DC bus voltage equalization in single-phase split-capacitor three-level neutral-point-clamped half-bridge inverters for PV applications

In this paper the issue of dynamic equalization of the dc bus voltages unbalance is considered in the case of single-phase split-capacitor three-level neutral-point-clamped (NPC) half-bridge inverters for photovoltaic (PV) inverters. The lack of a proper equalization strategy would lead to a rapid drift of dc bus voltages, causing stability problems to the inverter output current control loop and eventually to failure of the power stage. One common solution is based on the injection of a variable dc component superimposed to reference current during each half-cycle of the grid voltage. Unfortunately even a small dc component in the output current is not tolerable and is often limited by standards. The novel proposal presented in this paper considers the injection of a 2nd harmonic current component (patent pending). The presence of a 2nd harmonic component on the grid current is tolerable as far as EMC standards limitations are satisfied, being relatively high when compared to the dc current injection constraints. The proposed equalization strategy has been implemented and tested on a commercial PV inverter. Analytical developments as well as a complete simulation and experimental investigation are presented to prove the effectiveness of the proposal.