Calculation of transformer saturated leakage inductance based on field test data

In a black start of a power system, at times, an overvoltage could be caused by core saturation on the energization of a transformer with residual flux. Such an over-voltage might damage some equipment and delay power system restoration. Through an actual field test and EMTP simulations, we have found that such phenomena cannot be accurately simulated using a normal transformer model with inductance data measured during the factory test. This paper proposes a new calculation scheme of transformer inductance by using actual field test data in order to grasp the saturated transformer characteristics accurately. We also show an analytical scheme for calculating the leakage inductances from shunt currents in delta windings that give extensive influence to over-voltage under flux saturation.