MMC-LCC混合直流输电系统分段下垂控制策略

由模块化多电平换流器(modular multi-level converter,MMC)和电网换相换流器(line commutated converter,LCC)构成的混合直流输电系统中,LCC换相失败严重影响系统的安全稳定运行。文中首先分析MMC-LCC混合直流输电系统换相失败时的电流特性以及交直流电压特性。其次,考虑调制比对半桥型MMC的影响,采用MMC电压改善控制策略拓展电压调制比的可行域。然后,提出MMC电压分段控制策略,根据交流电压跌落程度的不同,分别设计直流电压参考值的调节方法,优化混合直流输电系统电压控制逻辑,实现MMC电压在正常运行与故障情况下的有效切换。最后,在MATLAB/Simulink中搭建MMC-LCC混合直流输电系统模型,对交流电压不同跌落程度进行仿真,结果表明所提控制策略能在实现故障穿越的同时提高直流电压控制精度,增强系统稳定性。

Segmented droop control strategy of MMC-LCC hybrid DC transmission system

In the hybrid direct current (DC) transmission system composed of modular multi-level converter (MMC) and line commutated converter (LCC),the commutation failure of LCC seriously affects the security and stability of the system. Firstly,the current characteristics and alternating current (AC)/DC voltage characteristics of MMC-LCC hybrid DC transmission system are analyzed when commutation failure occurs. Secondly,considering the influence of modulation ratio on half-bridge MMC,the feasible region of voltage modulation ratio is extended by using enhanced control strategy of MMC voltage. Then,the segmented control strategy of MMC voltage is proposed. According to the different degrees of AC voltage sag,the adjustment methods of DC voltage reference value are designed respectively,and the voltage control logic of hybrid HVDC transmission system is optimized to realize the effective switching of MMC voltage between normal operation and fault condition. Finally,the model of MMC-LCC hybrid DC transmission system is built in MATLAB/Simulink,and the simulation results show that the proposed control strategy can improve the DC voltage control accuracy and system stability while achieving fault ride through.