Control of three-level VSIs with a LQR-based gain-scheduling technique applied to DC-Link neutral voltage and power regulation

A simple and effective method to design a regulator for the complete large-signal control of the Three-Level Neutral-Point-Clamped (NPC) VSI is presented. For the regulator calculation, a comprehensive multivariable model of the NPC-VSI converter is used. The control strategy is based on Linear Quadratic Regulator (LQR) and Gain-Scheduling control techniques. Any state variable can be regulated using the proposed method, either in small and large-signal operation, including state variables related to DC-link voltages. With the model and regulator presented, DC-link unbalance regulation is also achieved by means of the controller action instead of the switching strategy. A controller is designed to keep DC-link neutral point voltage balanced and to regulate the power delivered to the load. Simulation results confirm the validity of the proposed controller design. Steady-state and transient operation experimental results are obtained using a NPC-VSI prototype and a PC-embedded DSP board for control implementation.