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Elimination of Transformer Inrush Currents When
Energizing Unloaded Power Transformers
John H. Brunke
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Presentation Outline
Basics of transformer inrush transients Three phase transformer core flux
transients based on model studies New controlled closing strategies Statistical performance
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Transformer Core Characteristics
1
2
Current
Flux
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Energizing a Transformer
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Transformer Inrush Current
Time 0.05 ms/div
4000
0.0
4000
Amperes
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Optimal controlled closing
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Three Phase Transformers
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Core Flux - No residual
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Prospective and Dynamic Flux
58.054.050.0 46.042.0
Time ms1
Flux, zero residual0.80
0.0
-0.80
FluxkV
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Three phase residual flux
Time 3 ms/div
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Core flux with residual flux
Time 6 ms/div
Core flux, EMTP Study 800
0.0
-800Vs
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Prospective and Dynamic Flux
58.054.050.046.042.0
Time ms
Flux, residual 0%, -70%, 70%0.80
0.0
-0.80
Flux kVs
ACB
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Prospective and Dynamic Flux
Time 6 ms/div
800
0.0
-800Vs
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Closing Strategies
Rapid closing strategy – Requires detailed transformer data and
look up table
Delayed Closing strategy – Generalized approach
Three phase strategy – Limited to high residual flux scenarios
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Verification - Laboratory Tests Rapid Closing Strategy
Time 20.5 ms/div
A Phase Flux .8000
0.0
-.8000Vs
Time 20.5 ms/div
B Phase Flux .8000
0.0
.8000Vs
Time 20.5 ms/div
C Phase Flux .8000
0.0
-.8000Vs
Time 20.5 ms/div
Neutral Current20
0.0
-20
Amperes
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Verification - Laboratory Tests Delayed Closing Strategy
Time 6 ms/div
.8000
0.0
-.8000Vs
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Effect of Closing Error
Model Study Results, Peak Inrush Current for a 70% Residual as a Function of Closing Angle
-1000-500
0500
100015002000250030003500
0 5 10 15 20
Closing Time ms
Pea
k In
rush
Cur
rent
A
mpe
res
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Prestrike
Time 2 ms/div
800.
400.
0.0
-400.
-800.
kV
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Statistical Studies
Benchmarks - linear bias runs with statistical pole span 1000 studies/run
Controlled studies - 200 studies/run 0.5, 1.0, 1.5, 2.0 ms timing error ranges
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Statistical Performance
CLOSING TIMESCATTER, 3 SIGMA
LEVEL EXCEEDING2% AMPERES
PERCENTREDUCTION FROMUNCONTROLLED 2%LEVEL
0.5 62 98%1.0 140 95.31.5 350 88.3%2.0 620 79.3Table 5.3. The improvement in performance using controlled closing with a70% residual flux. .
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Transformer Model
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Effect of Capacitance
Time
Flux
Time
Flux
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State of the Art Peak Inrush Current as a Function of Opening
Time
0
200
400
600
800
1000
1200
0 0.2 0.4 0.6 0.8 1
Opening Time over One Power Frequency Cycle
Peak
Inru
sh C
urre
nt
Ampe
res
Model
Moraw
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Delayed Performance Peak Inrush Current (2% Probability of occurrence)as a
Function of Closing Time Scatter
0
100
200
300
400
500
600
700
800
0 0.5 1 1.5 2 2.5
Closing Time Scatter (3 sigma) ms
Wor
st Ph
ase P
eak I
nrus
h Cu
rrent
Amp
eres 70,-70,0
0,0,0
0,-70,70
0,70,-70
0,35,-35
70,-40,-30
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Three Phase Performance Peak Inrush Current as a Function of Closing Time
Scatter
0
200
400
600
800
1000
1200
0 0.5 1 1.5 2 2.5
Closing Time Scatter (3 sigma) ms
Wor
st P
hase
Pea
k In
rush
Cur
rent
Am
pere
s
0,-70,70%
0,-35,35%
0,-.5,.5
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Rapid Closing Performance Peak Inrush Current (2% Probability of occurrence) as a
Function of Closing Time Scatter
0
100
200
300
400
500
600
700
0 0.5 1 1.5 2 2.5
Closing TIme Scatter (3 sigma) ms
Wor
st P
hase
Pea
k In
ruch
Cu
rrent
Am
pere
s
70,0,-70%
0,-70, 70%
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Conclusions
It is possible, by considering residual flux together with the appropriate closing strategy, to eliminate transformer inrush currents in most transformer configurations
With consideration of breaker closing scatter, a reduction of typically 90% of worst, can be achieved