This paper presented an inrush current reduction strategy
which sets the residual flux of a single-phase transformer to a
large magnitude and specific polarity in a method known as
“prefluxing” and then energizes the transformer at a specified
system voltage angle based on the flux polarity. This strategy
has advantages over some of the presently suggested reduction
strategies, including removing the need for residual flux measurements
during transformer de-energization.
The prefluxing device that sets the flux of the transformer is
simple in form and flexible to apply to any range of transformer
sizes. In addition, the device can operate at low-voltage levels,
such as the substation ac or dc supply, regardless of the voltage
rating of the transformer.
The results of energizing a prefluxed transformer were
presented, including accounting for breaker deviation, all with
inrush current levels below the transformer’s rating, proving
the effectiveness of the reduction procedure. These greatly
reduced inrush currents mean reduced stress and degradation
of the transformer windings and reduced system effects, such
as harmonic overvoltages or sympathetic inrush in neighboring
transformers.
Ultimately, the prefluxing inrush reduction technique described
in this paper provides the basis for reducing inrush
current in a three-phase transformer using a three-pole CB,
which the authors have successfully tested in the lab and intend