基于混合SVPWM方法的NPC三电平逆变器中点电压平衡控制
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摘要
多电平逆变器具有功率开关管上的电压应力低、输出电压中含有的谐波含量少和电压变化率低等优点，在大功率场合得到了广泛的应用。其中，NPC三电平逆变器是应用最广泛的结构之一，它对硬件的要求较低。但是NPC三电平逆变器存在一个固有缺点，即在调制系数较高且功率因数较低的条件下，中点电压存在低频振荡，并且在不平衡负载运行条件下，中点电压低频振荡幅值增加、振荡频率发生变化。
针对中点电压低频振荡问题，本文介绍了NPC三电平逆变器的拓扑结构和工作原理，利用三电平空间矢量图描述空间矢量的分布。在此基础上，详细分析最近三矢量法，并对加入中点电压平衡控制前后的最近三矢量法进行对比分析。同时，定义虚拟空间矢量，研究虚拟空间矢量法的具体实现过程，并对最近三矢量法和虚拟空间矢量法的优缺点进行对比分析。
针对虚拟空间矢量法开关频率较高问题，分析冗余小矢量占空比系数变化规律，确定最近三矢量法能够实现中点电压平衡控制的区域。在此基础上，综合最近三矢量法和虚拟空间矢量法的优点，提出比例可调混合SVPWM方法，该方法通过拓展最近三矢量法的作用区域，在限制中点电压振荡幅值的同时，能够有效减小开关频率。同时，定义一个比例参数，研究中点电压振荡幅值和总谐波含量与比例参数的关系，为实际应用中比例参数的选择提供理论依据。实验验证理论分析的正确性。
针对不平衡负载对中点电压的负面影响，利用对称分量理论将负载电流进行分解，研究负序电流对中点电压平衡的影响，得出不平衡负载会使振荡幅值增加、振荡频率发生变化的结论。在此基础上，具体分析不平衡负载作用下最近三矢量法可平衡控制区域的分布，提出适用于不平衡负载的改进比例可调混合SVPWM方法，仿真结果表明，该方法能够获得较好的中点电压平衡控制性能。
利用中点电流可得到的最大值和最小值，确定最近三矢量法在加入中点电压平衡控制后的中点电流。利用此中点电流，提出最优拓展方式，将最优拓展方式与其它四种拓展方式进行对比分析。同时对实验中出现的相电压输出波形不对称和线电压电平不恒定现象进行分析研究，并阐述上述现象出现的原因。
Multilevel converters have attracted more and more attention in high-powerapplications due to low voltage across the power device, improved output voltagequality and low dv/dt. Among them, the neutral-point-clamped (NPC) three-levelinverter is a most widely used multilevel converter topology, which has low hardwarerequirement. The disvantage of NPC three-level inverter is the presence oflow-frequency voltage oscillations on the NP under the conditions of high modulationindex and low power factor. And the ripple amplitude increases and the ripplefrequecncy changes under unbalanced load. This paper explores above issues.
This paper introduces the topology and the principle of NPC three-level inverter,and analyzes the vector distribution based on the space vector diagram. Then thenearest-three-vector (NTV) technique is described and the NTV methods before andafter adding balance control are compared. The nearest-three-virtual-vector (NTV2)technique is introduced based on the definition of virtual vector.
Based on the analysis of the duty-cycle coefficients of the redundant smallvectors, the intervals where NTV can control the NP voltage balance in a fundamentalcycle are obtained. This paper combines NTV2with NTV and proposes a hybridSVPWM method. This method can limit the NP ripple amplitude and reduce theswitching frequency by extending the NTV action area. A proportional parameter isproposed which represents the combination of the two techniques. The systemperformance depends on the proportional parameter and this paper gives therelationship between the performance and the proportional parameter to provide thebasis for selecting the combination of the two modulation methods.
The impact of unbalanced load on the NP voltage balance is analyzed and theconclusion that the unbalanced load can increase the ripple amplitude and change theripple frequency is obtained. Then the intervals where NTV can control the NPvoltage balance under unbalanced load are explored, and the hybrid SVPWM methodwhich is applicable to the unbalanced load is presentd.
Last, the maximum and minimum values of the NP current during the wholefundamental cycle are explored. Base on that, the NP current, when the balancecontrol is put into the NTV method, can be obtained. Various extension modes areanalyzed and the optimal extension mode is presented. Then, this paper compares five typical extension modes. This paper explains the asymmetry of the output phasevoltage, and the reason that the output voltage level of inverter is not constant isindicated by comparing the ideal operation condition and the practical operationcondition.
针对中点电压低频振荡问题，本文介绍了NPC三电平逆变器的拓扑结构和工作原理，利用三电平空间矢量图描述空间矢量的分布。在此基础上，详细分析最近三矢量法，并对加入中点电压平衡控制前后的最近三矢量法进行对比分析。同时，定义虚拟空间矢量，研究虚拟空间矢量法的具体实现过程，并对最近三矢量法和虚拟空间矢量法的优缺点进行对比分析。
针对虚拟空间矢量法开关频率较高问题，分析冗余小矢量占空比系数变化规律，确定最近三矢量法能够实现中点电压平衡控制的区域。在此基础上，综合最近三矢量法和虚拟空间矢量法的优点，提出比例可调混合SVPWM方法，该方法通过拓展最近三矢量法的作用区域，在限制中点电压振荡幅值的同时，能够有效减小开关频率。同时，定义一个比例参数，研究中点电压振荡幅值和总谐波含量与比例参数的关系，为实际应用中比例参数的选择提供理论依据。实验验证理论分析的正确性。
针对不平衡负载对中点电压的负面影响，利用对称分量理论将负载电流进行分解，研究负序电流对中点电压平衡的影响，得出不平衡负载会使振荡幅值增加、振荡频率发生变化的结论。在此基础上，具体分析不平衡负载作用下最近三矢量法可平衡控制区域的分布，提出适用于不平衡负载的改进比例可调混合SVPWM方法，仿真结果表明，该方法能够获得较好的中点电压平衡控制性能。
利用中点电流可得到的最大值和最小值，确定最近三矢量法在加入中点电压平衡控制后的中点电流。利用此中点电流，提出最优拓展方式，将最优拓展方式与其它四种拓展方式进行对比分析。同时对实验中出现的相电压输出波形不对称和线电压电平不恒定现象进行分析研究，并阐述上述现象出现的原因。
Multilevel converters have attracted more and more attention in high-powerapplications due to low voltage across the power device, improved output voltagequality and low dv/dt. Among them, the neutral-point-clamped (NPC) three-levelinverter is a most widely used multilevel converter topology, which has low hardwarerequirement. The disvantage of NPC three-level inverter is the presence oflow-frequency voltage oscillations on the NP under the conditions of high modulationindex and low power factor. And the ripple amplitude increases and the ripplefrequecncy changes under unbalanced load. This paper explores above issues.
This paper introduces the topology and the principle of NPC three-level inverter,and analyzes the vector distribution based on the space vector diagram. Then thenearest-three-vector (NTV) technique is described and the NTV methods before andafter adding balance control are compared. The nearest-three-virtual-vector (NTV2)technique is introduced based on the definition of virtual vector.
Based on the analysis of the duty-cycle coefficients of the redundant smallvectors, the intervals where NTV can control the NP voltage balance in a fundamentalcycle are obtained. This paper combines NTV2with NTV and proposes a hybridSVPWM method. This method can limit the NP ripple amplitude and reduce theswitching frequency by extending the NTV action area. A proportional parameter isproposed which represents the combination of the two techniques. The systemperformance depends on the proportional parameter and this paper gives therelationship between the performance and the proportional parameter to provide thebasis for selecting the combination of the two modulation methods.
The impact of unbalanced load on the NP voltage balance is analyzed and theconclusion that the unbalanced load can increase the ripple amplitude and change theripple frequency is obtained. Then the intervals where NTV can control the NPvoltage balance under unbalanced load are explored, and the hybrid SVPWM methodwhich is applicable to the unbalanced load is presentd.
Last, the maximum and minimum values of the NP current during the wholefundamental cycle are explored. Base on that, the NP current, when the balancecontrol is put into the NTV method, can be obtained. Various extension modes areanalyzed and the optimal extension mode is presented. Then, this paper compares five typical extension modes. This paper explains the asymmetry of the output phasevoltage, and the reason that the output voltage level of inverter is not constant isindicated by comparing the ideal operation condition and the practical operationcondition.
引文
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