Optimal regulator with integral action and gain-scheduling for the comprehensive control of three-level NPC VSI

A new method to design a regulator for the complete large-signal control of the three-level neutral-point-clamped (NPC) VSI is presented in this work. For the regulator calculation, a comprehensive multivariable model of the NPC-VSI converter is used. The referred model is general and can be applied to generic types of sources, filters and loads. No information about the system is lost with this model. Optimal multivariable control law based on linear quadratic regulator (LQR) has been selected for the three-level VSI control strategy. This type of controller is essentially a proportional regulator, hence proper integral action has been added to the regulator, in order to cancel steady-state errors. Also, gain-scheduling control technique allow to extend the application of the controller from small-signal to large-signal operation. Both control techniques work concurrently in the regulator. Then, any state variable can be regulated using the proposed method, either in small and large-signal operation, including state variables related to the DC-link voltages. With the model and regulator presented, DC-link compensation is also achieved by means of the controller action. Subsequently, a specific switching strategy to control DC-link neutral point voltage is not required. A carrier-based PWM has been used for the switching strategy. A controller is designed to keep DC-link neutral point voltage balanced and to regulate the output voltage. The method can be used for any application, since the multivariable structure makes possible the control of any parameter. A NPC-VSI prototype has been realized and a PC-embedded DSP board is used for the controller implementation. Simulation and experimental results confirm the validity of the proposed controller design.