Space Charge Evolution in XLPE with Long-term Aging under DC Voltage - the Effect of Temperature and Polarity Reversals

DC field long-term breakdown characteristics of commercially available cable-grade cross-linked polyethylene (XLPE) and tree retardant XLPE (TR-XLPE) were investigated. At room temperature, under a unipolar field of 50 kV/mm, more than 90% of XLPE specimens survived 52, 000 h (~6 yrs) of aging, whereas all TR-XLPE specimens broke down within less than 1, 200 h. Reversing the polarity increased the life-time of TR-XLPE by more than 20 times but reduced significantly the life-time of the XLPE specimens. At 75 degC and with polarity reversals both materials survived almost 21, 000 h without any breakdown. These results are discussed in terms of the space charge accumulation and retention properties of the two materials with emphasis on the role of additives in determining insulation properties under DC fields.