Limitation of cross country fault currents in MV distribution networks by current limiting reactors connected between cable screens and primary substation earth electrode

Abstract

MV distribution networks with ungrounded or grounded neutral by compensating impedance are exposed to Cross-Country-Faults (CCFs). If CCFs occur in cable lines, fault current value can reach that of line-to-line fault current. This is because the large part of the fault current flows through cable screens and only a small amount through grounding systems of secondary and primary substations. Cable screens in the joints can reach very high local temperatures where screen and copper stocking connecting the sectioned screens are not pressed enough each other. In these small areas high electric contact resistances cause large joule losses, potentially leading to insulation burning. Successive temporary/switching over voltages can cause dielectric failure, i.e. faults to ground. A way to prevent the screen insulation thermal damage could be a drastic reduction of CCF currents, especially when they occur in different cable feeders of the MV network. The paper illustrates a countermeasure able to reduce CCF currents: the use of current limiting reactors (CLRs) connecting the cable screens to the earth electrode of the primary substation. The influence of the CLRs on the equivalent earth impedances of primary and secondary substations is investigated. The methodology is applied to an existing MV distribution network.