Space Charge Behavior of Oil-paper Insulation in Transformer Under AC Fields

The relationship of space charge behavior and aging degree of oil-paper insulation is investigated in this paper. The insulation paper was obtained from different windings of a decommissioned transformers with three different voltage levels, i.e. 10 kV, 35 kV and 110 kV. The windings were coaxial placed, where the 10kV level winding was innermost and the 110 kV level winding was outermost. The insulation papers were vacuum dried to eliminate the influence of aging oil and then impregnated into the unaged oil. The space charge behavior under 5 kV/ mm AC field was detected by a detection system based on the automatic equal phase shift (AEPS) principle. It shows that the space charge polarity inside the sample is consistent with the polarity of the semi-conductive electrode under AC field. The phase corresponding to the charge density peak lag the voltage phase by an angle. This hysteresis phenomenon is most obvious in the middle layer insulation, and the outer layer is the weakest. It seems that the positive charges are more difficult to detrap than negative charges after the voltage polarity reversion. Compared with heat dissipation condition of the outer layer insulation, the inner layer and middle layer have poor heat dissipation. More traps may be generated during thermal aging due to high temperature for inner layer and middle layer. More injected charge may be trapped with the increase of trap density, leading to the charge hysteresis phenomenon. Thus, thermal stress may have a stronger aging effect on oil-paper insulation and more attention should be paid to heat dissipation in transformer design.