Control of a micro grid supplied by renewable energy sources and storage batteries

A micro grid supplied entirely by renewable energy sources (photovoltaics and wind generators) and a battery storage system is investigated. The battery converter is controlled with variable-frequency, variable-amplitude sinusoidal PWM modulation. The system frequency is determined by the state of charge (SOC) of the battery while the voltage at the main bus is held constant. The PV inverter is controlled as a current source following a reference active and reactive power with the active power being a function of the system frequency. The induction machine of the wind generator (WG) is directly connected to the grid. Detailed simulation shows that with this type of control the grid voltage and frequency remain within the limits specified by the EN 50160 standard even under large variations in active and reactive power caused by the wind generator, the PV or the load. It is further shown that, by changing the operation mode of the battery converter to current-controlled, the system can provide, in case of faults, adequate current for the protective devices to operate. Since the frequency is used as control signal, a wireless communication among the renewable energy sources is achieved while the battery SOC and thus its state-of-health is continuously monitored.