MMC型VSC-HVDC系统电容电压的优化平衡控制

Optimized Capacitor Voltage Balancing Control for Modular Multilevel Converter Based VSC-HVDC System

介绍模块化多电平换流器(modular multilevel converter,MMC)电容电压平衡的原理以及子模块(sub-module,SM)电容的充放电过程。指出传统的电容电压平衡控制下,子模块投切较频繁,器件开关频率较高,会造成较大的开关损耗。针对传统方法的问题,提出一种适合MMC型直流输电系统的电容电压优化平衡控制策略,将平衡控制的重点放在电容电压越限的子模块上。对电容电压未越限的子模块,优化策略通过引入保持因子使具有一定的保持原来投切状态的能力,以降低开关器件的开关频率。使用PSCAD/EMTDC对优化控制策略进行仿真,结果表明优化平衡控制策略与传统方法相比,可以在基本不增加电容电压波动的前提下,显著降低器件的开关频率。

This paper investigated the capacitor voltage balancing problem of the modular multilevel converter（MMC） and the charge and discharge process of sub-module capacitors.The requirement to reduce the switching frequency of power electronic devices is not satisfied in the conventional capacitor voltage balancing control,since it results in high switching losses.To solve this problem,an optimized capacitor voltage balancing control for MMC based VSC-HVDC transmission system was proposed.The optimized voltage balancing control focuses on the sub-modules whose capacitor voltage exceeds the limits,while the switching states of the other sub-modules are maintained to some degrees by employing the maintaining factor.Therefore,the switching frequency of the power electronic device is reduced.The performance of the optimized capacitor voltage balancing control was evaluated with a time-domain simulation in PSCAD/EMTDC.The simulation results show that the optimized voltage balancing control,compared with the conventional one,can more significantly reduce the switching frequency of power device,without noticeablly increasing the capacitor voltage ripples.