Adaptive balancing of the neutral point voltage in a single-phase three-level rectifier

Keeping charge balance among its two capacitors is a very important issue in an ac-dc three- level Neutral-Point-Clamped (NPC) converter, apart from controlling the dc-link voltage and the input power factor. To this effect a new method for guaranteeing the voltage equalization of both capacitors in the case of a three-level ac-dc single-phase converter is presented that makes use of a nonlinear control loop and combinations of redundant switching functions. A set of simulations are carried out first with continuous and after with momentary imbalanced load in order to assess the good performance of this method. In the second case an adaptive gain of the control loop is revealed to be necessary for avoiding a possible transient current inrush during the balancing process. Simulations about this topic will be presented too.