Detection of cross-country faults in medium voltage distribution ring lines

The paper deals with the protective relaying of MV rings recently introduced by e-distribuzione, the largest Italian Distribution System Operator (DSO). The paper refers to an existing MV distribution system which includes a loop line. Attention is focused on Cross-Country Faults (CCFs), i.e. two Line-to-Ground Faults simultaneously occurring in two different phases at different locations. A large number of detailed ATP-EMTP simulations of CCFs are carried out, evidencing a few critical CCF combinations not detected by the presently adopted protective relays. Results point out that such maloperations only occur in mixed overhead-cable rings, while all-cable loop lines are not affected. Lastly, it is also shown that CCF detection is not a problem in mixed overhead-cable rings if the overhead lines are equipped with a ground conductor. The above results could be taken as guidelines for new ring lines.