论文主要对模块化多电平变流器（Modular Multilevel Converter，MMC）进行研究，首先分析了MMC的数学模型及运行原理，研究了MMC的调制策略、子模块电容均压策略以及系统控制策略，推导了不同调制策略下输出电压波形的傅里叶级数表达式，并计算了谐波成分。由于MMC通过模块级联易于实现高压，因此在高压大容量场合得到广泛应用。但由于全控器件工作电压比较低，因此要实现高电压，需要大量模块级联。这将导致变流器的体积大幅增加。而在中低压的配网中，更是因为MMC体积大而影响了其应用。如果采用更高电压等级的开关器件或基于器件串联提高MMC子模块的输出电压，则可以大幅减少子模块数量，从而大幅减小MMC变流器的体积，实现紧凑化。但子模块电压提高后，子模块数量减少，模块dv/dt增加，一方面导致交流电流波形变差，同时也造成严重的电磁干扰问题，引起电磁噪声增加，同时过大的dv/dt也增加了交流侧电抗器设计的难度。为了解决上述问题，论文深入研究了MMC变流器的电磁噪声干扰问题，提出了随机零序分量注入的PWM（Pulse Width Modulation, PWM）调制策略。基于随机过程理论，计算了载波移相调制、随机开关频率PWM、随机脉冲位置PWM及随机零序分量注入PWM的功率谱密度。在单端并网逆变器仿真模型上验证对比不同调制策略对谐波能量的分散程度，考察最高输出电平、响度级等电磁噪声干扰指标，证明了随机零序分量注入PWM方式的简洁性、实用性与有效性。最后，分析随机零序分量注入PWM调制策略的典型应用场合，并以南方电网永富直流工程为背景，针对工程出现的运行问题，探讨了高压大容量的混合直流输电方案。搭建了传统直流与混合直流的仿真模型，对比分析了逆变侧发生不对称短路故障后的直流响应特性与逆变侧交流暂态过电压及畸变特性，验证了混合直流输电方案的有效性。在混合直流输电方案中对比了不同调制策略的性能，验证了论文所提调制策略抑制电磁噪声干扰的有效性。本文针对MMC运行时产生的电磁噪声干扰问题，提出了基于随机零序分量注入PWM调制策略。并在提出的混合直流仿真模型中，对比不同调制策略对谐波能量的分散程度，验证了所提策略具有良好降噪效果。
Power electronic converters have many advantages such as small size, high efficiency and large capacity. Therefore, it is widely used in high voltage and high capacity area such as industry, transmission and distribution. The traditional line commutated converter has the problems such as commutation failure, voltage and current ripple is large and so on. However, the modular multilevel converter which is based on the fully controlled power electronic device can be controlled flexibly, so it can be easily applied to high voltage large capacity transmission occasions, which has good prospects.At first, the related theory of modular multilevel converter is studied. The operating theory and the mathematical model of the converter are analyzed. The modulation strategy, the capacitor voltage balancing stategy and the control strategy of MMC are studied. The expressions of the Fourier series of the output voltage waveforms under different modulation strategies are deduced and the harmonic components are calculated.Then, because it is easy for the modular multilevel converter to increase voltage level, the modular multilevel converter is widely used. But due to the low voltage grade of the fully controlled electronic device, it requires a large number of modules cascade to achieve high voltage. It would result in a significant increase in the volume of the converter. Even more in the low-voltage distribution grid, MMC’s big size limits its application. If a higher voltage level device or a device in series to increse the output voltage of MMC sub-modules, the number of sub-modules can be significantly reduced, then the size of MMC converter is greatly reduced to achieve compacting. But when the sub-module voltage increases, the number of sub-modules would decrease, and the dv/dt of MMC would increase. On one hand, it leads to deterioration of the AC current waveform, on the other hand, it causes serious electromagnetic interference problems, and increases electromagnetic noise. What’s more, too large dv/dt also increased the difficulty in the design of the AC side reactor. In order to solve the above problems, this paper deeply studies the electromagnetic noise interference problem of MMC converter, and puts forward the PWM (Pulse Width Modulation) modulation strategy of random zero sequence component injection.Based on the stochastic process theory, the power spectral density of the carrier phase shift modulation, the random switching frequency PWM, the random pulse position PWM and the random zero sequence component injection of PWM are calculated. The dispersion degree of harmonic energy with different modulation strategies is verified on the simulation model of single-ended grid-connected inverter. The electromagnetic noise interference indexs of the highest output level and loudness level are investigated, which proves the simplicity, practicality and effectiveness of the random zero sequence component injection PWM.Finally, the application of random zero sequence component injection PWM is analyzed, and a high pressure and high capacity hybrid HVDC transmission system is put forward according to the problem of Yongfu HVDC project of South China Power Grid. The traditional DC and hybrid DC simulation model are built, and the DC response characteristics and the transient overvoltage and distortion characteristics of the inverter side with asymmetrical short circuit fault are compared and analyzed. The effectiveness of the hybrid DC transmission system is verified. Different modulation strategies are applied, which verifies the reliability of the proposed modulation strategy to suppress electromagnetic noise interference in the real project.In this paper, an optimized modulation strategy is proposed to solve the problem of electromagnetic noise interference when the modular multilevel converter is running. Combined with the fault recovery characteristics of the HVDC system, a hybrid HVDC system based on the modular multilevel converter is proposed. Then, the degree of dispersion of the harmonic energy is compared with different modulation strategies. The proposed strategy is verified that it can reduce the electromagnetic noise interference.
