Reliability calculation of MV-distribution networks with regard to ageing in XLPE-insulated cables

Failure statistics of German utilities show that more than 80% of the supply interruptions of medium voltage (MV) customers are caused by component outages located in the MV network itself. For these networks a model as near to reality as possible is required to correctly determine customers' quality of supply. One aspect so far neglected in reliability calculations of MV networks is that the failure rate of most components is not constant for the whole lifetime, but rises over the years according to the increasing right wing of the well-known bathtub curve. Due to the water-treeing phenomenon, XLPE-insulated MV cables manufactured during the 1970s show heavy ageing problems after some years of operation, resulting in a strongly increasing failure rate. An approach is presented to take these increasing failure rates into account in reliability calculations of MV networks. The cable-failure models used in the investigation are described in detail as well as the data received from a special evaluation of the VDEW-statistics of XLPE-insulated MV cables reflecting the ageing phenomenon. Several calculations of an actual 10 kV cable network supplying parts of a German city are presented to show the impact of increasing cable failure rates on the reliability of MV customers' power supply.