Per-phase vector control strategy for a four-leg voltage source inverter operating with highly unbalanced loads in stand-alone hybrid systems

This paper proposes a new per-phase vector (dq) control scheme for a four-leg grid-forming inverter operating in a three-phase four-wire hybrid stand-alone power system (mini-grid). The challenge is to provide balanced output voltages to highly unbalanced loads, very common in such systems, with a fast dynamic response. By using dq control in a per-phase basis one avoids the use of slow filter-based symmetrical components calculators (SCCs) while achieving zero error, i.e. voltage balancing, in steady-state. Further improvement in the dynamic response is achieved by using fictive axis emulation for obtaining the orthogonal current components required for the per-phase dq control. Simulation and experimental results are provided, to demonstrate the good performance of the system. It works very well, even when the grid-forming inverter supplies active power in two phases and absorbs in the other one, due to the presence of a single-phase PV inverter in the hybrid mini-grid.