Grid reconnection detection for synchronous distributed generators in stand-alone operation Hessamoddin Jouybari-Moghaddam * ,† , Seyed Hossein Hosseinian and Behrooz Vahidi Electrical Engineering Department, Amirkabir University of Technology, No. 424, Hafez Ave, 1591634311, Tehran, Iran SUMMARY Industrial plants/commercial facilities with synchronous generation, seek reliable and continuous operation even at the time of grid outages. Thus, islanding detection methods have been proposed in the literature that can be used to change the operating mode of the employed distributed generators from grid-parallel to stand- alone. Among the few solutions previously presented for switching the control mode in the reverse event of reconnecting to the grid, the method presented in this paper is unique. This grid-reconnection detection technique is low cost, fast, and does not interact with the power quality of the island although it only uses one, locally measured, variable. The method implies processing of oscillatory modes in frequency signal and is independent of the autonomous island’s boundary. The investigation also includes a discussion on similar event of merging of adjacent islands. Simulations confirm the successful discrimination between reconnection event and other switching/power quality events. Copyright © 2013 John Wiley & Sons, Ltd. key words: distributed generation; governor; grid reconnection detection; island merging; microgrid; oscillation 1. INTRODUCTION Economical benefits for distributed generation (DG) owners and higher supply reliability for reduction of outage intervals have attracted industrial plants to install local generators, which are mostly synchro- nous machines [1]. For the fact that successful islanded (stand-alone) operation of DG units helps in maintaining power quality and reliability and enhances system security, stand-alone operation has become an important topic in the research. When working in parallel (grid-connected), the DG control should not respond to changes in utility’s frequency and voltage magnitude deviations; otherwise, instability and equipment damage may threaten the DG unit [1,2]. However, in stand-alone mode (grid-disconnected), the DG unit should maintain frequency and voltage of the microgrid due to absence of utility support, thus emerging the need for a change in control mode of the DG. Any of the already presented methods in the literature for islanding detection could be used for switching the control mode of DG from parallel mode to stand-alone. Review papers such as [3,4] give thorough explanation of these methods. The inverse issue, detection of reconnection to the main grid and the proper control mode activation, is introduced in [3,5–7]. Although grid reconnection detection could be performed via breaker status communication (transfer trip) or power line carrier-based methods, introduced in [8,9], these methods are considered to be expensive for DGs on a utility circuit. This is mainly for expensive signal generators used in power line carrier-based methods of [8,9] and similarly for the cost of the transfer trip scheme and its communication channel [2,3,10]. Moreover, schemes based on breaker status communication must adapt to topology changes in the system, and the adaptation results in poor reliability of the system while it requires many communication links [11]. Islanding scenarios in which *Correspondence to: H. Jouybari-Moghaddam, No. 424, Hafez Ave, Electrical Engineering Department, Amirkabir University of Technology, 1591634311, Tehran, Iran. † E-mail: h.jooybari@aut.ac.ir Copyright © 2013 John Wiley & Sons, Ltd. INTERNATIONAL TRANSACTIONS ON ELECTRICAL ENERGY SYSTEMS Int. Trans. Electr. Energ. Syst. (2013) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/etep.1829