RESEARCH ARTICLE Fuzzy‐controlled energy storage system for improving reliability of electricity supply during islanding of photovoltaic systems Ace Lin Yi Khaw | Jianhui Wong | Yun Seng Lim Lee Kong Chien Faculty of Engineering and Science, Lee Universiti Tunku Abdul Rahman, Jalan Sungai Long, 43000 Kajang, Selangor, Malaysia Correspondence Ace Lin Yi Khaw, Lee Kong Chien Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Jalan Sungai Long, 43000 Kajang, Selangor, Malaysia. Email: acekhaw@gmail.com Funding information Collaborative Research in Engineering, Science and Technology (CREST) Centre, Grant/Award Number: P01C1‐14 Summary Photovoltaic (PV) systems are expected to grow on the distribution networks in the future. Under the current regulatory frameworks, islanding operation of PV systems is not permissible because the frequency and voltage of the islanded networks are not regulated and maintained within the statutory limits during islanding operation. Consequently, PV systems must be disconnected immedi- ately in the event of the grid outage as per Malaysian Standards, MS 1873. However, it is proven that the islanded operation of PV systems would enable the owners to consume the available solar energy and help the utility company improve the reliability of electricity supply. To enable the islanded operation of PV systems, a fuzzy‐based controller is developed to drive the operation of the energy storage system to maintain the frequency and voltage within the toler- ances during the transition from grid‐connected to islanded operation. Several experimental case studies are performed to show that the fuzzy‐based control- ler is able to maintain the frequency and voltage within the statutory limits before the islanded networks are reconnected to the grid. It is also shown that the fuzzy controller performs more effective than the proportional‐integrator controller. KEYWORDS energy storage system, fuzzy controller, islanding operation, low‐voltage distribution network, photovoltaic system Notation The following symbols are used in this paper: ΔP, Active power mismatch between load and PV; ΔQ, Reactive power mismatch between load and PV; P LOAD , Active power of controllable load; Q LOAD , Reactive power of controllable load; P PV , PV active power output; Q PV , PV reactive power output; P BESS , BESS active power output; Q BESS , BESS reactive power output; P GRID , Active power from main grid; Q GRID , Reactive power of main grid; ΔP BESS , BESS active power output changes during the transition from grid‐connected to islanded operation; ΔQ BESS , BESS reactive power output changes during the transition from grid‐connected to islanded operation; P * BESS , Steady‐state active power output of BESS during islanded operation; Q * BESS , Steady‐state reactive power output of BESS during islanded operation; ΔP BESS. ref , Operating boundary limit of NDZ; K c , Proportional gain; T i , Integral time; T d , Derivative time Received: 31 October 2017 Revised: 22 February 2018 Accepted: 26 March 2018 DOI: 10.1002/etep.2604 Int Trans Electr Energ Syst. 2018;e2604. https://doi.org/10.1002/etep.2604 Copyright © 2018 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/etep 1 of 14