The two major technical challenges facing widespread deployment of microgrids are the impacts of microsources and active loads on control instruments and protective devices. Classical protective devices operate through detection and response. Detection is performed by measuring parameters of the system and comparing with set values. Response is performed through isolation of faulted segment if the result of comparison dictates isolation. These classical protective devices as well as modified classical devices reported in literature malfunction under complex operating conditions of microgrids that have multiple controls regimes and are capable of grid interconnection and islanding. In this book chapter, a quad-variable rule-based relay is described. The relay meets both plug-and-play and peer-to-peer requirements of microgrids. The interaction of four critical parameters of complex microgrids is modeled using fuzzy rule. Using the rules, sub-controllers are designed for microsource and feeder in the distribution system. Combinational logic components are used to realize the hardware of the sub-controllers in SimPowerSystems. Reliability of the relay is established through offline and online response tests. The relay marks a departure from classical or modified classical protection wherein protection is based on a defined short circuit capacity. In this work, protection is based on nominal parameters of two members of the distribution system – microsources and feeders.
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