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Elimination of Transformer Inrush Currents When

Energizing Unloaded Power Transformers

John H. Brunke

Presentation Outline

Basics of transformer inrush transients Three phase transformer core flux

transients based on model studies New controlled closing strategies Statistical performance

Transformer Core Characteristics

1

2

Current

Flux

Energizing a Transformer

Transformer Inrush Current

Time 0.05 ms/div

4000

0.0

4000

Amperes

Optimal controlled closing

Three Phase Transformers

Core Flux - No residual

Prospective and Dynamic Flux

58.054.050.0 46.042.0

Time ms1

Flux, zero residual0.80

0.0

-0.80

FluxkV

Three phase residual flux

Time 3 ms/div

Core flux with residual flux

Time 6 ms/div

Core flux, EMTP Study 800

0.0

-800Vs

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

Prospective and Dynamic Flux

Time 6 ms/div

800

0.0

-800Vs

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

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

Verification - Laboratory Tests Delayed Closing Strategy

Time 6 ms/div

.8000

0.0

-.8000Vs

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

Prestrike

Time 2 ms/div

800.

400.

0.0

-400.

-800.

kV

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

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. .

Transformer Model

Effect of Capacitance

Time

Flux

Time

Flux

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

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

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

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%

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