Breakdown characteristics of converter transformer insulation under composite AC and DC voltage

In HVDC transmission systems, the oil and oil-impregnated pressboard insulation of converter transformers is subjected to composite AC and DC voltage. Therefore, a study on breakdown characteristics of this insulation under AC, DC and composite AC and DC voltage is essential. In this paper, breakdown tests on transformer oil and oil-impregnated pressboard insulation under all kinds of those voltages have been completed and the relationship between breakdown peak voltage and AC ratio is discussed. It is shown that the breakdown field strength of transformer oil decreases with the increases of oil gap under any waveform. Breakdown peak voltage of a given oil gap has little change with the variation of AC ratio, but evident inflection point occurs in their relation curve. Breakdown peak voltage of a given oil-impregnated pressboard decreases remarkably with the increases of AC ratio.