This paper describes a hitherto unstudied malfunction that can occur in the protection system of distribution networks (DNs) in the presence of large photovoltaic plants (LPVPs). The focus here is on the phasebalance current (PBC) protection of overhead or blended feeders back-fed by LPVPs. This study used the DNs belonging to ENDESA Distribución Eléctrica S.L.U. In the ENDESA utility company, PBC protection (ANSI 46) is an obligatory DN protection practice for overhead or blended feeder types. Low power quality or two ferroresonant states were found to be the causes of the malfunction of PBC protection, which led to its untimely tripping.The results of this study highlight the need to modify current DN protection practices and to elaborate new power quality requirements for PV-distributed generation (PV-DG). Since PV-DG penetration in DNs is steadily increasing, regulators and DN operators should implement more effective practices and guidelines to offer better power quality to customers. because of low power quality and ferroresonant states. This research has been carried out in DNs belonging to ENDESA Distribución Eléctrica S.L.U., a large utility company with long experience in the connection of LPVPs to DNs. In ENDESA, all overhead/blended feeders have phase-balance current (PBC) protection. Our study focuses on the untimely tripping of the PBC protection of feeders in certain LPVP operation conditions.Before the causes of this malfunction became known, ENDESA had to contend with an excessive number of untimely trippings in its feeders. It was found that the protection system of these DNs in the presence of PV-DG units malfunctioned because of the following: (i) low power quality of the LPVP current injected into the DN at low output power levels (high harmonic or unbalance level); (ii) sudden output power changes of the LPVP at low output power levels, which led to an unexpected jump to ferroresonant states with high unbalanced currents. These types of malfunction had not previously been observed in any other DG type.The second section of this paper gives an overview of PBC protection in overhead feeders. The third section describes ferroresonance as a contextualization for the special ferroresonance that occurs in an MV unearthed feeder back-fed by an LPVP. It also outlines a unified framework for analysing three-phase systems subjected to unbalance and waveform distortion. This helps to quantitatively determine the extent to which unbalance and distortion components of the three-phase currents cause network protection conflicts. Sections 4-7 present three case studies involving the malfunction of the PBC protection of MV feeders with LPVPs. Problems are discussed, and the possible solutions are presented.
PBC PROTECTION (ANSI 46)2.1. Description of the PBC protection unit PBC protection (negative-sequence protection) detects the unbalance of currents in phases. It measures the positive, negative, and zero sequence of the circulating current (I 1 , I 2 , and I 0 ) (Figure 1). The PBC detector picks up when...
