Voltage and frequency regulation strategies in isolated AC micro-grids

In ac power systems, including micro-grids, it is important to regulate the amplitude and frequency of the voltages throughout the system. Many of the existing and proposed control strategies for micro-grids are patterned after the classic ac power system. That is, frequency regulation is achieved by designing micro-sources (commonly called Distributed Energy Resources or DERs) to exhibit an output-frequency-versus-power characteristic similar to the speed-versus-power (droop) characteristics of conventional turbo- and hydro-generators. Moreover, voltage regulation strategies are patterned after the output-voltage-versus-reactive-power (droop) characteristics of the automatic voltage regulators (AVRs) used in conventional turbo- and hydro-generators. In this paper, established approaches of frequency and voltage regulation are reviewed. Alternative strategies that utilize modern communication and control technologies are presented and discussed.