三电平NPC逆变器的SHEPWM和DPWM切换策略研究

Study on Switching Strategy of SHEPWM and DPWM for Three-level NPC Inverter

三电平中性点箝位(neutral point clamped,NPC)逆变器在中压大容量场合中广泛应用。中压开关器件的开关损耗较大,开关频率较低,采用特定谐波消除脉宽调制(selective harmonic elimination PWM,SHEPWM)可消除输出电压中大部分低次谐波,提高电流波形质量。但三电平拓扑存在中点电压偏移的问题,该调制实现中点电位调节较困难。利用注入零序电压能够实现断续脉宽调制(DPWM),注入不同的零序电压会使中点电位偏移不同。提出SHEPWM和DPWM切换策略,稳态下使用SHEPWM实现高质量电流输出,出现中点偏移时再切换到不同的DPWM实现调节。结合两者优点,在维持开关频率恒定的前提下实现高质量的稳态电流和良好的中点电位调节能力。最后,搭建2.5 MW三电平NPC逆变器仿真模型,验证所提切换策略的正确性和有效性。

Three-level NPC inverter is widely used in medium-voltage and large-capacity applications.Medium voltage switching devices have high switching loss and low switching frequency.The use of specific harmonic elimination pulse width modulation(SHEPWM)can eliminate most of the low harmonic in the output voltage and improve the quality of current waveform.But the problem of midpoint voltage offset exists in three-level topology,so it is difficult to realize midpoint potential regulation with this modulation.Discontinuous pulse width modulation(DPWM)can be realized by injecting zero sequence voltage.The midpoint potential will be offset differently by injecting different zero sequence voltage.This paper proposes a switch strategy between SHEPWM and DPWM.SHEPWM is used in steady state to achieve high quality current output,and then switched to different DPWM to achieve adjustment when midpoint deviation occurs.Combining the two advantages,this paper realizes high quality steady state current and good midpoint potential regulation ability under the premise of maintaining constant switching frequency.Finally,a simulation model of a 2.5 MW three-level NPC inverter is built to verify the correctness and effectiveness of the proposed switching strategy.