A Study of Dual-loop Control Scheme and Corresponding Neutral-point Voltage Balance Control Method for Three-level PWM Rectifiers

The mathematical model of the three-level PWM rectifiers is set up in this paper. By comparing it with the model of DC motors, some similarities are established. Based on the state feedback decoupling, the dual-loop close control scheme of DC motors is implemented to three-level PWM rectifiers. As a result, the three-level rectifier has favorable dynamic and steady state performance, sinusoidal input current and unity power factor. Besides, a method is proposed to control the neutral-point voltage charge unbalance, which is an inherent problem of three-level inverter. The redundant positive switching states or negative switching states of small vectors are selected according to the direction information of input current of each phase as well as the ripple of neutral-point voltage. Finally, experimental results verify the feasibility and validity of dual-loop control scheme and neutral-point (NP) voltage balancing control method.