To make sure whether the standards for evaluating and measuring the deterioration of oil-paper insulation in electric transformer are applicable for converter transformer

an oil-paper insulation partial discharge（PD） process measurement platform was constructed by adopting the step-ladder voltage method.The characteristics of the normal impulse discharge from initial discharge to insulation failure were compared by recording the real-time PD developing processes on plane-plate model under AC

DC and AC-DC combination voltages. Thus the information of the characteristic quantities that can characterize the PD developing processes was extracted. The results show that: 1 the sudden increase in total discharge quantity and discharge number is the external characteristic of insulation deterioration of oil-paper sample;2for different applied voltages

the characteristics of initial discharge

total discharge quantities

discharge repetition rates

discharge times

and corresponding evolutionary relations exhibit apparent differences.