背靠背三电平中点箝位型（NPC）变换器在海上风电、轧钢、油田等场合得到广泛应用。然而PWM变换器产生的高频、高幅值的共模电压会对电机轴承造成严重腐蚀，影响整个系统的安全稳定运行。鉴于此，本文主要针对背靠背三电平变换器的共模电压抑制问题展开深入研究。本文首先分析了背靠背双PWM变换器共模电压的产生机理，并分别从零序电压、脉冲相位、调制信号三个自由度出发分析了共模电压抑制的基本思路，为后续背靠背三电平NPC变换器共模电压抑制方法的研究奠定了基础。将上述基本思路扩展到背靠背三电平NPC变换器，首先在载波同相层叠PWM的基础上充分利用零序电压自由度，提出了一种基于整流和逆变双侧零序电压注入的共模电压抑制和中点电位平衡综合控制方法。第一步找出各个载波周期中可抑制共模电压幅值至Vdc/6的所有关键零序电压组，并以中点电位平衡控制为目标选取一个最佳关键零序电压组来抑制共模电压；第二步在不影响共模电压的前提下，通过在两侧再次注入相同的零序电压来进一步优化中点电位平衡的效果，最终实现了共模电压抑制和中点电位平衡的统一优化控制。为了进一步提高共模电压的抑制能力，继续利用脉冲相位自由度提出了一种在载波反相层叠PWM基础上进行脉冲移相的共模电压消除方法，通过调整每相电压脉冲的相位，使得任意正电平脉冲的上升沿和下降沿都与负电平脉冲的下降沿和上升沿对齐，理论上可实现共模电压的完全消除。同时，继续采用在整流侧和逆变侧同时注入相同零序电压的方法控制母线中点电位，可实现中点电位平衡与共模电压消除的解耦控制。其不足之处是谐波性能较差。为了进一步优化共模电压消除方法的谐波性能，最后提出了一种基于调制信号分解的共模电压消除方法。将每相实际参考电压分解为两个虚拟参考电压之差，并将三电平相电压用这两个虚拟参考电压与三角波比较之后得到的虚拟相电压相减来合成，最终实现了背靠背三电平变换器共模电压的完全消除。这种调制方法将实际三电平参考电压与两个层叠三角载波的比较转换为两个两电平虚拟参考电压与一个三角载波的比较，控制更加简单，而且在相同载波频率下谐波性能也更好。综上所述，本文针对背靠背三电平变换器从不同的自由度出发提出了多种共模电压抑制方法，在共模抑制效果、谐波性能、开关动作次数等方面各具特色。随着以SiC为代表的宽禁带半导体器件的推广应用，其高开关速度带来的共模电压问题也会越来越突出，本文的研究成果必将有广阔的应用前景。

Back-to-back three-level neutral point clamped (NPC) converters are widely used in offshore wind power generation, steel rolling, oilfields, and other applications. However, the high-frequency and high-amplitude common-mode voltage (CMV) generated by the PWM converter will cause serious corrosion to the motor bearings, affecting the secure and stable operation of the overall system. In view of this, this dissertation mainly conducts in-depth research on the CMV reduction of back-to-back three-level NPC converters.The CMV generation principle of the back-to-back dual PWM converter is firstly analyzed in this dissertation, and the basic ideas of CMV reduction are studied from three degrees of freedom, i.e., zero-sequence voltage (ZSV), pulse phase, and modulation signal, which lay the foundation for the research on CMV reduction methods of back-to-back three-level NPC converters.These basic ideas are extended to back-to-back three-level NPC converters. First of all, the ZSV degree of freedom is fully utilized on the basis of phase disposition PWM (PDPWM), and a comprehensive control method for CMV reduction and neutral-point potential (NPP) balance by injecting ZSV into the rectifier-side and the inverter-side is proposed. The first step is to find out all the key ZSV groups that can reduce the CMV amplitude to Vdc/6 in each carrier cycle, and select an optimal key ZSV group to reduce the CMV with the goal of NPP balance. The second step is to further optimize the effect of NPP balance by injecting the same ZSV into both the rectifier-side and the inverter-side, under the premise of not affecting the CMV reduction performance. Finally, the unified control of CMV reduction and NPP balance is achieved.In order to further improve the CMV reduction performance, utilizing the pulse phase degree of freedom, a CMV elimination method by shifting the pulse phase under phase opposition disposition PWM (PODPWM) is proposed. By adjusting the phase of each voltage pulse to align the rising and falling edges of the positive pulse with the falling and rising edges of the negative pulse, the CMV can be eliminated in theory. At the same time, the method of injecting the same ZSV into both the rectifier-side and the inverter-side is adopted to control the NPP balance, which can realize the decoupling control of NPP balance and CMV elimination. However, the deficiency is the relatively poor harmonic performance.In order to further optimize the harmonic performance for CMV elimination, a CMV elimination method based on the modulation signal decomposition is proposed at last. In this method, the actual reference voltage of each phase is decomposed into the difference between two virtual reference voltages, and the actual phase voltage of the back-to-back three-level NPC converter is synthesized by subtracting two virtual phase voltages obtained by comparing the virtual reference voltage with the triangular wave. In this way, the CMV of the back-to-back three-level converter is eliminated. This modulation method converts the comparison of an actual three-level reference voltage with two stacked triangular carriers into the comparison of two two-level virtual reference voltages with a triangular carrier, which is simpler to control. In addition, this modulation signal decomposition method has better harmonic performance under the same carrier frequency.In summary, this dissertation proposes a variety of CMV reduction methods for the back-to-back three-level NPC converter from different degrees of freedom, each has different characteristics in CMV reduction effect, harmonic performance, switching frequency, etc. With the popularization and application of wide-bandgap semiconductor devices represented by silicon carbide, the CMV problem caused by its high switching speed will become more and more serious, and the research results of this dissertation will have broad application prospects.