An improved finite control-set model predictive control for nested neutral point-clamped converters under both balanced and unbalanced grid conditions

Finite control-set model predictive control (FCS-MPC) is an alternative strategy, in particular, for multi-level converters. However, the computational burden is rapidly increased with the number of voltage vectors to be predicted and objectives to be controlled. This will hinder the development of the FCS-MPC strategy. In this paper, an improved FCS-MPC is proposed for nested neutral point-clamped converters (NNPCs) under both balanced and unbalanced grid conditions. Specifically, to reduce the computational burden, an optimized voltage vector selection process based on deadbeat technique is embedded into the proposed design. Meanwhile, a simplified cascade-free control strategy is presented to directly govern the DC-link voltage and reactive power in NNPCs while the flying capacitor voltages are maintained balanced. Moreover, the inclusion of a power compensation approach is presented for the FCS-MPC method in an attempt to obtain desired grid-side currents under nonideal grid-side voltage conditions. The novelty of this work lies not only in a compatible reference design that directly allows to formulate the optimal control problem using a simplified cascade-free control strategy, but also in the utilization of combining the proposed FCS-MPC and the power compensation technique for suppressing the distorted grid-side currents under unbalanced grid conditions. The performance of the proposed strategy for NNPCs is evaluated by simulation studies in various operating conditions.