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REDUCTION OF TRANSFORMER INRUSH CURRENT BY CONTROLLED SWITCHING METHOD

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					  REDUCTION OF TRANSFORMER
INRUSH CURRENT BY CONTROLLED
      SWITCHING METHOD




                               1
               CONTENTS
• Introduction
• Why is the flux asymmetrical when a
  Transformer is energized ?
• How does the asymmetrical flux lead the
  Transformer to operate in saturation region ?
• Why does the Transformer operating in
  saturation region draw heavy inrush current ?
• Controlled switching of Transformers
• Conclusion
• Reference
                                              2
                Introduction
• On energisation, the transformer takes heavy
  current for its magnetization, which flows in the
  primary for 5 to 10 cycles.
• It sags the system voltage causes the production of
  harmonics and leads the power system to the
  instability.
• Controlled transformer switching can eliminate
  the inrush current.


                                                    3
Relation between voltage and flux -
continuously-operating Transformer

e = dφ/dt
φ=e dt




                                      4
 Relation between voltage and flux –
Transformer energized at voltage peak




                                        5
 Relation between voltage and flux –
Transformer energized at voltage zero




                                        6
                 Doubling effect

• Since the flux can not instantaneously rise to its
  peak value, it starts from zero and reaches 1p.u.
  after ¼ cycle and continues to to increase until it
  becomes 2p.u. ½ cycle after switching.

causes the formation of asymmetrical flux



                                                        7
How asymmetrical flux leads Transformer to
operate in saturated region?
a)symmetrical flux b)asymmetrical flux




                                         8
Why Transformer operated in saturation
    region causes inrush current?




                                         9
Controlled switching of 1-phase Transformer
             with residual flux




                                        10
Prospective and dynamic core fluxes




                                      11
    Simulation Conditions without
      considering residual flux

Transformer       Simulation Conditions
   Model
  250MVA,      Simultaneous     Sequential
400kV/110kV       Closing        Closing
    Y-Δ
              Without   With   Without With
                Rn      Rn       Rn    Rn
   Simulation circuit of
Sequential Closing Method




                            13
                  Inrush Currents in 3 Phases in
                     simultaneous switching
Current in Amps




                             Time in sec
                                                   14
                  Simulated output of Sequential Switching
Current in Amps




                                    Time in sec
                                                         15
Simulation circuit with Neutral
    Resistance Method




                                  16
                   Reduced inrush current with
                  external resistance connected
Current in Amps




                            Time in sec
                                                  17
    Optimum value of Neutral
          Resistance
 Neutral        Ia     Ib      Ic
Resistor(Ω)   (kA)   (kA)    (kA)
   100        1.10   0.17    0.16
   200        0.85   0.14    0.18
   400        0.60   0.11    0.20
   600        0.45    0.12   0.24
   800        0.35   0.135   0.26
   900        0.32    0.14   0.27
  1000       0.30   0.145    0.3
   1500       0.24    0.15   0.36
   2000       0.18   0.155   0.40
Effect of delay time between each phase
energization on inrush current reduction

Time(s)   Current(A)   Current(A)   Current(A)
           I phase      II phase     III phase
 0.02        300          150          300

  0.1        300          150          300

  0.2        300          150          300

  0.5        300          150          300

  1.0        300          150          300
  Control Strategies in 3 Phase
          Transformers
1. Rapid Closing Strategy
2. Simultaneous Closing Strategy
3. Delayed Closing Strategy




                                   20
     Core Flux Equalization
• The phenomenon that flux in phase B
  and C rapidly equalizes eliminate the
  effect of residual flux.




                                      21
Controlled switching of 3- phase
 Transformer with residual flux




                                   22
Prospective and dynamic core fluxes




                                  23
              Simulation Conditions with
                Effect of Residual Flux
                                  Simulation Conditions
Transformer                                                Simultaneous
    Model       Rapid Closing       Delayed Closing
                                                               Closing
                    Strategy             Strategy
                                                              Strategy
                                                          Phase A, B & C
               Phase A closed     Phase A closed at t
                                                               closed
                    at t = 0               =0
                                                           simultaneously
 250 MVA,
               Residual Flux at     Not considering       Residual Flux at
400kV/110kV
                     70%                  directly              70%
  Y-
                Phase B & C                                  Switching
                                   B & C closed after
                    closed at                                     time
                                      5 cycles (0.1s)
                 t = 0.1414872                              t = 0.1414872
                   Simulated Output by
                  Rapid Closing Strategy
Current in Amps




                          Time in sec
                    Simulated Output by
                  Delayed Closing Strategy
Current in Amps




                           Time in sec
                      Simulated Output by
                  Simultaneous Closing Strategy
Current in Amps




                             Time in sec
 Summary of Simulated Output
                           Current Current Current
                             in       in      in
Type of closing strategy   Phase A Phase B Phase C
                           (Amps) (Amps) (Amps)

1     Rapid Closing         1495     233     233

2    Delayed Closing        1495     520     300

3 Simultaneous Closing      143      153     460
                Conclusion
• Closing each winding, when the prospective
  and dynamic core fluxes are equal, results in
  an optimal energization, without core
  saturation or inrush currents.
• It is possible by considering residual flux
  together with the appropriate closing strategy
  to eliminate transformer inrush current in
  most transformer configurations.
• By simultaneous closing strategy the
  transformer inrush current can be reduced.
• Stability of the power system can be
  improved.                                    29
                        Reference
[1] Mukesh Nagpal, Terrence G. Martinich, Ali Moshref, Kip
    Morison and P.Kundur, “Assessing and Limiting of
    Transformer Inrush current on power quality” IEEE
    Trans.Power Del., vol.21, no.2, pp. 890-896, Apr.2006.
[2] J.H.Brunke and K.J.Frohlich, “Elimination of transformer
    inrush currents by controlled switching-Part I :Theoretical
    considerations” IEEE Trans. Power Del., vol.16, no.2, pp.
    276-280, Apr.2001.
[3] J.H.Brunke and K.J.Frohlich, “Elimination of transformer
    inrush currents by controlled switching - Part II:
    Application and performance considerations” IEEE
    Trans.Power Del., vol.16, no.2, pp. 28-285, Apr.2001.
                                                          30
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DOCUMENT INFO
Description: REDUCTION OF TRANSFORMER INRUSH CURRENT BY CONTROLLED SWITCHING METHOD On energisation, the transformer takes heavy current for its magnetization, which flows in the primary for 5 to 10 cycles. It sags the system voltage causes the production of harmonics and leads the power system to the instability. Controlled transformer switching can eliminate the inrush current