Wavefront dependent dielectric strength of transformer oil under double-exponential impulse voltage

Lightning impulse withstand test is extremely important to the insulation examination of oil-immersed equipment, but the increase of the entrance capacitance of ultrahigh voltage equipment usually makes the front time T
₁
of test impulses exceed the upper limit regulated by IEC 60060-1. In order to evaluate the equivalency between the withstand tests under standard lightning impulses and impulses with longer front time (T
₁
<; 6 μs), as well as to explore the effect of wavefront on dielectric strength of transformer oil, the breakdown characteristics of transformer oil under impulses with different front time in quasi-uniform and non-uniform field were studied. The results show that, in quasi-uniform field, the 50% breakdown voltages U
₅₀
% are almost independent of the front time, but the minimum breakdown voltages slightly increase as the front time increases. In a highly inhomogeneous field, the front time has remarkable effects on the 50% breakdown voltages under positive impulses, contrary to the weak influences under negative impulses. Light signals and shadow images of streamers were recorded to analyze the influence of wavefront on the initiation and propagation characteristics. It is found that an increase in front time would increase the initiation voltages of both positive and negative streamers, but their shapes do not change much as front time varies. The dependence of light signals of streamers on front time was only observed under positive impulses. The mechanism underlying the differences between the front time related breakdown characteristics of transformer oil under positive and negative impulses is discussed, as well as the waveform equivalency for impulse withstand tests of oil-immersed equipment.