Use of time limited ratings in distribution systems to improve asset utilization

Distribution networks are facing potentially dramatic increases in loads as new low carbon technologies appear on the network. Many 11kV circuits in the UK and elsewhere are N-1 redundant, leaving large amounts of capacity unused most of the time. This paper describes how time-limited (or emergency) ratings can be applied to underground cables during contingencies and the effect doing this will have on distribution networks. By applying these time-limited ratings it is possible allow more load to be reconnected during fault repairs and therefore more customers to be reliably supplied by existing network assets. These time-limited ratings utilise the thermal inertia inherent in underground cables, by which it is possible to continue to use cable systems beyond 100% firm capacity while maintaining acceptable network performance. Furthermore, this work shows how degrading performance due to increasing loading can be compared to degrading performance due to ageing equipment. This comparison shows that by limiting the maximum age of a cable system, loading beyond firm capacity may be acceptable.