IEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING IEEJ Trans 2016 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI:10.1002/tee.22384 Paper Load Shedding Scheme Based on Rate of Change of Frequency and Ranked Stability Index for Islanded Distribution System Connected to Mini Hydro Norazliani Md. Sapari * , Non-member Hazlie Mokhlis * a , Non-member Ab H. Abu Bakar ** , Non-member Hasmaini Mohamad *** , Non-member Javed A. Laghari * , Non-member Mohd R. M. Dahalan **** , Non-member This paper presents a load shedding scheme for a distribution network connected to mini hydro generations. The proposed scheme uses the rate of change of frequency (ROCOF) to estimate the power imbalance and voltage stability (VS) to prioritize the load that is to be shed. The load with the most critical voltage stability (which is based on a special index) is given the first priority to be disconnected from the network in order to avoid voltage collapse. By performing load shedding based on the combination of the ROCOF and VS techniques, a better system frequency and enhanced voltage magnitudes can be achieved. The performance of the proposed scheme is validated on existing Malaysian network interconnected with a mini hydro generator. Simulation results show that the proposed scheme not only successfully stabilizes system frequency, but also simultaneously stabilizes the voltage magnitude of the buses within an acceptable limit.  2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. Keywords: voltage stability index; under-frequency load shedding; distributed generation; islanding; rate of change of frequency Received 3 November 2015; Revised 22 January 2016 1. Introduction Electrical energy is an essential component for daily human activities. Its demand keeps increasing due to the needs in various sectors such as in industrial, telecommunication, and banking. It is predicted that in 2020, the energy demand will increase to 116 Mtoe with annual growth of 8.1% [1]. Thus, more energy sources are needed to cope with the increase in energy demand. Malaysia, for example, is dependent on the mix of energy generation especially from 1995 to 2009 [1]. Fossil fuels dominate the major power generation with almost 94.5%, and the remaining is generated by hydroelectric generation [1]. The dependence on fossil fuels hit a barrier with sudden increasing of oil price in July 2008 [2]. Besides the cost factor, there was realization on the negative impact of fossil fuels on emission of greenhouse gases. Because of this, alternative sources such as hydrogenerators have become important to countries such as Malaysia. Under 10th Malaysian Plan (2011–2015), the gov- ernment promoted the utilization of renewable energy resources [3]. This policy consequently contributed to an increase in the use of renewable energy generation in the mixed generation of a Correspondence to: Hazlie Mokhlis. E-mail: hazli@um.edu.my *Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia **University Malaya Power Energy Dedicated Advanced Centre, Wisma R&D UM, University of Malaya, 59990, Kuala Lumpur, Malaysia ***Faculty of Electical Engineering, University of Technology Mara, 40450, Shah Alam, Selangor, Malaysia ****Malaysian Institute of Marine Engineering Technology (UniKL MIMET), Universiti Kuala Lumpur, 32200, Lumut, Perak, Malaysia national electricity [2]. The widespread use of renewable energy as distributed generations (DGs) has raised some technical issues. One of the major concerns in operating DGs in the distribution system is the possibility of islanding occurrences. According to IEEE standard-1547, DGs are required to be disconnected during islanding to avoid system instability and safety issues [4,5]. This practice may prove uneconomical, since DGs could be used to energize some of the loads in the islanded system. Intentional islanding operation with proper control can be an option to avoid these issues. Various studies have been conducted, and some are going on, all of which intending to make islanding operation a reality [6 – 10]. When islanding occurs, the voltage and frequency in the distribution network can vary significantly in cases of large power mismatch between the total generation and load demand. Therefore, it is essential to control the frequency and voltage of the islanded network within the allowable limit. Otherwise, it may cause the islanded network to experience blackouts. One possibility is to implement load shedding. By shedding some of the loads, a close match between the total power generation and load demand can be achieved, and frequency can be brought back to permissible limits. Generally, there are two types of load shedding: Under- frequency load shedding (UFLS) and under-voltage load shedding (UVLS) scheme. UFLS is based on the changes of the system frequency, while UVLS is based on the changes of the voltage magnitude. In the past, load shedding schemes were commonly applied for transmission systems. However, with the high penetration of DGs, studies on load shedding schemes for distribution systems have been attracting interest. Some of the work based on load shedding schemes focused on the distribution network is reported  2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.