Conflicts in the distribution network protection in the presence of large photovoltaic plants: the case of ENDESA J. C. Hernández 1 * ,† , J. De La Cruz 2 , P. G. Vidal 1 and B. Ogayar 1 1 Grupo de Investigación IDEA, EPS, University of Jaén, Campus de las Lagunillas s/n, Edificio A3, Jaén 23071, Spain 2 Endesa Distribución Eléctrica S.L.U., Escudo del Carmen 31, 18009-Granada, Spain SUMMARY 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 phase- balance 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. Copyright © 2012 John Wiley & Sons, Ltd. key words: Photovoltaic-distributed generation; electrical distribution networks; distribution protection; power quality; ferroresonance 1. INTRODUCTION The penetration of PV-distributed generation (DG) in distribution networks (DNs) has increased all over the world. In fact, its growth is now interfering with the normal operation of traditional protection practices in DNs. These DNs were originally planned for passive, radially operated networks rather than for PV-DG units. Conflicts in the DN protection system have been amply studied (DG [1–4] and PV-DG [5–9]). Such protection conflicts mainly involve the following: (i) fault current level modification; (ii) lack of coordination between protective equipment; (iii) ineffective feeder reclosing after a fault; and (iv) undesired islanding after the untimely tripping of interconnection protection. Effective practical methods for solving such conflicts have been proposed for DG [1,10] and particularly for PV-DG [4,6–8,11]. However, somewhat less attention has been paid to network protection conflicts stemming from the presence of PV-DG in DNs, due to other factors such as low power quality or ferroresonant states [1]. The problem of different kinds of electrical resonance in a PV inverter when interfaced with the network has been previously studied. Research on this topic has focused on the following: (i) effective control laws for grid-connected inverters with inductor-capacitor-inductor-filters, which take into account system resonances [12–14]; (ii) system resonance conditions excited by harmonics generated by grid-side power converters when capacitors in power systems interact with system inductances [3,15–22]. Accordingly, this paper describes a network protection conflict that has systematically occurred in more than ten overhead/blended feeders back-fed by large photovoltaic plants (LPVPs) *Correspondence to: J. C. Hernández, Grupo de Investigación IDEA, EPS, University of Jaén, Campus de las Lagunillas s/n, Edificio A3, Jaén 23071, Spain. † E-mail: jcasa@ujaen.es Copyright © 2012 John Wiley & Sons, Ltd. EUROPEAN TRANSACTIONS ON ELECTRICAL POWER Euro. Trans. Electr. Power (2012) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/etep.1623