is total gas concentration (% by vol)

Additional RisksAdditional Risks

Expansion of oil beyond capacity Expansion of oil beyond capacity of tankof tank

Deterioration of gaskets and sealsDeterioration of gaskets and seals Delamination of composite Delamination of composite

insulating materialsinsulating materials Reduced mechanical strength of Reduced mechanical strength of

conductor bonding materialsconductor bonding materials

Ancillary ComponentsAncillary Components

BushingsBushings LTCsLTCs DETCsDETCs LeadsLeads Bushing CTsBushing CTs

BushingsBushings

Oil-impregnated, paper-Oil-impregnated, paper-insulated, capacitance-insulated, capacitance-graded bushingsgraded bushings

Designed with 105C Designed with 105C bushing hot spot limit at bushing hot spot limit at rated load and 95C top oil rated load and 95C top oil temperature average over temperature average over 24 hours24 hours

Overload is possibleOverload is possible

Bushing (cont)Bushing (cont)

Overload risks of bushings include:Overload risks of bushings include:– Pressure build-up due to oil expansionPressure build-up due to oil expansion– Deterioration of gaskets and sealsDeterioration of gaskets and seals– Thermal deterioration of paper Thermal deterioration of paper

insulationinsulation– Increase in dielectric loss, possibly Increase in dielectric loss, possibly

resulting in thermal runawayresulting in thermal runaway– Gas evolution at extreme hot spotsGas evolution at extreme hot spots

Bushing (cont)Bushing (cont)

Overload limits:Overload limits:– 40C ambient40C ambient– 110C transformer top oil 110C transformer top oil

temperaturetemperature– 2x rated bushing current2x rated bushing current– 150C bushing hot spot temperature150C bushing hot spot temperature

Load Tap ChangerLoad Tap Changer

Designed with a contact rise Designed with a contact rise over oil of less than 20C at over oil of less than 20C at 1.2 times rated load1.2 times rated load

Designed to break twice Designed to break twice rated current at least 40 rated current at least 40 timestimes

Load Tap Changer Load Tap Changer (cont)(cont) Overload risks of LTCs include:Overload risks of LTCs include:

– Increased contact wear and ablation with Increased contact wear and ablation with increased load during break operationsincreased load during break operations

– Increased contact temperatures increases Increased contact temperatures increases probability and rate of coking of contacts probability and rate of coking of contacts (>120C)(>120C)

– Higher overloads result in prolonged Higher overloads result in prolonged arcing during break operation. Dragging arcing during break operation. Dragging the arc across the contacts could result in the arc across the contacts could result in short-circuiting the regulating windingshort-circuiting the regulating winding

Load Tap Changer Load Tap Changer (cont)(cont) Overload limits:Overload limits:

– 120C contact temperature (higher is 120C contact temperature (higher is OK, but may result in greater OK, but may result in greater maintenance)maintenance)

– 2x LTC rated load current (limit 2x LTC rated load current (limit breaking operations at high load breaking operations at high load level to few times/year)level to few times/year)

OtherOther

DETC similar in rating to OLTCDETC similar in rating to OLTC Leads can be a concern. Same hot Leads can be a concern. Same hot

spot limits apply for leads as for spot limits apply for leads as for windings. However, leads are usually windings. However, leads are usually not limiting.not limiting.

Bushing CTs are limited by Bushing CTs are limited by transformer top oil temperature. transformer top oil temperature. Maintaining a top oil < 110C should Maintaining a top oil < 110C should avoid excessive temperaturesavoid excessive temperatures

Example Lead HeatingExample Lead Heating

Operation ConcernsOperation Concerns

Difference between tested rises Difference between tested rises and design limitsand design limits

Age & loading historyAge & loading history Prior operational records Prior operational records

(maintenance history)(maintenance history) Overall Condition (moisture, DGA, Overall Condition (moisture, DGA,

oil quality)oil quality)

UpratingUprating

Add fans and pumpsAdd fans and pumps– Practical maximumPractical maximum– Caution must be exercised to avoid large Caution must be exercised to avoid large

hot spot gradienthot spot gradient Increase size of heat exchangerIncrease size of heat exchanger

– Same caveats as aboveSame caveats as above Water spray during peak loadingWater spray during peak loading

– Careful to avoid excessive local Careful to avoid excessive local mechanical forces and steam may cause mechanical forces and steam may cause flashover of bushingsflashover of bushings