To investigate the characteristic of oil-paper insulation under DC transmission voltage

a high voltage frequency-variable test power source is designed and constructed. The typical oil-paper insulation partial discharge models are established according to the discharge in inner air gap of insulating press board

the discharge on surface of insulating press board in oil

and the corona discharge at metal tip in oil. The partial discharge is measured for the same test sample under different frequency voltages to find the frequency and energy of partial discharge increasing with the frequency and amplitude of AC power source.The analysis report indicates that the phase position

amplitude

and frequency of partial discharge are affected by different frequencies

namely

oil-insulation structure holds heavy electrical field while the frequency increases. This study may be helpful in designing insulation structure of converter transformer and smoothing reactor for DC transmission system.