International Review of Electrical Engineering (I.R.E.E.), Vol. 7, N. 4 ISSN 1827- 6660 July - August 2012 Manuscript received and revised July 2012, accepted August 2012 Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved 4992 A Fuzzy Based Under-Frequency Load Shedding Scheme for Islanded Distribution Network Connected with DG H. Mokhlis 1, 2 , J. A. Laghari 1, 3 , A. H. A. Bakar 2 , M. Karimi 1, 2 Abstract – The frequency of power system is very sensitive to load changing when operating in islanded mode. This may causes overloading or loss of generation cases. Under-Frequency Load Shedding (UFLS) Scheme is commonly applied to stabilize the frequency during these cases. Conventional UFLS scheme operates successfully in interconnected grid system and may not work well when applied to DG based system operating in islanded mode. This paper presents a new fuzzy logic based under- frequency load shedding scheme implemented on mini hydro type-DG operating in islanded mode. The proposed strategy is based on frequency, rate of change of frequency and load prioritization. In proposed UFLS scheme, a fuzzy logic load shedding controller (FLLSC) with Load Shed Controller Module (LSCM) is modelled. FLLSC measures amount of load to be shed and LSCM shed the respective load to stabilize frequency. The proposed scheme is validated on different event-based and response-based cases. Simulation results show that proposed scheme is effective in shedding optimal number of loads while stabilizing the frequency. Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved. Keywords: Distributed Generation (DG), Fuzzy Logic Load Shedding Controller (FLLSC), Load Shed Controller Module (LSCM), Islanded Distribution Network Nomenclature f c Frequency of the center of inertia f Frequency deviation df/dt Rate of change of frequency H Generator inertia constant P Active power at new voltage/frequency P 0 Active power at base voltage/frequency Q Reactive power at new voltage/ frequency Q 0 Reactive power at base voltage/ frequency P Imbalance power DG Distributed Generation FLLSC Fuzzy logic load shedding controller RCB Remote Circuit Breaker UFLS Under-frequency load shedding LSCM Load shedding controller module I. Introduction Distributed generation (DG) refers to small type of electric power generation having capacity less than 10 MW [1]. DG units are located near load centers in order to avoid expansion of the existing network and supply new load areas. Due to market deregulation and environmental constraints, the use of DG resources has been widely employed in power industry[2]. The increasing trend of DG penetration in power system network has opened the new challenging issues in the field of power system. The use of DG has provided benefits to the end user, power utilities and DG’s owner in terms of reliability, efficiency of power and economics [3]-[5]. However, penetration of DG causes various problems to the existing network and power system need to be modified. One of the constraints is operation of DG during islanded mode, in which DG is electrically isolated from the main grid [6], [7]. This causes overloading or loss of generation cases and requires load shedding technique to stabilize the frequency within acceptable range. The conventional load shedding scheme employs frequency relay to stabilize frequency under abnormal conditions. In this scheme, the under frequency relay operates when system frequency falls below a certain threshold value. Conventional UFLS scheme shed a fixed amount of electrical power in fixed steps. This scheme is unreliable in shedding the optimal number of loads [6]-[9]. Efforts have been made to improve the existing conventional UFLS scheme by combining the generator tripping with UFLS scheme in order to stabilize power system during unstable faults[10]. Another effort for improving the conventional load shedding scheme is presented by simultaneously studying the transmission and distribution network [11]. Since, conventional load shedding scheme shed the load in fixed steps without estimating the amount of load to shed. Hence, it often shed more load than required. This problem is solved by employing power swing equation to estimate the amount of load to be shed. The technique is known as an adaptive UFLS scheme.