(0.89 p.u.) at the 210 energizing instant. This difference in the
two fluxes results in the transformer’s negative flux peak exceeding
the rated 18 mWb peak, which results in the negative
polarity peak. It is worthwhile to note that since the maximum
residual flux achievable in the transformer is 12 mWb (0.67
p.u.), even under perfect prefluxing conditions, the transformer
would still experience some inrush current.
The controlled switch used in the laboratory was very precise,
but this is not necessarily the case with an actual substation CB.
At 60 Hz, a 1-ms deviation in closing time results in approximately
22 of variation. Studies have shown that spring-mechanism
SF breakers typically experience a breaker deviation of
2.5 ms [10]. Fig. 12 plots some of these variations and, as expected,
the variations result in magnitude and polarity changes
of the inrush current. Note that advancing the breaker by 2.5 ms
resulted in roughly the same inrush current as the desired closing
instant because the symmetry of the prospective flux waveform
at these two time instances results in the same prospective flux
value.