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On the choice of voltage regulators for droop-controlled voltage source converters in microgrids to ensure stability

Microgrids are emerging to be an attractive means to cluster various distributed generation systems with local loads while operating either in a grid-connected mode or in an islanded mode in a semi-autonomous fashion. Voltage source converter (VSC) based distributed generation devices are often an essential component in such microgrids, particularly in interfacing dc energy storage devices. While system level control of VSCs that incorporate frequency and voltage droop have been shown to be essential in ensuring autonomous system operation in a distributed manner, the impact of VSC internal voltage regulator dynamics on system stability has not been definitively established. This paper presents a systematic comparison of voltage regulators using a dynamic phasor based model of an inverter system interfaced to an infinite grid; the model is used to examine system stability boundaries. The impact of various options for realizing VSC voltage regulators in conjuction with varying interconnection system impedance parameters are studied to establish stability boundaries for each case. A dynamic phasor model is used to evaluate operating point stability and the results are verified using computer simulation results.