Influence of moisture on space charge dynamics in multilayer oil-paper insulation

The main insulation system in HVDC transformer consists of oil-paper insulation. The formation of space charge in oil/paper insulation system can not only affect the dielectric strength, but also relate to the stability and security of HVDC transformer. Moisture has detrimental effect on the electrical breakdown strength and insulation life of oil-paper insulation. Thus a better understanding the influence of moisture on the charge distribution in multi-layer oil-paper insulation system becomes increasingly practical importance. In the present study, the space charge dynamics in three layers oil-paper insulation system with three different moisture concentrations (0.28, 1.32 and 4.96%) were investigated using the PEA technique under 6 kV. The influence of moisture on the charge distribution in multi-layer oil-paper insulation system was provided. Results show that the higher the moisture content of the oil-paper sample, the higher the mobility of the trapped charges, and the more obvious the positive charge injection and negative charge injection were observed. The negative charges can inject across the paper-paper interface near to the cathode when the moisture content increases to 4.96%. The higher moisture content of the oil-paper sample, the faster the dissipation rate of the charges trapped in the sample. Moreover, the higher mobility of the trapped charges leads to less slow moving charges trapped in the sample with higher moisture content.