2088 IEEE TRANSACTIONS ON SMART GRID, VOL. 3, NO. 4, DECEMBER 2012 A Communication-Assisted Protection Strategy for Inverter-Based Medium-Voltage Microgrids M. Amin Zamani, Student Member, IEEE, Amirnaser Yazdani, Senior Member, IEEE, and Tarlochan S. Sidhu, Fellow, IEEE Abstract—This paper proposes a communication-assisted pro- tection strategy implementable by commercially available micro- processor-based relays for the protection of medium-voltage mi- crogrids. Even though the developed protection strategy benefits from communications, it offers a backup protection strategy to manage communication network failures. The paper also intro- duces the structure of a relay that enables the proposed protec- tion strategy. Comprehensive simulation studies are carried out to verify the effectiveness of the proposed protection strategy under different fault scenarios, in the PSCAD/EMTDC software environ- ment. Index Terms—Communication-assisted protection strategy, dis- tributed generation, electronically coupled distributed generators, microgrid, microprocessor-based relays, smart grid. I. INTRODUCTION E LECTRONICALLY coupled distributed generators, pow- ered by such microsources as photovoltaic arrays, micro gas turbines, wind power systems, and fuel cells, have been gaining popularity among the industries and utilities due to their higher reliability, improved operational efficiency, and reduced greenhouse gas emission level. The increasing penetration of distributed generators (DGs) and the existence of multiple DGs in electrical proximity to one another have brought up the con- cept of the microgrid [1]. A microgrid is a relatively small area of a power system which includes sufficient generation and is capable of operating either in parallel with or independent from the utility grid, while providing continuous service to end-users, and improving power quality, reliability, and operational opti- mality [1], [2]. The practice of operating microgrids, however, disturbs the traditional control and protection strategies which are based on the assumption of a radial network structure that features large fault currents and unidirectional power flows [3], [4]. Manuscript received August 03, 2011; revised October 14, 2011; accepted July 23, 2012. Date of publication September 07, 2012; date of current version December 28, 2012. Paper no. TSG-00315-2011. M. A. Zamani is with the Department of Electrical and Computer En- gineering, University of Western Ontario, London, ON N6A 5B9, Canada (e-mail: mzamani2@uwo.ca). A. Yazdani is with the Department of Electrical and Computer Engineering, Ryerson University, Toronto, ON M5B 2K3, Canada (e-mail: yazdani@ryerson. ca). T. S. Sidhu is with the Faculty of Engineering and Applied Science, Univer- sity of Ontario Institute of Technology, Oshawa, ON L1H 7K4, Canada (e-mail: tarlochan.sidhu@uoit.ca). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TSG.2012.2211045 The main issue concerning the microgrid protection arises in the islanded mode of operation where the microgrid is operated in isolation from the main utility grid. In this mode, fault cur- rents are relatively small, due to the limited current ratings of sil- icon switches employed in the converters of electronically cou- pled DGs (EC-DGs). Thus, the conventional overcurrent pro- tection is not adequate for the protection of islanded microgrids [5]–[10], [16]. In the grid-connected mode of operation, how- ever, the host grid contributes to fault currents and, thus, the fault currents are comparatively large. Although the use of over- current relays is possible for the protection of grid-connected microgrids, the existing relay settings should be carefully re- vised as the presence of DGs may compromise the protection coordination [4], [11]–[15]. Reference [5] has proposed an admittance relay for protection of microgrids in both modes of operation. However, it does not present a reliable method for measuring the accurate value of the line admittance for different fault scenarios; the issue especially manifests itself when the distribution lines are short. Moreover, the coordination of the proposed relays has not been fully dis- cussed in the paper. Reference [6] proposes to equip each DG with its own relay. The proposed method relies on the current sequence components and islands the microgrid for each fault incident. Although the strategy works well for single-phase-to- ground and phase-to-phase faults, it neither offers protection against three-phase faults nor does it provide a systematic co- ordination technique. Monitoring the network voltage for the protection of mi- crogrids has been proposed in some references [7], [8]. The method presented in [8], for example, employs - and -axis components of the network voltage to detect and isolate faults within a microgrid. However, it neither ensures protection against symmetrical and high-impedance faults nor does it consider grid-connected mode of operation. In [9], a communi- cation-based strategy implemented by digital relays is proposed for the micrgrid protection. Although an effective strategy, it is very costly and in need of technical features that are absent in the present-day electrical equipment. Moreover, the strategy relies on differential currents while ignoring the error and mismatches amongst current transformers. Reference [10] proposes the employment of energy storage devices with high short-circuit capacities in a microgrid. The proposed strategy enables the use of overcurrent protection algorithms for microgrids, even in the islanded mode of opera- tion; however, it is expensive and in need of adaptive protection schemes. Several communication-assisted protection strategies have also been proposed for distribution networks with a 1949-3053/$31.00 © 2012 IEEE