The Meaning of LIFE
1
Transformer Life
The Meaning of LIFE!
IEEE C57.91 Guide for Loading Mineral-Oil Immersed Transformers
Thermal Profile
air
oil
Transformer
Height
h
Top Oil
core & coil
NL (core) Losses
I2R
NL (core) Losses
Bottom Oil
Oil Temperature
Top Oil Temperature
air
oil
Transformer
Height
h
Note: The FLASH POINT of Transformer
Oil is about 140 Deg C!
Top Oil
core & coil
Bottom Oil
Oil Temperature
Average Winding Rise
Resistance - ohms
P S
S P P S
S P
Average Winding Rise :
Measured by the Resistance of
the windings at the shut-down
of a heat run...
(t = 0)
shut down
time
Limited by ANSI to <= 55/65 deg C...
i.e., the basic definition of the kVA Rating of a Transformer!
Effective kVA
A good approximation for calculating the Effective kVA
over the range of parameters normally encountered in
Distribution Transformers is...
Effective kVA = Rated kVA x
65
Average Winding Rise
For example, a Transformer rated at 25 kVA with an AWR
of 50 deg C would have the capacity of a 30.4 kVA...
0.75
Hottest Spot
P S
S P P S
S P
Transformer Hot Spot -
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...
Limited to 140-170 deg C for Mineral Oil
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…
Temperature - Deg C
Transformer Temperatures
Hottest Spot
hottest spot gradient (HSG)
Top Oil
top oil rise (TOR)
Ambient
time
Under Steady-State Load and Ambient Conditions, the temperatures
stabilize to constant values...
Steady-State Load Conditions
Temperature - Deg C
Transformer Temperatures
Hottest Spot
HSG = rated HSG x K2m
TOR = rated TOR x
K2R+1
n
R+1
Top Oil
Ambient
time
The TOR and HSG at any value of Load is a function of the Rated
values of TOR and HSG, the Load (K), the Ratio of Losses
(R = LL/NL), and the winding (m) and oil (n) exponents...
Steady-State Load Conditions
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 1980’s
in actual tests done by several manufacturer’s
working with the IEEE Transformers Committee.
Thermal Mass
Load
K2
Tank
K1
Oil
C&C
t=0
time
The Temperature response to a change in LOAD is a
function of the MASS of the System reflected in the
TIME CONSTANT...
Oil Time Constant
K2
Load
for Top Oil...
K1
tcoil = C x
TOR
Total Losses
C = .06 x C&C + .04 x Tank + 1.33 x Oil Gallons
(note: for OA ratings with Copper Windings...)
t=0
time
The Top Oil Time Constant is a function of the Capacity
Factor, the Rated Top Oil Rise, and the Total Losses...
Oil Time Constant
K2
Load
for Top Oil...
K1
tcoil = C x
TOR
Total Losses
C = .06 x (WtCC-WtAL) + .14 x WtAL + .04 x WtTANK + 1.33 x Oil Gal
(note: for OA ratings with Aluminum Windings...)
t=0
time
The Top Oil Time Constant is a function of the Capacity
Factor, the Rated Top Oil Rise, and the Total Losses...
Hottest Spot Time Constant
Load
K2
for the Hottest Spot ...
K1
tchs = 5 to 15 minutes depending
on the winding material and
construction...
t=0
time
The Winding Time Constant is a function of Coil Construction
and is usually supplied by the Manufacturer...
Top Oil Rise – Increasing Load
Load
K2
K1
tu
TOR = ( tu - ti ) ( 1 - e -t/tc oil ) + ti
ti
t=0
time
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...
Hottest Spot – Increasing Load
HSG = (gu - gi) (1-e -t/tc hs) + gi
gu
K2
Load
tu
K1
gi
TOR = ( tu - ti ) ( 1 - e -t/tc oil) + ti
ti
t=0
time
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...
Top Oil – Decreasing Load
Load
K1
K2
ti
t=0
tu
TOR = ( ti - tu ) ( e -t/tc oil) + tu
time
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...
Hottest Spot – Decreasing Load
Load
K1
K2
HSG = (gi - gu) (e -t/tc hs ) + gu
ti
gu
t=0
TOR = ( ti - tu ) ( e -t/tc oil) + tu
tu
time
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...
Insulation Life
The Insulation LIFE of a Transformer
is a function of Temperature
and Time at Temperature...
Insulation Life – Montsinger’s 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...
This work was done in the 1920’s and later refined to 6 to 10
degrees based on the temperature range involved...
End-of-Life was defined as the point where the tensile
strength was reduced by 50% of the initial value...
Insulation Life – Retained Tensile Strength
Life vs. Temperature
Temperature
In the 1940’s, Dakin postulated that transformer insulation deteriorated
following a modification of Arrhenius’ chemical reaction rate theory.
Log LIFE = A + B/T
where T is the ABSOLUTE Temperature
of the Hottest Spot within the Winding...
T
LIFE
Hours to Failure
note: shown as a log-log
relationship...
Hottest Spot Temperature - Deg C
Life Testing
Based on AIEE Transformer’s Committee tests done in
the mid-1950’s, LIFE curves for Transformers were established...
220
group 1
Log LIFE = A + B/T
group 2
180
group 3
140
Lockie Tests
Hours to Failure
Insulation Life Testing - ANSI C57.100
log-log
relationship...
Life Testing
Insulation Life Testing - ANSI C57.100
Hottest Spot Temperature - Deg C
Life Testing
Based on AIEE Transformer’s Committee tests done in
the mid-1950’s, LIFE curves for Transformers were established...
220
group 1
Log LIFE = A + B/T
group 2
180
group 3
140
Lockie Tests
Hours to Failure
Insulation Life Testing - ANSI C57.100
log-log
relationship...
Hottest Spot Temperature - Deg C
Life Testing
Based on AIEE Transformer’s Committee tests done in
the mid-1950’s, LIFE curves for Transformers were established...
group 1
220
Log LIFE = A + B/T
1/5
180
140
C5
7.9
1-1
981
group 2
Dis
trib
uti
on
group 3
Lockie Tests
Hours to Failure
Insulation Life Testing - ANSI C57.100
log-log
relationship...
Hottest Spot Temperature - Deg C
Life Testing
Based on AIEE Transformer’s Committee tests done in
the mid-1950’s, LIFE curves for Transformers were established...
group 1
220
Log LIFE = A + B/T
1/5
180
140
1/2
C5
7.9
1-1
981
C5
7.9
2-1
981
Pow
er
group 2
Dis
trib
uti
on
group 3
Lockie Tests
Hours to Failure
Insulation Life Testing - ANSI C57.100
log-log
relationship...
Standard Life Curves
Hottest Spot - Deg C
1960 - 1980’s
C57.91-1981
110
100
90
C57.92-1981
10
30
50
70
Relative Insulation Life - years
ANSI Loading Guides - 65 deg C Life Equations
Insulation Life
Based on work done on model power transformers
in the 1980’s 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 C57.91.1995
b) 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...
ANSI Loading Guide - C57.91-1995
Per-Unit Life
B
Per Unit Life = A exp
ΘH + 273
A = 9.8 x 10 -18
B = 15000
Per Unit of Normal Life
1000
(based on 180,000 hours,
i.e., 20.55 years at 110 Deg C
for 1 NORMAL Life)
100
10
1.0
0.1
0.01
0.001
50
110
Hottest Spot Temperature - deg C
ΘH = Hottest Spot Temperature - Deg C
ANSI Loading Guide - C57.91 1995
190
Normal Insulation Life
for a well-dried, oxygen free 65 deg C average winding temperature
rise insulation system at the reference temperature of 110 deg C...
Hours
Years
50% retained tensile strength of insulation
(former C57.92-1981 criterion)
65,000
7.42
25% retained tensile strength of insulation
135,000
15.41
200 retained degree of polymerization in
insulation
150,000
17.12
Interpretation of distribution Transformer functional
life test data ( C57.91-1981 criteria)
180,000
20.55
ANSI Loading Guide - C57.91 1995
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 High
Performance Liquid Chromatography. The most significant compound is 2Furfuraldehyde 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.
Limitations
• The flash point of standard transformer oil is about 135-140 deg C. ANSI
C57.91 recommends that the 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) in
the 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.
All Transformers will eventually fail…
Lots of problems can be avoided by reducing the risk of an eventful failure…
Transformer Case Studies
36
Transformer Applications – FECA Agenda
June 11, 2009
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 3Φ + harmonics
- Vault 3Φ + 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
Case Studies
Case Studies :
1. Overhead Residential – OH/Res
3. Padmounted Residential – PM/Res
5. Commercial Padmount – PM/3Φ Com
7. Commercial Vault – Vault/3Φ Com
9. Substation/Power
Case Study – OH/Res
Case Study – OH/Res Ambient
Case Study – OH/Res Load
Case Study – OH/Res Transformer
Case Study – OH/Res Load Summary
Case Study – OH/Res Secondary
Case Study – PM/Res
Case Study – PM/Res
Load
Case Study – PM/Res
Transformer
Case Study – PM/Res
Bayonet Fusing
Case Study – PM/Res
Load Summary
Case Study – PM/Res
Touch Temps
Case Study – PM/Res
SCA & VR
Case Study – PM/Res
Customer Letter
Case Study – PM/3Φ Com
Case Study – PM/3Φ Com
Load
Case Study – PM/3Φ Com
Transformer
Case Study – PM/3Φ Com
Transformer
Case Study – PM/3Φ Com
Harmonics
Case Study – PM/3Φ Com
Harmonics
Case Study – PM/3Φ Com
w/Harmonics
Case Study – VAULT/3Φ Com
Case Study – VAULT/3Φ Com
Transformer
Case Study – VAULT/3Φ Com
FUSING
Case Study – VAULT/3Φ Com
Load Summary
Case Study – VAULT/3Φ Com
Secondary
Case Study – VAULT/3Φ Com
Secondary
Case Study – VAULT/3Φ Com
Secondary
Case Study – Substation
Case Study – Substation
AMBIENT
Case Study – Substation
LOAD
Case Study – Substation
TRANSFORMER
Case Study – Substation
OPTIONS
Case Study – Substation
Load Summary
Case Study – Substation
NATURAL ESTER
Case Study – Contingency Modeling
North American Transformer 200//333/373 55/65C MVA Pershing Sub - #6
Case Study – Contingency Modeling
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…
Case Study – Contingency Modeling
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
Q/A?
Don A. Duckett, P.E.
Technical Sales Engineer
HD Supply Utilities
(407) 402-0944
Don.Duckett@ieee.org