Transient Voltage Distribution Characteristics of Ultrahigh-Voltage Transformer Windings under Impulses with Different Front Time

Lightning impulse withstand voltage test is conducted to examine the dielectric strength of turn-to-turn insulation under the circumstance of uneven voltage distribution, but large entrance capacitance of ultrahigh-voltage transformers makes it difficult to generate standard lightning impulse (SLI) in the on-site test, especially for the front time. In this paper, to evaluate the feasibility of substituting test impulses with longer front time for SLI, the transient voltages of the outmost turn of each disc of the 1000kV transformer windings under impulses with different front time were measured by the coupling capacitive sensor. The results showed that, for the low-voltage winding and the high-voltage winding, whose high voltage leads were located at the top and middle, respectively, the maximum voltage distributions were almost independent of the front time; while for the mid-voltage winding, whose lead was located at the bottom, the maximum voltage distribution changed as the front time varied. For all three windings, the front time of applied impulse had a little effect on the frequency components of the transient voltages, which could significantly influence the dielectric behavior of oil-paper insulation. Based on those results, the equivalence between the SLI and impulse with longer wavefront was discussed and so was the effect of very fast transient overvoltage on transformers.