Modulation Techniques for Three-Phase Three-Level NPC Inverters: a Review and a Novel Solution for Switching Losses Reduction and Optimal Neutral-Point Balancing in Photovoltaic Applications

A number of modulation strategies have been proposed in literature in the particular case of three-phase three-level NPC inverter, each one focusing on the optimization of specific aspects and performance of the converter. Nevertheless a comprehensive analysis of state-of-art techniques and their specific features suitable for a photovoltaic (PV) applications is still missing, such a study being highly desirable. Both carrier-based and space vector modulation techniques are analyzed in this paper, highlighting specific features and limitations, especially related to PV applications. Basic issues are considered and compared among modulation strategies, namely: switching losses, low-frequency oscillations of the neutral-point (NP) voltage, total harmonic distortion (THD) and weighted total harmonic distortion (WTHD) of the phase currents and line voltages, dynamic response of the neutral-point voltage control loop under imbalance conditions and modulation algorithm complexity. A hybrid modulation strategy assisted by an optimal neutral-point voltage controller is then proposed aiming at both the reduction of the switching losses with a limited deterioration of the output voltage/current quality and fast dynamics control of the neutral point voltage. Those features allow the development of reduced dc bus capacitance PV inverters with optimized efficiency and quality of the output waveforms. Simulation and experimental results based on a TMS320F28069 DSP controller and a PV inverter are presented to confirm the effectiveness of the proposal.