I. INTRODUCTION
Converter transformer is a key point of hub device in
HVDC transmission system and operation of power system.
Since oil-paper insulation is the main insulation form in the
converter transformer, its insulating properties directly affect
the system’s safe operation [1,2]. In the HVDC transmission
system, converter valve will produce pulse voltage that has
high amplitude and steep wave. This pulse voltage
superimposed on the winding voltage will lead that the voltage
withstood by the valve side winding on converter transformer
is a pulsating voltage similar to sinusoidal waveform. And this
voltage is created by DC voltage and seriously distorted AC
voltage [3,4].
A large number of researches about insulating properties of
oil-paper insulation under pure AC, DC has been carried out at
home and abroad. However, the research about insulating
properties of the oil-paper insulation under pulsating voltage,
composed by DC component and AC component, is relatively
few. S. Grzybowski firstly researched the breakdown
characteristics of oilpaper insulation under pulsating
voltage[5], obtain the result that the dependence of the AC