Calculation and experiment on transformer reactance in relation to transients of restriking voltage

In this paper it is shown that the effective leakage inductance of transformers, which in many cases determines transients of restriking voltage, cannot be regarded as a constant, but varies very considerably with time from the start of the transient and with the frequency of the transient. In extreme cases the effect of this phenomenon may be to reduce the rate of rise to 70 %, and the first peak of restriking voltage to 50 %, of the corresponding values calculated using the power-frequency leakage inductance of the transformer. It is shown that the laws governing these variations can be calculated with reasonable accuracy from, the dimensions of the transformer. In illustration of the importance of this phenomenon in practical cases, the paper describes short-circuit site tests for the measurement of transients of restriking voltage at the medium-voltage busbars of a large substation. The work is, however, equally applicable to higher-voltage busbars.