Mechanism of space charge distribution in XLPE used in HVDC cables under temperature gradient

It is reported that temperature gradient can enhance hetero-charge accumulation at the low temperature end of polyethylene, while at the high temperature side only a small quantity of homo-charge accumulates. To understand this observation, we discussed the mechanism of how the temperature gradient influences hetero-charge formation based on two theories, respectively, namely the ionization of impurities in insulation and the blocked extraction of injected carriers, and found that temperature gradient across the sample thickness would influence the space charge distribution through influencing carrier mobility, and temperature difference between two electrodes through charge injection and extraction at the electrode ends. In order to verify this theory, we measured the space charge distribution in samples of different thicknesses under different temperatures. The results show that, with the same temperature difference but different temperature gradients, more hetero-charge accumulates in thin samples, however, it more accumulates in thick samples with the same temperature gradient but different temperature differences. It is concluded that both temperature gradient and the temperature difference between electrodes accelerate the hetero-charge accumulation at the lower temperature end.