Neutral point control for multi-level inverters: theory, design and operational limitations

This paper describes the theory, design, and operation of a neutral point voltage control system for a three level inverter drive system. The paper specifically addresses: (1) the mathematical derivation of the neutral point control principle described, defining the operating limits imposed; (2) design of a closed loop control system; (3) and modifications to the control system to enable operation throughout the full range of the drive. Experimental results are provided, and control for a five level inverter is also discussed.