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1

The Meaning of LIFE

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Transformer Life

IEEE C57.91 Guide for Loading Mineral-Oil Immersed Transformers

The Meaning of LIFE!

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Thermal Profile

h

core & coil

oil

air

Bottom Oil

Oil Temperature

Transformer

Height

Top Oil

NL (core) Losses

I2R

NL (core) Losses

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Top Oil Temperature

h

core & coil

oil

air

Bottom Oil

Oil Temperature

Note: The FLASH POINT of TransformerOil is about 140 Deg C!

Transformer

Height

Top Oil

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Average Winding Rise

P P P PS S S S

Resistance - ohms

timeshut down

Average Winding Rise :

(t = 0)Measured by the Resistance of

the windings at the shut-down

of a heat run...

Limited by ANSI to

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Effective kVA

Effective kVA = Rated kVA x65

Average Winding Rise

0.75

A good approximation for calculating the Effective kVAover the range of parameters normally encountered in

Distribution Transformers is...

For example, a Transformer rated at 25 kVA with an AWR

of 50 deg C would have the capacity of a 30.4 kVA...

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Hottest Spot

P P P PS S S S

Transformer Hot Spot - Limited to 140-170 deg C for Mineral Oil

Typically, the Hot Spot Gradient

(HSG) is 1.6 - 2 times the difference

between the Average Winding Rise

(AWR) and the Top Oil Rise (TOR)...

Attempts to measure the Hottest Spot

involve embedding thermocouples inside

the windings... this of course is not

typically done on production units...

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Hottest Spot Disk Wound

Typically, the Hot Spot Gradient (HSG) for Power

Construction is 1.1 to 1.2 times the difference between

the Average Winding Rise (AWR) and the Top Oil Rise

(TOR)...

Due to the Oil Flow inside the windings, the Hottest Spot

is typically near the top

Forced Oil (Directed or Non-directed) flow can effect

this relationship

For Core-Type Construction, Embedding Fiber Optic Temperature Transducers inside

the disk wound windings can be done when the unit is Unblocked...

Typically these are placed near the top (1-3 layers) down near the Center of the

Low Voltage winding

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Transformer Temperatures

Steady-State Load Conditions

time

Temperature-DegC

top oil rise (TOR)

hottest spot gradient (HSG)

Under Steady-State Load and Ambient Conditions, the temperaturesstabilize to constant values...

Hottest Spot

Top Oil

Ambient

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Transformer Temperatures

Steady-State Load Conditions

time

Temperature-DegC

TOR = rated TOR x

HSG = rated HSG x K2m

The TOR and HSG at any value of Load is a function of the Ratedvalues of TOR and HSG, the Load (K), the Ratio of Losses

(R = LL/NL), and the winding (m) and oil (n) exponents...

Hottest Spot

Top Oil

Ambient

K2R+1

R+1

n

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Transformer Thermal Calculations

The Calculations of the Hottest Spot, Top Oil, and

Insulation Life are defined in ANSI C57.91.

These procedures were verified in the late 1980s

in actual tests done by several manufacturers

working with the IEEE Transformers Committee.

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Thermal Mass

time

Load

The Temperature response to a change in LOAD is a

function of the MASS of the System reflected in the

TIME CONSTANT...

t = 0

K1

K2

C&C

Oil

Tank

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Oil Time Constant

time

Load

t = 0

K1

K2

for Top Oil...

tcoil = C xTOR

Total Losses

C = .06 x C&C + .04 x Tank + 1.33 x Oil Gallons

The Top Oil Time Constant is a function of the Capacity

Factor, the Rated Top Oil Rise, and the Total Losses...

(note: for OA ratings with Copper Windings...)

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Oil Time Constant

time

Load

t = 0

K1

K2

for Top Oil...

tcoil = C xTOR

Total Losses

C = .06 x (WtCC-WtAL) + .14 x WtAL + .04 x WtTANK+ 1.33 x Oil Gal

The Top Oil Time Constant is a function of the Capacity

Factor, the Rated Top Oil Rise, and the Total Losses...

(note: for OA ratings with Aluminum Windings...)

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Hottest Spot Time Constant

time

L

oad

t = 0

K1

K2

for the Hottest Spot ...

tchs = 5 to 15 minutes dependingon the winding material and

construction...

The Winding Time Constant is a function of Coil Construction

and is usually supplied by the Manufacturer...

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Top Oil Rise Increasing Load

time

Load

t = 0

K1

K2

ti

tu

TOR = ( tu - ti ) ( 1 - e-t/tc ) + ti

The TOR at any point in time is a function of both the Initial

and Ultimate Top Oil Temperatures and the Top Oil Time

Constant...

oil

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Hottest Spot Increasing Load

time

Lo

ad

t = 0

K1

K2

ti

tu

TOR = ( tu - ti ) ( 1 - e-t/tc ) + ti

The HSG at any point in time is a function of both the Initial

and Ultimate Hot Spot Temperatures and the Hot Spot Time

Constant...

oil

HSG = (gu - gi) (1-e-t/tc

) + gi

gi

gu

hs

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Top Oil Decreasing Load

time

Lo

ad

t = 0

ti

tu

TOR = ( ti - tu ) ( e-t/tc ) + tu

The TOR at any point in time is a function of both the Initial

and Ultimate Top Oil Temperatures and the Top Oil Time

Constant...

oil

K1

K2

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Hottest Spot Decreasing Load

time

Lo

ad

t = 0

K1

K2

tuTOR = ( ti - tu ) ( e-t/tc ) + tuoil

The HSG at any point in time is a function of both the Initial

and Ultimate Hot Spot Temperatures and the Hot Spot Time

Constant...

ti

HSG = (gi - gu) (e-t/tc ) + gu

gu

hs

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Insulation Life

The Insulation LIFE of a Transformeris a function ofTemperature

and Time at Temperature...

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Insulation Life Montsingers 8 degree Rule

The LIFE of varnished cambric tape, as defined by a

reduction in the tensile strength, is reduced by one half

for each 8 deg C increase in continuous temperature...

End-of-Life was defined as the point where the tensile

strength was reduced by 50% of the initial value...

This work was done in the 1920s and later refined to 6 to 10

degrees based on the temperature range involved...

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Insulation Life Retained Tensile Strength

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Life vs. Temperature

Temperature

Hours to Failure

Log LIFE = A + B/Twhere T is the ABSOLUTE Temperature

of the Hottest Spot within the Winding...

note: shown as a log-log

relationship...

T

LIFE

In the 1940s, Dakin postulated that transformer insulation deterioratedfollowing a modification of Arrhenius chemical reaction rate theory.

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Life Testing

Insulation Life Testing - ANSI C57.100

HottestSpotTemperature-DegC

Hours to Failure

Log LIFE = A + B/T

log-log

relationship...

140

180

220 group 1

group 2

group 3

Lockie Tests

Based on AIEE Transformers Committee tests done inthe mid-1950s, LIFE curves for Transformers were established...

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Life Testing


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Life Testing



Hours to Failure

Log LIFE = A + B/T

log-log

relationship...

140

180

220 group 1

group 2

group 3

Lockie Tests


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Life Testing



Hours to Failure

Log LIFE = A + B/T

log-log

relationship...

140

180

220 group 1

group 2

group 3

C57.91-1981Distribution

1/5

Lockie Tests


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Life Testing



Hours to Failure

Log LIFE = A + B/T

log-log

relationship...

140

180

220 group 1

group 2

group 3

C57.91-1981Distribution

C57.92-1981Power

1/5

1/2

Lockie Tests


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Standard Life Curves

ANSI Loading Guides - 65 deg C Life Equations

90

100

110

HottestSpot-DegC

10 30 50 70

Relative Insulation Life - years

C57.91-1981

C57.92-1981

1960 - 1980s

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Insulation Life

ANSI Loading Guide - C57.91-1995

Based on work done on model power transformersin the 1980s by EPRI...

a) It was decided that the insulation life curves for both Power

and Distribution Transformers are similar...C57.91, C57.92

and C57.115 were combined into C57.91.1995b) ANSI C57.100 should be used for the evaluation of the thermal

life of new insulation systems...

c) The chemical test measurement of degree of polymerization (DP)

is a much better indication of cellulose insulation mechanical

characteristics than loss of tensile strength...

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Per-Unit Life

ANSI Loading Guide - C57.91 1995

Per Unit Life = A exp + 273

= Hottest Spot Temperature - Deg CH

A =

B = 15000

9.8 x 10 -18

(based on 180,000 hours,

i.e., 20.55 years at 110 Deg C

for 1 NORMAL Life)

0.001

0.01

0.1

1.0

10

100

1000

PerUnitofNormalLife

50 110 190

Hottest Spot Temperature - deg C

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Normal Insulation Life

ANSI Loading Guide - C57.91 1995

for a well-dried, oxygen free 65 deg C average winding temperaturerise insulation system at the reference temperature of 110 deg C...

50% retained tensile strength of insulation

(former C57.92-1981 criterion)

25% retained tensile strength of insulation

200 retained degree of polymerization in

insulation

Interpretation of distribution Transformer functional

life test data ( C57.91-1981 criteria)

Hours Years

65,000

135,000

150,000

180,000

7.42

15.41

17.12

20.55

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Non-Invasive Testing - Furans

The DP of paper can be determined utilizing ASTM method D-4243.

Acquisition of a paper sample is an invasive and expensive procedure that requires

taking the unit off line. A non-invasive alternative has been developed recently based

on the determination of oil soluble cellulose decomposition products called furans.

These compounds can analyze these compounds from an oil sample with HighPerformance Liquid Chromatography. The most significant compound is 2-

Furfuraldehyde where concentrations of this compound have been correlated with DP.

Chendong has developed an empirical correlation between the concentration of

2-Furfuraldehyde and the DP. Log [Fur] = 1.51 - 0.0035 DP

where [Fur] = conc. of 2-Furfuraldehyde in PPM.

This equation, though not exact, allows one to estimate the DP of cellulose insulation.

Knowing the DP value, one can estimate the remaining insulation life of a transformer.

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Limitations

The flash point of standard transformer oil is about 135-140 deg C. ANSIC57.91 recommends thatthe maximum allowable temperature for Top Oil

should not exceed 110 deg C! (Mineral Oil)

Due to the potential of gas bubble generation in mineral oil, the Hottest Spot

temperature should be limited to a maximum of 140 deg C(Mineral Oil)inthe Spring/Fall and summer seasons. Temperatures up to 170 deg C can

typically be allowed in the winter seasons.

The thermal limitations of equipment and components such as bushings,

gaskets, tap changers, switches, etc. should be considered.

For normal 55/65C Oil-Paper insulation systems, Thermal run-away begins

around 200 deg C.

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All Transformers will eventually fail

Lots of problems can be avoided by reducing the risk of an eventful failure

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36

Transformer Case Studies

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Transformer Applications FECA Agenda

08:30-10:00 Transformer Overview- Basic Construction

- Lightning / LV Surges- Voltage Regulation / Flicker- Life Cycle Costing / DOE Efficiency Ruling

10:00-10:15 Break

10:15-12:15 Insulation Life (C57.91) + TAP Simulations- The Meaning of Life-

the C57.91 Loading Guide- Over Head Residential + flicker- Padmount Residential + fusing + Short Circuit- Padmount 3 + harmonics- Vault 3 + vault restrictions- Substation/Power Transformers- Ratings/Cooling Modes Settings- Contingency Modeling

1:00 2:45 Power Transformer Maintenance, Monitoring & DGA

2:45 3:00 Exams/Discussions/wrap-up

Revised May 29, 2009 dad

June 11, 2009

C S di

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Case Studies

1. Overhead Residential OH/Res

3. Padmounted Residential PM/Res

5. Commercial Padmount PM/3 Com

7. Commercial Vault Vault/3 Com

9. Substation/Power

Case Studies :

C St d

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Case Study OH/Res

C St d

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Case Study OH/Res Ambient

C St d

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Case Study OH/Res Load

C St d

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Case Study OH/Res Transformer

C St d

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Case Study OH/Res Load Summary

C St d

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Case Study OH/Res Secondary

C St d

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Case Study PM/Res

Case Study /

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Case Study PM/Res Load

Case Study PM/R T f

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Case Study PM/Res Transformer

Case Study PM/R B t F i

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Case Study PM/Res Bayonet Fusing

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Case Study PM/R T h T

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Case Study PM/Res Touch Temps

Case Study PM/R SCA & VR

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Case Study PM/Res SCA & VR

Case Study PM/Res C stomer Letter

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Case Study PM/Res Customer Letter

Case Study PM/3 Com

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Case Study PM/3 Com

Case Study PM/3 Com Load

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Case Study PM/3 Com Load

Case Study PM/3 Com Transformer

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Case Study PM/3 Com Harmonics

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Case Study PM/3 Com w/Harmonics

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Case Study PM/3 Com w/Harmonics

Case Study VAULT/3 Com

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Case Study VAULT/3 Com

Case Study VAULT/3 Com Transformer

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Case Study VAULT/3 Com Transformer

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Case Study VAULT/3 Com Load Summary

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Case Study VAULT/3 Com Load Summary

Case Study VAULT/3 Com Secondary

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Case Study Substation

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Case Study Substation

Case Study Substation AMBIENT

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y

Case Study Substation LOAD

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y

Case Study Substation TRANSFORMER

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y

Case Study Substation OPTIONS

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y

Case Study Substation Load Summary

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y y

Case Study Substation NATURAL ESTER

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y

Case Study Contingency Modeling

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y

North American Transformer 200//333/373 55/65C MVA Pershing Sub - #6


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y

With 85% of peak top rating applied (317050 kVA) during the peak season (extreme summer) a

switching operation adds an additional 130550 kVA (about 120% of Peak) for 15 minutes the Hottest

Spot peaks to 89.8 degrees C and is still within the NERC specified requirements (max is 105C). The

losses, Top Oil Rise and Hottest Spot Gradients used to develop the review on this unit is based on

supplied manufacturer test reports this is using Clause 7 of C57.91


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y

Using the Annex G procedure, with 85% of peak rating applied (317050 kVA) during the peak season

(extreme summer) the switching operation (120% of peak) shows the Hottest Spot peaks to 95.43

degrees C which is still within the requirements (max is 105C). Note the difference between the results

of the Clause 7 procedure vs. Annex G note that Annex G routines provide a metric on duct oil

which might be useful in predicting potential gassing

Case Study Summary

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y

Q/A?Don A. Duckett, P.E.

Technical Sales EngineerHD Supply Utilities

(407) 402-0944

[email protected]

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