The interface charge characteristics of oil-pressboard composite insulation and its impact on surface flashover under combined AC/DC voltages

The interface charges of oil-pressboard insulation could distort the local electric field and even weaken the insulating strength of converter transformer. This paper employs the electrostatic capacitive probe to capture the accumulation characteristics of interface charges in oil-pressboard insulation with needle-plate electrode under AC, DC and combined AC/DC voltages. The research results indicate that, under AC voltage, the polarity of interface charge is the same to that of the instantaneous value of applied voltage, and density of interface charge remains constant with the AC voltage application prolonging. In the context of DC voltage, the density of negative interface charge is about 1.2-1.5 times of that of the positive one. Under AC/DC combined voltage, the time-dependent dynamic accumulating process of interface charge behaves similarly to the waveform of AC voltage, and the larger the amplitude of DC component in combined voltage, the weaker the fluctuation. The tests for the surface flashover of oil-pressboard insulation were also conducted, aiming at discovering the impacting mechanism of interface charge accumulation on flashover voltage. The surface flashover voltage of test model under negative DC superimposed AC voltage is 1.3 times higher than that of positive DC combined with AC voltage. The built interface polarization model made an explanation for the interface charge accumulation, and the impacting mechanism of interface charge on flashover voltage, namely the positive effect of homopolar interface charges, is also proposed.