摘要 提出了一种模块组合多电平变换器 (MMC) 应用于静态无功发生器 (Static Var Generator, SVG) 的新技术. MMC 由于具有公共的直流母线, 可以实现四象限运行. 又因为其具有良好的可扩展性, 故障无损性, 故必将成为未来中, 高压功率变换的重要研究方向. 然而, 很少有人对其在静止无功补偿方面进行研究. 本文基于 MMC 电路的特点和基本工作原理, 提出一种基于 MMC 的静态无功生器 (M-SVG) 及其相应的控制策略. 通过一台三电平单相 MSVG 样机的小功率实验结果, 详细地分析了其动态特性, 表明了本文控制策略的有效性以及 M-SVG 的可行性.

The floating dc capacitor voltage of modular multilevel converter based STATCOM (M-STATCOM) contains a large amount of low-order harmonic components, which produce current ripple in leg currents, and then greatly affect the output performance and safe operation of M-STATCOM. However, fewer papers have made explicit harmonic analysis on the floating capacitor voltage of M-STATCOM. Detailed floating capacitor voltage computation has been developed using the switching functions concept. Moreover, a quasi-harmonic elimination technique has been proposed to achieve low-order harmonic elimination of leg currents without any external circuit. When the modified switching function is applied, the output leg currents of M-STATCOM are independent from the voltage fluctuation of floating capacitors. The effectiveness of the proposed quasi-harmonic elimination technique and the feasibility of theoretical analysis were verified by the simulation results and experimental results.

This paper presents the structural characteristics and operating principles of modular multilevel converters. Considering the actual requirements, SPWM pulse sequence suitable for modular multilevel converter is analyzed in detail. Two pulse modulation techniques, carrier phase shifted (CPS) technique and pulse phase shifted (PPS) technique, are proposed based on the existing modular multilevel converter prototype and control system hardware. Moreover, the principles of the two corresponding modulation schemes are studied by means of simulation and experiment. Study results indicate the difference between CPS and PPS, that is, CPS generates better SPWM pulse and features with higher dc voltage utilization compared with PPS. Meanwhile, CPS will take up more resources of control chips, and has limitations caused by limited expandability of DSP. Compared with CPS, PPS has better expandability, more simply control logic, as well as less control resources requirements, so it is suitable for multiplex SPWM pulse generating.

Modular multilevel converter (MMC) is one of the excellent selections for high voltage and high power conversion applications without transformers. It provides the advantages such as high modularity, availability, low generation of harmonics, etc. However, the circulating current distorts the leg currents and increases the rated current of power devices, which further increases system cost. By controlling the role of circulating current as current controlled voltage sources (CCVS), this paper derived the circulating current model of MMC, which shows that both the upper and lower legs of the same phase can be controlled separately as two independent cascaded multilevel converters. Then the quantitative relationship between the circulating currents and dc-bus current had been derived and modified considering the actual characteristics of the circulation system. Further, this paper also revealed that MMC is suitable for the need to achieve negative sequence balancing applications.

A new modular multilevel converter (MMC) based static synchronous compensator (STATCOM) is proposed and hereafter called M-STATCOM. The new shunt compensator, features with the characteristics such as high modularity, availability, low generation of harmonics, etc., is well suitable for medium- or high-voltage utility grid without line-frequency transformers. M-STATCOM can not only be controlled in static var generating condition for reactive power compensation purpose, but also be suitable for the comprehensive mitigation of all the power quality problems under the three-phase unbalanced power systems. The structural characteristics and operating principles of M-STATCOM for both static var generating condition and comprehensive compensating condition are analyzed in detail. Moreover, two corresponding control schemes were proposed. The effectiveness of the proposed control strategy and feasibility of analysis is validated by the simulation results.

This paper presents a new technology of modular multilevel converter (MMC) for static var generator(SVG) application. MMC has a common DC bus, which makes it possible for achieving four-quadrant operation easily. Due to its scalability, and nondestructive fault behavior, it will become an important research direction in the field of future medium-voltage or high-voltage application. However, few papers have made explicit discussion on its reactive compensation applications. Based on analysis of the structural characteristics and operating principles of MMC, a control strategy of MMC based SVG(M-SVG) is proposed in this paper. The dynamic characteristics are explained in detail on a three-level three-phase M-SVG. The effectiveness of the proposed modulation strategy and the feasibility of M-SVG are verified by the low power experimental results.

本文提出一种基于模块组合多电平变换器(Modular Multilevel Converter,MMC)的新型综合补偿装置M-STATCOM,具有模块化、体积小、损耗低、等效开关频率高等优点,无需工频变压器即可直接应用于中高压电网。M-STATCOM不仅能工作于无功发生工况实现对电网的无功补偿,也适用于三相不平衡系统电能质量问题的综合治理应用。本文详细分析了M-STATCOM的结构特点和工作原理,在此基础上提出了无功发生工况和综合补偿工况两种工况下的控制方案。实验结果表明了本文提出控制策略的有效性和理论分析的正确性。

Modular multilevel converter (MMC) is one of the excellent selections for high voltage and high power conversion applications without transformers. It provides the advantages such as high modularity, availability, low generation of harmonics, etc. However, the circulating current distorts the leg currents and increases the rated current of power devices, which further increases system cost. By controlling the circulating current as CCVS (current controlled voltage sources), this paper derived the circulating current model of MMC. Then the quantitative relationship between the circulating currents and dc-bus current is derived and modified considering the actual characteristics of the circulation system. Further, this paper also reveals the existence of low-frequency components in the circulating currents. It means that MMC is suitable for the need to achieve negative sequence balancing applications.

A new modular multilevel converter is proposed for static synchronous compensator (STATCOM), which is hereafter called M-STATCOM. The new shunt comprehensive compensator, features with the advantages such as high modularity, availability, low generation of harmonics, etc. is suitable for medium- or high-voltage utility grid without line-frequency transformers. M-STATCOM can not only be controlled in static var generating condition for reactive power compensation purpose, but also be suitable for the comprehensive mitigation of all the power quality problems under the three-phase unbalanced power systems. The structural characteristics and operating principles of M-STATCOM for both static var generating condition and comprehension compensating condition are analyzed in detail. Moreover two corresponding control schemes are proposed. The effectiveness of the proposed control strategy and feasibility of analysis is validated by the experimental results.