Hitachi Relion 670 Series Applications Manual page 282

Section 12
Control
• Switching transients
• Out-of-phase synchronization of a generator
• Clearing of an external fault
When saturated, a transformer absorbs a magnetization current, also known as inrush current, which
can reach several times the nominal current of the transformer. Energization of a transformer from
the network by uncontrolled/random closing by a switching device produces inrush currents since the
flux in the core can reach a maximum theoretical value of 2 to 3 times the rated peak flux. This can
have undesirable consequences.
The flux-linkage/current relation is non-linear, as shown in Figure 135, and it is determined by the
saturation curve of a transformer. Therefore, the magnetization current of the transformer contains
harmonics.
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 135.
IEC15000177 V1 EN-US
Figure 135:
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
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
276
Time
Voltage
Flux
Saturation Curve
Current when Voltage
Zero switching
Current when Voltage
Peak Switching
Flux-linkage/current relation
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