Space charge dynamic properties of oil-paper insulation during aging process at pulsating DC voltage

Valve side windings of the converter transformer withstand pulsating DC voltage. Space charges accumulate in oil-paper during HVDC converter transformers operation, which is often responsible of insulation failure. To clarify the dynamic properties of space charge, this paper processed the oil-paper with the electrical-thermal aging test at 90 °C and7.6 kV pulsating DC and AC voltage respectively. The behaviors of space charges in aged oil-paper samples were investigated by pulsed electro-acoustic (PEA) technique. It is concluded that the DC component of pulsating DC voltage facilitates the negative charges injection from the cathode, which leads to the accumulation of hetero-charges near anode. Moreover, deep traps excited by the DC component of the pulsating DC aging voltage slightly outnumber by the AC aging voltage, and also the probability of space charges falling into deep traps is bigger when the oil-paper suffers the pulsating DC voltage.