Turn-to-turn 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 propagating into the transformer winding may cause resonance and excite high potential gradient, consequently leading to the breakdown of the winding insulation. In this paper, two electrode models are established to simulate the typical insulation configuration of windings in oil-immersed transformers. The influence of front time and oscillation frequency of impulse voltages on the breakdown characteristics are studied. The experimental results indicate that the breakdown strength increases with the decrease of front time. For the turn-to-turn insulation, its time lag is greatly influenced by the sickness of insulation layer and front time. Moreover, the sensitivity of time lag to the front time enhances with the increase of number of insulation layer. The turn to turn insulation can withstand a much more severe oscillation lightning impulse in comparison to standard lightning impulse. In addition, its 50% breakdown voltage increases with the increase of the oscillation frequency. Furthermore, while the total insulation paper thickness of turn-to-turn remains the same, the breakdown strength is reinforced as the number of insulation layers increasing.