NPC Converter-Based Grid Current Quality Improvement through Adaptive Phase-Shift at Single-Phase AC Ports

This paper proposes a method to improve grid-side current quality in a three-phase neutral point converter (NPC) connecting a hybrid DC-AC microgrid (MG) to a utility grid, by single-phase MG-connected inverters’ voltage reference phase shift, while NP voltage keeps balanced. Three-level NPC converters with bipolar DC bus are a good solution to connect hybrid DC-AC MG to the main grid. The split DC bus voltage balancing, and the MG DC bus power quality are fundamental for the life-long operation of capacitors and converters as well as for the grid-side power quality. The algorithms to maintain the neutral-point potential lead to distortions in grid side currents when working under unbalanced loads connected to the split DC-bus. The proposed algorithm improves the grid current quality without any additional circuitry, using a proper voltage phase reference angle for each single-phase inverter connected to the split bus. The proposed algorithm improves grid side currents while the neutral-point voltage maintains its balance under unbalanced load conditions. The proposed method is being validated in HIL real-time simulation.