Characteristics of trapped charge distribution in LDPE and XLPE used in DC cables based on isothermal surface potential decay method

Space charge accumulation is one of the major factors threatening the safe operation of DC cables. Quantitatively characterizing the trapped charge distribution and understanding the root causes of the traps within polyethylene materials used in DC cables are of great significance to suppress space charge accumulation and secure DC cable operation. Therefore, we charged low-density polyethylene (LDPE) and crosslinked polyethylene (XLPE) films using the DC corona charging method to simulate the injection process of space charges. Taking the de-trapping process of trapped charges in polymeric materials into consideration,we proposed a model based on isothermal surface potential decay (ISPD). The distribution of trapped charges was obtained by analyzing the ISPD data of the LDPE and XLPE films, and then the effects of morphological characteristics of LDPE and XLPE and the effects of additives on the formation of traps were analyzed. The results show that there are two trap centers, i.e. a shallow trap center and a deep trap center for each film. In the samples, the density of hole-type deep trapped charges is higher than that of shallow ones, i.e. the hole-type trapped charges are mainly deep traps for both LDPE and XLPE. The quantity difference between shallow and deep hole-type charges is smaller in XLPE and the density of shallow trapped hole-type charges in XLPE is higher than that in LDPE. The crosslinking byproducts have an important influence on the formation of internal traps in XLPE, and the depth of the traps formed by additives is not exactly the same.