Bidirectional DC-AC converter for isolated microgrids with voltage unbalance reduction capabilities

This paper presents a three-phase bidirectional DC-AC converter suitable for operating as an interface between an energy storage system (ESS) based on a battery bank and an isolated microgrid with distributed generation. The converter consists of two stages: a DC-DC stage implemented with a bidirectional half-bridge converter that operates on buck or boost mode (during charge or discharge of the ESS, respectively), and a DC-AC full-bridge three-phase bidirectional inverter. The two stages are connected through a DC link, which also works as a power-decoupling element. Isolation from the microgrid is obtained with a Δ-Y low frequency transformer. A control strategy is proposed where the DC-DC stage regulates the DC link voltage level while the DC-AC controls the three-phase output voltage and frequency, therefore acting as a grid-forming converter (GFC). A modified droop strategy is implemented to control the power generated inside the microgrid to avoid overcharging the ESS. Voltage unbalance reduction capabilities were implemented in order to deal with unbalanced loads. A 15kW prototype was successfully built and tested in different situations and the experimental results are shown.