Unbalanced Operation of Per-Phase Vector Controlled Four-Leg Grid Forming Inverter for Stand-Alone Hybrid Systems

Electricity in stand-alone systems (mini-grids) is usually provided by diesel generator sets (gensets). Hybridizing such systems with renewable energy sources (RESs) and storage units helps reducing the environmental impact and cost of electricity generation. In general, RESs inject as much power as possible while the diesel power plant operates as the grid forming element, balancing active and reactive power in the hybrid mini-grid. Whenever there is enough renewable power available to meet the forecasted load, it is possible to shut down the diesel power plant leading to operation with minimum cost. In such a case, a battery inverter with appropriate storage capacity takes over the grid forming task. Mini-grids often present a three-phase four-wire configuration allowing the connection of three-phase and single-phase loads and RESs. This imposes a significant burden on the grid forming battery inverter which has to cope with issues such as highly unbalanced loads while ensuring good power quality. This paper proposes a new per-phase vector control strategy for a four-leg grid forming inverter that provides balanced voltage with enhanced dynamic response even when it has to supply active power in two phases and absorb in the other one, due to the presence of a single-phase RES.