Load voltage and output current control of three-level voltage source inverters

A load voltage and output current control for the three-level neutral-point-clamped (NPC) VSI is presented in this work. In usual operation, the regulator controls the voltage applied to the load. If an overcurrent occurs, the regulator will switch to current mode controller and the current will be driven to an acceptable value. A multivariable model of the NPC-VSI is used for the regulator calculation. The controller used in this paper is based on linear quadratic regulator (LQR) and gain-scheduling control techniques. With this control approach, any state variable can be regulated, either in small and large-signal operation, including state variables related to DC-link voltages. Then, a switching strategy to control DC-link neutral point voltage is not required. The controller is designed to keep DC-link neutral point voltage balanced and to regulate the load voltage or the output current. Simulations and experimental results agree and confirm the validity of the proposed controller