Neutral Point Potential Control Under Three Level Selective Harmonic Elimination PWM Considering Low Frequency Fluctuation Interference

Selective harmonic elimination pulse width modulation (SHEPWM) is crucial for three level neutral point clamped converters used widely in middle voltage high power applications. However, since the PWM pattern is determined by switching angles offline, the neutral point potential (NPP) control under SHEPWM becomes difficult. In particular, the low frequency pulsation in NPP cannot be eliminated from the principle, which can lead to misadjustment of the conventional NPP control based on switching angle adjustment. This makes it necessary to set the adjustment amount of switching angle very small and the start-up threshold of the algorithm large in the conventional method. In order to overcome the weak NPP control capability of the conventional method, an improved NPP control strategy is proposed in this paper. The proposed method deeply investigates the generation mechanism of the low frequency pulsation and the law of its harmonic distribution, and then designs a recursive least square filter to eliminate the effect of the low-frequency pulsation. In turn, the proposed method can improve the NPP control capability and ensure a good steady-state output performance under SHEPWM. Finally, the simulation results demonstrate the correctness and effectiveness of the proposed method.