Improvement of SAIDI and SAIFI reliability indices using a shunt circuit-breaker in ungrounded MV networks

This document has been created after the report published at CIRED in 2005, 2009 and 2011 (Shunt Circuit-Breaker in Ungrounded Medium Voltage Systems - ref. 0028, 0875 and 0182), ref.[1]. Established in the cantons of Fribourg, Neuchatel and Vaud, the Swiss company Groupe E currently operates its medium voltage networks in ungrounded mode. To eliminate insulation faults, in the case of single lineto-earth fault, we use shunt circuit-breaker systems with unipolar closure connected to the MV busbars of our substations. This system facilitates fault elimination without the necessity to open the faulty line. As for any extinction or current limitation systems, Petersen coil or earth resistance, the power quality is affected. Initially we had favoured the operation of our networks without interruption, acting solely to eliminate fugitive ground faults, occurring in our aerial networks. After this stage and with an increased rate of installation of the arc suppression systems (shunt circuit-breaker), we wanted to extend this philosophy across our networks, including underground systems, with the aim of maintaining operational continuity in the event of permanent ground faults According to our estimations, this extension of the basic application must improve the quality of our supply. An improvement can be seen from analyses of the events recorded over five years and the calculation of the IEEE continuity indices, ref.[2]. However we also envisage some operating constraints. The changeover to the new operating method cannot occur without a review of the protection settings. It will also be necessary for certain installations to be adapted including the operating and intervention processes during the course of the fault clearance work installations. The purpose of this study is to verify these suppositions.