NPC三电平变换器电流谐波最优PWM策略

Current Harmonic Optimal PWM Strategy for NPC Three-level Converter

当应用特定谐波消除脉宽调制(SHEPWM)于中点钳位型(NPC)三电平变换器以降低开关频率时,将引起更为严重的中点电位(NPP)波动,因为SHEPWM无中点电位控制自由度。对此,提出了一种基于3次和9次谐波优化的新型电流谐波最优PWM策略,以抑制NPP低频脉动。传统SHEPWM通过谐波优化目标离线计算开关角来提高谐波性能,由于很难在线求解开关角,故中点电位实时控制效果不佳。而新方案中通过分析3次和9次谐波与NPP波动之间的数学关系,得到了最优谐波表达式,从而直接应用即可实现最小NPP低频脉动,而无需在线更改开关角。在NPC三电平变换器样机上开展了实验测试,实验结果验证了新型PWM策略的有效性。

When applying selected harmonic elimination pulse width modulation(SHEPWM)to a neutral-point clamped(NPC)three-level converter to reduce the switching frequency,it will cause more serious neutral-point potential(NPP)fluctuations because there is no degree of freedom for SHEPWM to control the NPP.In this regard,a new current harmonic optimal PWM strategy based on 3rd and 9th harmonic optimization was proposed to restrain low-frequency NPP fluctuation.The traditional SHEPWM improves the harmonic performance by calculating the switching angles off-line through the harmonic optimization target.Since it is difficult to solve the switching angles online,the NPP cannot be controlled well in real time.In the new scheme,by analyzing the mathematical relationship between the 3rd and 9th harmonics and NPP fluctuations,the optimal harmonic expression was obtained,so that the minimum low-frequency NPP fluctuation can be achieved directly without changing the switching angles online.The experimental tests were carried out on a prototype NPC three-level converter,and the experimental results verify the effectiveness of the new PWMstrategy.