Equalization of dc bus voltage in three-level NPC half-brige inverters for PV applications by even harmonics injection or fundamental phase modulation

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. The novel proposal presented in this paper considers the injection of a 2^nd harmonic current component (patent pending) or an equivalent modulation of the fundamental current reference phase angle. The presence of a 2^nd 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 method, which is normally adopted. The proposed equalization strategy has been implemented and tested on a commercial PV inverter. Analytical developments as well as a simulation and experimental investigation are presented to prove the effectiveness of the proposal.