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

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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
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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

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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

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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.
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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
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