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

Abstract

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

An Equivalent Circuit for the Evaluation of Cross-Country Fault Currents in Medium Voltage (MV) Distribution Networks

“…The CCFs may occur both in two phases of the same feeder and in two different feeders, as described in Figure 1. It should be pointed out that the correct selection of CCFs in MV networks operated with nonsolidly grounded neutral is not always straightforward, especially in case of non-radial network topologies (e.g., closed loop) [2,3]. CCFs occur much less frequently in HV and EHV networks and they may affect the performance of distance relaying schemes; however, there are some research papers addressed in the identification and elimination of CCFs in HV and EHV networks [4][5][6][7][8][9][10][11].…”

“…In [2], a simple equivalent circuit and formulas are proposed to evaluate CCFs occurring in overhead lines of an MV distribution network. In [14], a comparison between the symmetrical components method presented in [12] and the three-phase network solution is reported while considering low voltage distribution networks and showing a full agreement between the two solutions.…”

Evaluation of the limitations of a permissive overreach protection scheme in a distribution loop in case of cross country faults and proposal of alternative solutions

“…As part of the POI‐P4 Operating Interregional Plan project, financed by the European Commission, which studies medium‐voltage (MV) networks operated in loop configuration (both loop line ends connected to the same MV substation), a loop permissive overreach blocking logic protection scheme based on directional overcurrent protection relays has been proposed: MV/low‐voltage (LV) substations along the loop are equipped with circuit breakers supervised by digital protection relays (the so‐called RGDM) which are interconnected by a fibre‐optic network; the fault direction detection is based on the zero sequence polarisation method [1]. The largest Italian Distribution System Operator, e‐distribuzione, is currently testing five MV loops configured with this protection scheme.…”

“…The largest Italian Distribution System Operator, e‐distribuzione, is currently testing five MV loops configured with this protection scheme. The behaviour of the protection system of the Taurianova substation MV loop has been studied in [1, 2]. In [1] it was observed that the position of cross country faults (CCFs) were being incorrectly determined.…”

“…The behaviour of the protection system of the Taurianova substation MV loop has been studied in [1, 2]. In [1] it was observed that the position of cross country faults (CCFs) were being incorrectly determined. This finding was confirmed by Borgnino and Castillo [2] which additionally found that the problem can arise for multiple fault combinations in the case of high impedance (> 10 Ω) faults.…”