多端柔性直流输电系统直流电压控制无缝切换方法

Seamless Switching Method for DC Voltage Control of Multi-terminal Flexible DC Transmission System

维持直流侧电压稳定是多端柔性直流输电系统可靠运行的关键。针对某些MMC需要灵活改变控制方式的系统,本文提出一种新型的控制策略,其将定直流电压控制模式、定有功功率控制模式以及下垂控制模式相结合设计出多模式控制器,只需通过改变控制模式选择系数,即可实现三种控制模式间的无缝切换与下垂控制模式自身的转换。该策略不仅克服了下垂控制方法存在的无法使MMC功率稳定在某一需求值的缺点,而且减小了利用硬件开关切换时产生的波动与振荡问题,进而提升了系统的暂态特性。最后,搭建了五端柔性直流输电系统RT-LAB仿真模型,验证了所提控制策略的有效性。

Maintaining the stability of DC voltage is the key to the reliable operation of a multi-terminal flexible DC transmission system.For some systems that need to flexibly change the control modes of a modular multi-level converter(MMC),a novel control strategy is proposed in this paper,under which a multi-mode controller is designed by combin-ing constant DC voltage control mode,constant active power control mode,and droop control mode.By only changing the selection coefficients of the control mode,this controller can realize the seamless switching of these three kinds of control modes,as well as the self-transformation of droop control mode.In this way,the proposed strategy not only over-comes the disadvantage of the droop control method(i.e.,the power of MMC cannot be stabilized at a preset value),but also reduces the fluctuation and shock generated when switching a hardware switch,thus improving the transient characteristic of the system.Finally,an RT-LAB simulation model of a five-terminal flexible DC transmission system is built,which verifies the effectiveness of the proposed control strategy.