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

Wenjie Wu,Dan Wang

doi:10.1109/access.2019.2925467

Abstract

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.

Highlights

Multilevel voltage source converters (VSCs) have attracted much attention and widely accepted for medium-voltage high-power applications over the past three decades
The neutral-point-clamped (NPC) converter, flying capacitor (FC) converter, and the cascaded H-bridge (CHB) converter are classified into the classical multilevel converters
In order to mitigate the aforementioned drawbacks, an optimal voltage-level based MPC (OVL-MPC) for the T-type nested neutral point clamped (NNPC) converter with reduced calculation burden is proposed in this paper

Summary

INTRODUCTION

Multilevel voltage source converters (VSCs) have attracted much attention and widely accepted for medium-voltage high-power applications over the past three decades. In [33], a voltage level based MPC is deployed, where the output voltage levels are considered as the control options instead of the switching states In this sense, the further reduction of calculation burden can be achieved while maintaining the steady-state and dynamic process performance. In order to mitigate the aforementioned drawbacks, an optimal voltage-level based MPC (OVL-MPC) for the T-type NNPC converter with reduced calculation burden is proposed in this paper. With aim of relieving the enormous calculation intensity, the novel OVL-MPC methodology is employed by evaluating all the available output levels rather than the combination of switching states In this sense, load currents, as the main control objective of T-type NNPC converter, are expressed in terms of the voltage levels. The principle and implementation of the proposed CVB criterion in this design are presented

MATHEMATICAL MODEL OF THE T-TYPE NNPC INVERTER

SIMULATION RESULTS

TRANSIENT-STATE PERFORMANCE