Effect of oscillation frequency on the breakdown characteristics of transformer oil in a highly nonuniform field under lightning impulse voltage

This study presents the influence of the waveform oscillation frequency on the breakdown of transformer oil in a highly nonuniform field under the lightning impulse voltage waveform. Four oscillating frequency waveforms, as per IEC 60060-3 standard, are applied and the standard lightning impulse (SLI) waveform is adopted as the test reference waveform. The experimental results show that breakdown under the oscillating lightning impulse (OLI) always occurs at the peak of the oscillation cycle, the voltage then rapidly decays to zero, followed by a small recovery with oscillation occurs when upon a breakdown. The breakdown voltages under the OLI waveforms are higher than those under the SLI waveform. Moreover, breakdown voltage rises with the increasing oscillating frequency. The analysis of breakdown distribution points and voltage time characteristics curves show that discharge time lag for OLI is longer than that for SLI and tends to decrease as the frequency increases. Weibull breakdown probability curves show that agglomeration occurs when the voltage reaches a high value. The breakdown mechanism is analyzed and the statistical time lags under different impulse voltages are compared. The results of this study can provide an important support for the field lightning impulse test of oil-immersed power transformers.