Research on Hybrid Control Strategy of SHEPWM and MPC Based on 3-Order Harmonic Optimization

Resolve the insufficient dynamic response involved in SHEPWM control of three-level inverters, this research proposed a hybrid control method that simulated the switching angle of SHEPWM through model predictive control, and verified that this method had lower switching frequency and better harmonic performance under steady-state conditions, as well as excellent dynamic response under dynamic conditions. However, the discreteness of MPC caused quantization errors in the switching angle of SHEPWM, resulting in certain low-frequency harmonic. To minimize quantization errors, this research utilized the third harmonic control to suppress the neutral point potential fluctuation and conducted Simulink simulation validation. The simulation results showed that the low-order harmonic content of the SHEPWM and MPC hybrid control method based on the third harmonic control was lower than that of the traditional hybrid control method under steady-state conditions.

I. Introduction

With the progress of high-voltage technology and the rise of the inverter power level, multi-level inverters are gradually being used in industrial production. Three-level neutral point clamped (NPC) inverters are widely used in medium-voltage high-power variable frequency converters owing to their low output harmonic distortion rate and small dv/dt [1]. As for high-power variable frequency converters, high switching loss will be caused by the non-simultaneous switching on or off between the voltage and current of switching devices, and the switching frequency is usually lower than 1 kHz [2]. In terms of the conventional modulation strategies of three-level NPC inverters, traditional carrier modulation and space vector modulation will cause fundamental voltage distortion at low switching frequencies, resulting in the quality decreasing of output current. Therefore, selected harmonic elimination pulse width modulation (SHEPWM) is commonly used in three-level NPC inverters under low switching frequencies [3]. Compared to traditional pulse width modulation (PWM), SHEPWM is characterized by better output waveform quality, lower switching loss of power switch, higher voltage utilization and more accurate selected harmonic elimination [4].