Evaluation of surge-transferred overvoltages in distribution transformers

The paper presents an analysis of very fast-transient overvoltages that occur because of the capacitive surge transfer from the high-voltage (HV) transformer winding to the low-voltage (LV) transformer winding.The study is done on a 6.6 kV single-phase test transformer. By applying a pulse with a short rise time at the HV terminal, the voltage at the LV side is measured and calculated. The voltage distribution along the LV winding is calculated by applying the transmission line theory, so that the foils of the LV winding are represented as transmission lines. For the studied transformer with a nominal transformer ratio of 95.6, a maximum voltage ratio of 3.3 was measured during the rate of rise of the applied impulse. The present paper also shows that in some specific cases, the mutual inductance between the primary and the secondary side can be ignored during determination of the transients along the secondary winding. The computation and measurement of the voltage at the LV side is validated by making use of the vector fitting method.