Effect of additives on the performance of cross-linked polyethylene subjected to long term single and periodically reversed polarity DC voltage

The performance of cross-linked polyethylene with and without tree-retarding additives, aged with single and reversed polarity DC voltage was investigated. Under single polarity field of a 50 kV/mm, cross-linked polyethylene without the tree retardant (XLPE) showed significantly longer time to breakdown than the material with a tree retardant (TR-XLPE). This is attributed to the difference in how the two materials accumulate and retain space charge. In the case of polarity reversals, TR-XLPE showed better performance than XLPE. This seeming contradiction was studied by measuring the dynamics of space charge evolution with time. The Thermal Step Method (TSM) of space charge measurements was used. It showed that after the polarity reversal the TSM current rapidly inversed in the TR-XLPE but not in the XLPE. This implies more rapid charge dissipation in TR-XLPE. The retained charges cause the electric field enhancement in the material when the polarity is reversed. Thus, after each polarity reversal XLPE was subjected to higher local electric stress for a longer time and thus broke down sooner than TR-XLPE. For both materials the space charge decreased faster when the polarity was changed from negative to positive than vice versa. This could be explained by more efficient electron than holes injection. Since the control of the power flow in DC networks requires reversals of voltage polarity, their detrimental effect on XLPE-insulated equipment, such as cables, cannot be avoided. The effect could be mitigated by the application of carefully designed additives. However, as the present study indicates, additives that improve insulation performance under polarity reversals can be harmful under single polarity voltage.