When a transformer is energized, the initial value of the flux may differ from the
prospective flux. This causes a DC offset of the flux-linkage and a higher
-than-
rated peak value. The result is an inrush current that may be several times the value
of the nominal current. Due to the low relative permeability of the ferromagnetic
material in saturation, a marginal increase in the peak of the flux-linkage results in
a magnification of the inrush current as shown in Figure
143.
Time
Voltage & Flux
Time
Current
Voltage
Flux
Current when Voltage
Zero switching
Current when Voltage
Peak Switching
Saturation Curve
IEC15000177.vsd
IEC15000177 V1 EN-US
Figure 143: Flux-linkage/current relation
The transformer energization control function (XENCPOW) is used to minimize
inrush currents by energizing the transformer near the voltage peak. Energizing the
transformer at a voltage zero crossing results in the most severe inrush current.
Energizing the transformer at voltage peak results in no DC offset other than that
caused by the initial residual flux.
12.12.3 Setting guidelines
GUID-CC37700A-5AA0-4A71-8073-0D63C79BBDFE v1
The controlled close operation of a transformer from the XENCPOW function is
based on the voltage waveform and circuit breaker operating time. The voltage
waveform is traced using the input signal fed to the function, and the breaker
operate time is taken from the setting. Incorrect operation may occur if the
tBreaker setting is set incorrectly. Hence the operation accuracy of the XENCPOW
function depends strongly on the accuracy of the breaker operate time setting.
1MRK 506 375-UEN A Section 12
Control
Railway application RER670 2.2 IEC 341
Application manual
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