International Journal of the Physical Sciences Vol. 6(16), pp. 3873-3881, 18 August, 2011 Available online at http://www.academicjournals.org/IJPS ISSN 1992-1950 ©2011 Academic Journals Full Length Research Paper Impact of distributed generations on power system protection performance Hadi Zayandehroodi 1 *, Azah Mohamed 2 , Hussain Shareef 2 and Marjan Mohammadjafari 1 1 Department of Electrical Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran. 2 Department of Electrical, Electronic and Systems Engineering, Universiti Kebangsaan Malaysia (UKM), Selangor, Malaysia. Accepted 20 June, 2011 Though distributed generators (DGs) have significant economic and environmental benefits, increased penetration of DGs will impose significant technical barriers for the efficient and effective operation of bulk power systems. Increased fault current contribution and load flow changes are the major two impacts on utility systems, and these will affect existing protective relaying, especially overcurrent relays. To ensure safe and selective protection relay coordination, the impact of DGs should be taken into account when planning DG interconnection. This paper presents an introduction of DGs and an overview of the impacts of DGs on system protection relay coordination. Several protection issues are identified to study the requirements for protection in the presence of DGs are also discussed. Key words: Distributed generator (DG), protection coordination, distribution network, overcurrent relay. INTRODUCTION The electric power system normally includes a generating system, a transmission system, power substations and the distribution network. The distribution networks were designed to extract power from the power substations and distribute it to the loads. It was not designed to have generators directly connected to it. The distribution networks topology, control, and protection are all designed assuming that power is flowing in one direction, from transmission to loads (Zayandehroodi et al., 2009). In the recent years, the electrical power utilities are undergoing rapid restructuring process worldwide. Indeed, with deregulation, advancement in technologies and concern about the environmental impacts, competition is particularly fostered in the generation side thus allowing increased interconnection of generating units to the utility networks. These generating sources are called as distributed generators (DGs). The increase in interconnection of DG to utility networks can lead to *Corresponding author. E-mail: h.zayandehroodi@yahoo.com. Abbreviations: DGs, Distributed generators; ac, alternating current; dc/DC, direct current; CB, circuit breaker; LV, low voltage. reverse power flow violating fundamental assumption in their design. This creates complexity in operation and control of existing distribution networks and offers many technical challenges for successful introduction of DG systems. Some of the technical issues are islanding of DG, voltage regulation, protection and stability of the network (Gaonkar, 2010; Zayandehroodi et al., 2011). A typical distribution protection system consists of fuses, relays and reclosers. An inverse overcurrent relay is usually placed at a substation where a feeder originates. Reclosers are usually installed on main feeders with fuses on laterals (Zayandehroodi et al., 2010, 2011). Reclosers are necessary in a distribution system as 80% of all faults that take place in distribution systems are temporary. It gives a temporary fault a chance to clear before allowing a fuse to blow. The coordination between fuses, reclosers and relays is well established for radial systems; however, when DG units are connected to a distribution network, the system is no longer radial, which causes a loss of coordination among network protection devices (Brahma and Girgis, 2004). The extent to which a DG affects protection coordination depends on the DG’s capacity, type and location (Burke, 1994; Doyle, 2002; Kumpulainen and Kauhaniemi, 2004). The introduction of a DG into a distribution system brings about a change in the fault current level of the