Abstract- Conventional power distribution system is radial in nature, characterized by a single source feeding a network of downstream feeders. Protection scheme for distribution system, primarily consisting of fuses and reclosures and, in some cases, relays, has traditionally been designed assuming the system to be radial. In last year extra attention applied in use of distributed generator units in distribution networks. The integration of distributed sources into existing networks brings up several technical, economical and regulatory questions. The connection of distributed generators (DG) to distribution networks also influences the performance of the networks and the impact depends on the number, location and size of injected DG. The presence of distributed generators in the distribution network can cause the mis-coordination of the protection system. In order to overcome this problem, one can change the relay setting based on the number and location of DGs in the network. In this paper, another approach is selected in which, the capacity of DG at each node is determined in such a way, that the mis-coordination does not happen. The proposed method is explained in two cases. In the first case, just one DG at each node is considered, but in the second case existence of two or more DGs in separate nodes is taken into account. The simulation result are presented and discussed in a typical distribution network. Index Terms- Distributed generator, distribution network, protection equipment coordination, DG capacity. I. INTRODUCTION onnection of distributed generators directly to distribution systems has become a common practice worldwide. The connection of DGs brings a great change to configuration of the utility distribution network. As a result, this leads to a big challenge for its control and protection system [1-3]. Capacity and location of DG in the network have much influence on the protection system. Some of the often-quoted benefits of DG include the following: 1) Emergency backup during sustained utility outages 2) Reduced voltage sags 3) Increased reliability 4) Potential utility capacity addition deferrals At present standards for interconnecting DG to network, mostly are based on the principle that DG shouldn’t bring _______________________________________________ Javad Sadeh is with Islamic Azad University, Gonabad Branch, Gonabad, Iran, M. Bashir and E. Kamyab are with the Department of Electrical Engineering Ferdowsi University of Mashhad, Mashhad, Iran (e-mail: sadeh@ieee.org; mohsenbashir@ieee.org and ebadkamyab@yahoo.com ) influence upon the normal operation performance for the utility protection and control system [4,5]. When a DG is connected into the network then levels of short circuit changed, so relay settings should be changed and if DG is disconnected relay setting should back to previous state, for doing these a lot of communication links require that usually not available in distributed network [6]. This paper presents an analysis to the protection for the radial distribution system with DG. A method for determining the distributed generators capacity has been suggested that these units do not cause loss of coordination of relays. II. RELAY COORDINATION OF CONVENTIONAL PROTECTION Fig. 1 shows a simple radial distribution power system, where GS is the grid system, A, B, C, D, E are the nodes of system and their corresponding loads are load1, load2, load3, load4, load5 respectively. When a fault occurs at the network, inverse over-current protection is adopted. Fig.1. Simple radial distribution power system The general operational characteristics of relays correspond to IEC standard which is expressed by the following equation: 1 14 . 0 02 . 0 i - ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ = i pickup i f i I I TMS t (1) Where: TMS i = time multiplier setting of relay i I fi = fault current seen by relay i I pickupi = pickup current of relay i Effect of Distributed Generation Capacity on the Coordination of Protection System of Distribution Network J. Sadeh, IEEE Member, M. Bashir, Student Member, IEEE, E. Kamyab C 110 2010 IEEE/PES Transmission and Distribution Conference and Exposition: Latin America 978-1-4577-0487-1/10/$26.00 ©2010 IEEE