Model Predictive Control Method with NP Voltage Balance by Offset Voltage Injection for Single-phase NPC Converters

Abstract

In single-phase three-level neutral point (NPC) converters, two major objectives are to generate sinusoidal source current and to balance two dc capacitor voltages. This paper proposes a model predictive control (MPC) method for the NPC converters to accomplish these multi-objectives using single reference voltage with offset voltage injection, without using a weighting factor in a cost function. The proposed MPC method determines a modified reference voltage signal which simply adds a reference voltage and an offset voltage, where a reference voltage deals with sinusoidal source current synthesis and an offset voltage accomplishes capacitor voltage balancing. By comparing the modified reference voltage with all possible voltage candidates producible in the single-phase three-level NPC converter, an optimal state to the dc voltage balance as well as the sinusoidal source current control with unity power factor is selected at every step. As a result, the proposed method is free from multi-objective optimization issue and empirical approaches to select a proper weighting factor accompanied by conventional MPC methods for the NPC converters. Therefore, the proposed method is easy to implement based on a deterministic approach for the capacitor voltage balance as well as the sinusoidal source current control, whereas fast dynamic performance is guaranteed.