Passivity-based Control of NPC Three-level Inverter

Unbalance of dc capacitor voltage is a key technical problem of NPC inverter. Through establishing the dc capacitor voltage balancing mathematical model of NPC three-level inverter, it can be proved that the NPC three-level inverter is a Passivity nonlinear system. By controlling the total charge of point injection is zero in a sampling period, and considering the Passivity dissipation characteristics of the nonlinear system, this paper calculated current of middle line and built up the passivity-based control model, and then kept the balance between voltages of dc capacitors, realized the low frequency current and dried the load transformer based on this. This control algorithm can achieve balance of dc voltages and output current at the same time, and the validity and feasibility of this control strategy is verified by simulation results, and very good effect has been proved by a practical engineering of 750kVA, NPC Three-level inverter. Based on this, a new idea of solving the unbalance of dc capacitor of diode clamped multi-level inverter is provided by this study.