Partial discharge activities of pressboard/oil insulation under AC plus DC voltages

In this paper, focus is made on partial discharge (PD) activity in oil-impregnated pressboard used in converter transformers. Voltages over the main insulation of the valve-side windings of the converter transformer were analyzed using the PSIM software. Next, transformer defects were simulated by insulation models to generate PD signals. The dependence of PD inception voltage (PDIV) on the AC plus DC voltages with and without pre-charging is reported along with PD patterns. Experimental results show that: a) using AC plus DC voltages, PDIV values need pre-charging to be estimated accurately, b) activities can be more energetic than those under sinusoidal AC voltages, although repetition rates are generally lower