New interpretation of long-term test results of MV cables. Material ageing or internal defect structure development

XLPE medium voltage cables aged in the long-term test for about 2 years were investigated by different structural characterization methods. The main focus was aimed at medium voltage cables with a low residual breakdown strength between 7U0 and 14U0. The formation of defects in the interface XLPE insulation and the inner semiconductive shield reduced the residual breakdown strength of the cable system remarkable already after 3 month. This formation and growing of vented trees in MV cable without protrusions in the interface was investigated by light microscopy, scanning and transmission, electron microscopy (SEM, TEM) and FTIR microscopy. The starting points of the trees were found inside the semiconductive layer. The defects are able to attack the XLPE insulation, if the reach the interface between insulation and semiconductive layer.