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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

    Elimination of Transformer Inrush Currents When Energizing Unloaded Power TransformersJohn H. BrunkePresentation OutlineTransformer Core Characteristics Energizing a Transformer Transformer Inrush CurrentOptimal controlled closingThree Phase TransformersCore Flux - No residualProspective and Dynamic FluxThree phase residual fluxCore flux with residual fluxProspective and Dynamic FluxProspective and Dynamic FluxClosing StrategiesVerification - Laboratory TestsRapid Closing StrategyVerification - Laboratory TestsDelayed Closing StrategyEffect of Closing ErrorPrestrikeStatistical StudiesStatistical PerformanceTransformer ModelEffect of CapacitanceState of the ArtDelayed PerformanceThree Phase PerformanceRapid Closing PerformanceConclusions

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