As one of the most important equipment in HVDC transmission system

the insulation state of converter transformer plays a vital role in the safe and stable operation of HVDC transmission system. The different positions of winding of its valve side need withstanding the combined voltage of different proportion of AC-DC components.However

the existing study mainly focuses on the ratio of 1:1 composite voltage and the study for other voltage ratio is less. In this paper

the oil-pressboard insulated partial discharge experimental platform under the composite volta

ge is set up on the basis of pulse current method and constant voltage method

the initial partial discharge voltage

spectrum characteristics and typical discharge characteristics of needle-plate electrode with oil gap model and needle-plate electrode surface with strong/weak vertical component model are studied at 1:1

1:3

1:5 and 1:7 AC-DC combined voltages ratio. The results show that the initial discharge voltage of the model with strong vertical component along the surface is obviously lower than that of the model with weak vertical component. The distribution range of partial discharge of PRPD is wide and most of the discharge is located around the rising edge and peak of AC voltage in positive half cycle. The Q

ave

、F

nc

and F

eqave

of different models of needle plate electrode models have the increase and decrease tread with the increase of DC component.