柔性直流输电系统MMC换流阀闭环充电策略

Closed-loop charging strategy of MMC in flexible DC transmission system

随着柔性直流输电系统的快速发展与广泛应用,其启动工况愈发具有多样性,MMC换流阀传统开环充电策略需要综合考虑系统状态变量进行参数配置,且配置难度大。由此提出了通用性的换流阀闭环充电控制策略,无需检测系统状态即能将子模块稳态电压控制在额定值,具有较好的适应性。首先,分析了柔性直流输电系统的三类充电回路及主动均压充电策略。其次,基于子模块特性不一致的均压需求分析,提出了完整的通用性闭环充电策略及对应的柔性直流输电系统的协调启动策略。最后通过PSCAD/EMTDC搭建了三端直流电网模型,仿真验证了闭环充电策略。结果表明闭合充电策略通用于各类换流站的启动充电过程,且可将子模块电压平稳充电至额定值,同时不存在桥臂过流及子模块过压等现象。

With the rapid development and wide application of the flexible DC transmission system, its starting conditions are more and more diverse. Traditional open-loop charging strategy of MMC valve requires comprehensive consideration of the system state variables for parameter configuration, and the configuration is difficult. Therefore, a general-purpose closed-loop charging strategy for the converter valve is proposed, which can control the steady state of the sub-module at a rated voltage without detecting the system state, and has a good adaptability. First of all, three kinds of charging loops and active voltage equalization charging strategies of flexible HVDC transmission systems are analyzed. Secondly, based on the analysis of equalization requirements of submodules with inconsistent characteristics, a complete universal closed-loop charging strategy and the coordination start strategy of multi-terminal flexible DC transmission system are proposed. Finally, a three-terminal DC grid model is built through PSCAD/EMTDC. Simulation results show that the closed-loop charging strategy can be generally applicable to the startup charging process of various converter stations, ensuring the submodules steadily charge to the rated voltage without bridge arm overcurrent and submodules overvoltage.