An Optimal Voltage-Level Based Model Predictive Control Approach for Four-Level T-Type Nested Neutral Point Clamped Converter With Reduced Calculation Burden

Four-level T-type nested neutral point clamped (NNPC) converter is a newly developed multilevel voltage source converter for higher power density systems in low and medium voltage applications. The T-type NNPC converter can operate over a wide range of voltages and own less proportion of components compared with other four-level topologies, which makes it attractive among multilevel converters. In this paper, an optimal voltage-level based model predictive control (OVL-MPC) with reduced calculation burden is proposed for the T-type NNPC converter. The main objectives of this paper are to achieve the T-type NNPC converter load current control and the flying capacitors voltages balancing while remaining computationally feasible. In our proposed method, the output voltage levels are considered as the control options rather than the switching states in the conventional MPC scheme. Meanwhile, a simplified capacitor voltage balancing approach is employed to regulate the voltages of the capacitors at their desired values without punishing the computational complexity. The contribution of this paper lies in the following two aspects: first, the finite control set is decreased in the optimization processes. Second, a simplified capacitor voltage balancing strategy is introduced for the T-type NNPC converter. The performance of the proposed OVL-MPC approach for the T-type NNPC converter under steady-state, transient-operation, and unbalance mode are verified by the simulation results.