Water ingress in high-voltage cross-linked polyethylene (XLPE) cable terminations

In Norway, there are 6000 to 7000 oil-filled high-voltage cable terminations in the transmission network, most of which are constructed following the old design shown in Figure 1. Although the number of reported failures of medium-voltage cross-linked polyethylene (XLPE) cables and cable accessories is low, during the last few decades some severe explosive failures with potential for personnel injury have been reported [1]. Examination of some of the failed cable terminations showed that the most likely cause was the presence of water in the housing. Water can enter the housing through defective O-rings or through an air ventilation screw on the top cover of the termination housing. As part of their regular maintenance programs, some Norwegian utilities measure the water content of the filling oil. Liquid water was found in some installations, and in one case, the stress cone was found to be covered in ice when the porcelain housing was removed [2] (Figure 2). Similar breakdowns have been reported in other European countries [3], [4]. The main purpose of the work described in this article was to investigate the effect of water ingress in oil-filled high-voltage cable terminations, in particular the associated breakdown mechanisms. The work was performed using full-size and model cable terminations.