Windfarms have underground cables and it is a regular occurrence, mainly at the joints. It has been suspected that as sometimes the cables are running flat out and than not carrying anything at other times the heating and cooling cycles cause the cables to grow and contract and the joints the weakest point. The cables had 3 phases in them and the joints heatshrunk? together. We ended up splitting up the cables for several metres at the joints and mounted each phase a metre or so apart to a frame underground. This seemed to fix the problem as we only had one failure in the last 6 or 7 years I was there. There a transmission lines in Australia that run up to 500kV. Portland Vic heading back towards the coal fired stations. 132, 275kV are the common ones.