A. Introduction
The innovation behind the prefluxing inrush current reduction
strategy lies in the prefluxing device itself. The prefluxing
device capacitor is charged to a user-specified voltage and then
discharged into the transformer when closing the device switch,
establishing the desired flux polarity.
It is necessary for the prefluxing device to set the residual flux
of a transformer as high as possible to minimize the inrush current,
but also to do so efficiently. Two flux-current curves
for the laboratory transformer operating at the open circuit are
shown in Fig. 3.
The curve on the left is with rated voltage applied to the transformer,
and the curve on the right is with slightly above-rated
voltage applied to the transformer (overexcited). Notice that
when it is overexcited, the transformer operates further in the
saturated region of the iron. The rated peak flux is labeled ,
and the maximum residual flux is labeled . (This is the maximum
residual flux achievable in the transformer core).
In each curve, the shaded regions indicate the energy transferred
to the magnetic field during a half cycle of operation. The
lightly shaded area is proportional to the energy that supports
the hysteresis and eddy current effects of the transformer core; a
portion of this energy establishes the value of residual flux. The
darker shaded area is proportional to the energy that the magnetic
field gives back to the circuit. Comparing the shaded regions
between the two plots, it is evident that a larger percentage
of the energy is returned to the circuit in the overexcited transformer
and is less efficient compared to the transformer under
rated operation.
Ultimately, the criteria of the prefluxing device, maximal
flux setting, and efficient operation, are satisfied by storing the
proper amount of energy in the device’s capacitor depending
on the size of the transformer it is applied to. The two design
variables used to control the amount of energy stored by the
prefluxing device are the device capacitance and the initial
capacitor voltage (refer to Fig. 2). The Volt-Second