Diagnostic testing of thermally aged medium voltage XLPE cable joints

Current loading of wind farm distribution network is very different from that observed for traditional underground cable systems. Variations in wind power results in a significant number of current load cycles during service. The current can increase from zero to maximum rated load in a short time. This can cause high temperature gradients in the cable systems inducing significant thermo-mechanical forces during heating and cooling, and thermal oxidation at high load. Several premature service failures of cable joints in wind farm distribution networks have occurred, mainly due to the combined action of bad metallic connectors and high load current. Partial discharge measurements are today performed on-site using different frequencies and magnitudes of the test voltage. As the oxidation process causes the conductivity of the bulk and surfaces of the materials involved to strongly increase, this will also likely impact the frequency dependence of the partial discharges. This work includes laboratory high temperature ageing of rubber type heat-shrink MV XLPE cable joints including temperature gradients by application of high load currents. The ageing has been electrically characterised by variable partial discharge measurements in the frequency range of 10 mHz to 100 Hz up to 12 kV (2 U₀). The joint material was examined by microscopy examinations after ageing. The results show that the partial discharges are strongly frequency and voltage dependent. For shorter ageing times of the rubber joints, the number of discharges per voltage cycle decrease to zero for frequencies below 10 Hz. At longer ageing times resulting in severe material ageing, the number of discharges per voltage cycle strongly increases by decreasing frequency. In this case the inception voltage at 0.1 Hz was equal or lower than that measured at higher frequencies. It is observed by SEM that especially the stress-control material was strongly degraded by the high temperature conditions.