The oil-paper insulation breakdown characteristics under non-standard lightning impulse voltages

The non-standard lightning impulse voltages have variable front time, duration time and may have high-frequency components superimposed on the waveforms due to the reflection in the substation and other factors. These surges can propagate to the transformer winding and may even lead to the breakdown of the winding insulation. To maintain the high reliability in insulation performance of the oil-immersed transformer and offer reference to the design of the insulation in the transformers, it is necessary to obtain the oil-paper insulation characteristics under non-standard lightning impulse voltages (non-standard LIWs). In this paper, two electrode models are established to simulate the insulation configuration of windings in oil-immersed transformers. The experiments were carried out on the breakdown characteristics of the electrode model in oil. The breakdown voltages of oil-paper insulation for different front time of impulses, oil-gap distance and insulation structure were measured and then compared with the breakdown voltages under standard lightning impulse waveform (SLIW). In the examined range, the dielectric breakdown values under steep front non-standard lightning impulse waveforms were much higher than those under standard lightning impulse waveforms in all cases. Breakdown voltage of turn-to-turn model under oscillating lightning impulse waveform (OLIW) is much higher than that under SLIW. The experiment results clearly showed that the same insulation structure could withstand higher steep front non-standard LIWs and OLIWs, and these findings may offer a reference to clarify the insulation margin between non-standard LIWs or OLIWs and SLIWs.