A Three-Phase Four-Leg Neutral-Point-Clamped Photovoltaic Inverter With Decoupled Active and Reactive Power Control and DC-Link Voltage Ripple Minimization Under Unbalanced Grid Operation

This article analyzes the performance of a three-phase four-leg three-level neutral-point-clamped-based photovoltaic (PV) inverter, which is connected to an unbalanced grid. The PV source terminals are connected to the dc link of the inverter through a three-level dc–dc boost converter, which helps in controlling the individual dc-link capacitor voltages. In this article, a space-vector-control-based voltage balancing strategy is introduced for balancing the individual dc-link capacitor voltages under unbalanced grid operation. In addition, the amplitude and phase angle of individual-phase currents are thoroughly studied to understand their dependence on the percentage of unbalancing in the grid voltage and, therefore, to decide the maximum allowable current that can flow through any particular phase leg under a given unbalancing condition. A control method for minimizing voltage ripple of the individual dc-link capacitors using the boost converter is then proposed, which ensures that the capacitors do not persist any voltage overshoot under limited neutral current operation. The PV-generated grid-tied system is implemented on a scaled-down laboratory-based experimental prototype to validate the aforementioned claims and substantiate the efficacy of the proposed control techniques.