Cross-country fault protection in ENEL Distribuzione's experimental MV loop lines

In the last years, high penetration of distributed generation (DG) has strongly affected public MV networks. In order to improve their reliability and availability, loop operation is being tested. This solution requires the installation of new line protection devices and the identification of appropriate settings against phase-to-phase and ground faults. This study refers to an experimental loop line operated by ENEL Distribuzione, the largest Italian Distribution System Operator (DSO). The paper deals with the behavior of the currently installed line protection during cross-country faults (CCFs), occurring both in radial and loop lines. MV network response has been preliminary evaluated by approximated formula (which allow a fast check of line protection performances), and then verified by accurate simulations based on a detailed Simulink network model. CCFs occurring inside and outside the loop line have been analyzed. Obtained results show that the zero-sequence and phase directional overcurrent (Blocking-Scheme supported) are generally able to selectively identify and clear CCFs. Nevertheless, if CCFs occur in the same overhead stretch of the loop line, the adopted protection scheme may fail.