多端柔性直流输电系统的启动控制策略

Start Control Strategy of MMC-MTDC System

为了平稳快速地启动基于模块化多电平换流器的多端柔性直流输电系统(MMC-MTDC),提出了一种启动控制策略。首先,为了最大限度地降低启动过程中直流电压的跌落,给出了一种直流侧预充电的有序解锁方案。其次,从充电等值电路的二阶RLC回路出发,推导出了限流电阻选取的表达式。随后,考虑交流系统的强弱情况,推导出了交流电压跌落比与系统短路比之间的关系,并给出了相应的启动控制策略。最后,在时域仿真软件PSCAD/EMTDC下搭建了5端系统进行仿真。结果表明:直流侧预充电的有序解锁方案能够最大限度地降低解锁过程中直流电压的跌落;以电压跌落0.1倍额定值为界限,当交流系统短路比(SCR)>3.5时,此系统连接的换流站适合交流侧预充电启动,否则,应采用直流侧预充电启动。

In order to start MMC-MTDC system smoothly and rapidly, we presented a start control strategy for the system, and proposed an ordered-unlock sub-modules method to minimize the DC voltage drop during the start-up stage. Accord- ing to the RLC equivalent charging circuit, we deduced an expression for selecting the current limiting resistor, and obtained the relationship between AC voltage drop ratio and short circuit ratio （SCR） considering the weakness of AC system. Then, the scheme framework and procedures of the proposed start control strategy were described. Finally, we conducted simulations using a digital model of five-terminal MMC-MTDC system in PSCAD/EMTDC. The results show that the proposed ordered-unlock sub-modules method can significantly decrease the voltage drop. Taking 0.1 times of rated voltage as the critical value, when the SCR of the AC system is higher than 3.5, the AC side pre-charging strategy is suitable for connecting the converter to this AC system. Otherwise, the converter should use the DC side pre-charging strategy.