power transformers thermal rating
TRANSCRIPT
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