International Review of Electrical Engineering (I.R.E.E.), Vol. 7, N. 2 ISSN 1827- 6660 March-April 2012 Manuscript received and revised March 2012, accepted April 2012 Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved 4155 A New Approach of Under-Frequency Load Shedding Technique in an Islanded Distribution Network M. Karimi 1, 2 , H. Mohamad 2, 3 , H. Mokhlis 1, 2 , A. H. A. Bakar 2 Abstract – Under-frequency load shedding is a common technique to maintain the power system stability by removing the overload in some part of the system. The existing under-frequency load shedding schemes operate successfully in the interconnected grid system. A particular approach of under-frequency load shedding scheme is required for an islanded system which is known to be not as strong as the grid. This paper presents a new approach of load shedding scheme in an islanded distribution system incorporating distribution state estimation. The scheme is based on an intelligent technique to reduce the effect of power deficiency in an islanded network. The disturbance magnitude is calculated based on rate of change of frequency by using the simplest expression of the generator swing equation. Moreover, the process for assessing the power consumption of the customer can be achieved by state estimation method. In this paper, state estimation for a distribution network is developed and enhanced by composite load model to obtain the most practical value of the system state variables. Finally, the calculated value of the disturbance magnitude will be shed according to the estimated value of loads in each bus. Copyright © 2012 Praise Worthy Prize S.r.l. - All rights reserved. Keywords: Distribution State Estimation, Under-Frequency Load Shedding, Distribution Network, Distributed Generation, Islanding Nomenclature H Generator inertia constant f c Frequency of the center of inertia P i Real power injection Q i Reactive power injection P ij Real power flow Q ij Reactive power flow h i (x) Measurement matrix of the nonlinear function H(x) Jacobian matrix of the measurement function P Imbalance power DG Distributed generation UFLS Under-frequency load shedding SE State estimation EMS Energy management system DSE Distribution state estimation WLS Weighted least square AUFLS Adaptive Under-frequency load shedding ROCOF Rate of change of frequency FCM Frequency calculator module LSCM Load shedding controller module DSEM Distribution state estimation module COI Center of inertia I. Introduction Nowadays, islanding operation of a distribution network is a topic of interests to improve security and reliability of a network. Implementing an intentional islanding operation of Distributed Generation (DG) will establish continuity of supply in the customer side [1]- [2]. Accordingly, IEEE 1547 group has produced a standard which serve as a guide for practicing an intentional islanding operation in an electric power system [3]. However, the operation of a DG in an islanded brings up some concerns. The most critical concern is to maintain the power quality of the islanded system. A sudden change in generation and load immediately after system is islanded poses a challenge in controlling the frequency response. A large excess of load over local generation in the islanded system could result in a rapid frequency drop. The smaller system inertia tends to cause a quick frequency drop. It also has a limited spinning reserve to cover for the shortfall of the generation. For such an imbalance power, DG would not be able to stabilize the frequency response. The only way to stabilize the frequency to its nominal value is to reject several loads. It is a common practice among the power utilities to employ a conventional Under-Frequency Load Shedding (UFLS) scheme. The UFLS relay is set to shed certain amount of loads based on a specified frequency threshold. Although the scheme is simple, it is widely