微电网直流输电MMC环流抑制研究

Research on MMC circulation suppression for microgrid DC transmission

模块化多电平换流器(Modular Multilevel Converters,MMC)具有维护方便,易扩容等优点,在柔性直流输电(High Voltage Direct Current,HVDC)、电网谐波治理等领域得到广泛应用。但其子模块中的分散电容,在实际使用中会产生谐波环流,影响了模块电容电压波动,降低换流器的安全裕度和经济性。文章首先介绍了MMC组成结构;然后通过建立桥臂瞬时功率方程,分析环流产生机理,并得出桥臂环流中存在二次谐波分量的结论,通过控制直流侧电压与桥臂等效输出电压之间的差值,调节环流中的谐波分量,进而将现阶段国内、外环流抑制方法分为间接环流抑制方法与直接环流抑制方法,并对环流的利用也做了相关阐述。最后对环流抑制存在的问题进行总结与展望。

With the development of offshore wind DC transmission technology,large-capacity converters have ushered in a period of rapid development.Modular multilevel converters(MMC)have developed rapidly in recent years due to their unique modular cascade approach,which brings advantages such as easy maintenance and easy expansion,and are widely used in flexible DC transmission High voltage direct current(HVDC),grid harmonic control and other fields are widely used.However,the dispersion capacitor in its sub-module,in actual use will produce harmonic loop current,loop current will affect the module capacitance voltage fluctuations,reducing the safety margin and economy of the converter.In this paper,the MMC composition structure is firstly introduced.Then,by establishing the instantaneous power equation of the bridge arm,we analyze the mechanism of loop current generation and conclude that there is a second harmonic component in the bridge arm loop current.Based on the existing theory,the harmonic components in the circulating current are regulated by controlling the difference between the DC side voltage and the equivalent output voltage of the bridge arm,and then the current domestic and foreign circulating current suppression methods are divided into indirect and direct circulating current suppression methods,and the utilization of the circulating current is also explained.Finally,the remaining problems of circulating current suppression are summarized and prospected.