抑制T型三电平逆变器共模电压的调制策略

Modulation Strategy With Reduced Common Mode Voltage for T-type Three-level Inverter

共模干扰和中点电位不平衡问题是三电平逆变器的研究热点之一。为此,提出了一种可在全调制比和全功率因数范围内实现共模电压抑制和中点电位平衡的调制方法。该方法优选共模电压小的基本矢量参与虚拟矢量的合成,将共模电压抑制在±U_(dc)/6范围内;将优选后的基本矢量构成虚拟矢量,进一步消除了中点电位波动大的问题;通过改进小区域划分方法,使得每个小区域内构成虚拟矢量的基本矢量平滑切换,避免了相邻基本矢量之间出现状态P和状态N的直接切换;使得每个小区域均包含幅值可调的虚拟矢量,通过调节虚拟矢量的幅值来改变每个载波周期内基本矢量的作用时间,以控制流出直流侧中点的平均电流,最终所有小区域均具备良好的中点电位平衡控制能力。实验结果表明,使用该方法可使得逆变器输出的共模电压为±U_(dc)/6,同时可以消除中点电位不平衡的问题。研究输出共模电压低、中点电位波动小且具备中点电位调节能力的调制策略对三电平逆变器具有重要意义。

Common mode voltage(CMV)and imbalance of neutral-point(NP)potential are the important issues in investigating three-level inverters.Therefore,a modulation method is proposed,which can be adopted to achieve CMV suppression and NP potential balance in the range of full modulation ratio and full power factor.In this method,the basic vectors with low CMV are selected to synthesize virtual vectors,which suppress the CMV within the range of±U_(dc)/6.The optimized basic vectors are formed into virtual vectors,which further eliminates the problem of large fluctuations in the NP potential.By improving the small area division method,the basic vectors which are comprised in the virtual vectors in each small area are smoothly switched,avoiding the direct switching of state P and state N between adjacent basic vectors.Each small area contains a virtual vector with adjustable amplitude.By adjusting the amplitude of the virtual vector,the action time of the basic vector in each carrier cycle is changed to control the average current flowing out of the NP of the DC side.Finally,all small areas have good midpoint potential balance control ability.The experimental results show that the CMV of the inverter is limited to±U_(dc)/6,and the neutral point potential imbalance can be eliminated.Research on the modulation strategy with low output CMV,small NP potential fluctuation and NP potential adjustment capability is of great significance to three-level inverters.