极端环境下MMC子模块故障高可靠旁路保护策略

Bypass Protection Strategy with High Reliability for MMC Sub-module Faults in Extreme Environments

摘要:

电力系统空间范围大且各环节联系紧密，雷击、地震、极寒等极端环境易对电力设备产生直接和间接影响，并进一步引起系统电气量的剧烈变化，研究极端环境下MMC子模块的故障保护对提高电力系统的安全稳定运行水平具有重要意义。文中分析了极端环境下MMC子模块的故障特性，采用基于子模块电容电压比较的故障诊断方法，提出一种在子模块输出端口间加装晶闸管的极端环境下MMC子模块高可靠旁路故障保护策略：配置冗余供能电路确保子模块状态可控，利用晶闸管过电压击穿作为后备被动保护，结合子模块的多级保护和热、冷备用冗余子模块的投入，实现极端环境下对故障子模块的高可靠旁路保护。以±800 kV特高压柔性直流输电系统为算例，经仿真验证所提策略具有良好的可行性，能够提高极端环境下子模块故障保护的可靠性和速动性，同时对晶闸管击穿后流过桥臂电流进行热仿真，发现已超过其工作温度限制，为满足长期旁路通流要求，加装晶闸管击穿后须通过散热器进行强制冷却。

Abstract:

Power system has a large spatial range and all links are closely connected. Extreme environments such as lightning strikes, earthquakes, and extreme cold are likely to have direct and indirect impacts on power equipment, and further cause drastic changes in electrical quantity of the system. Studying fault protection of MMC sub-modules in extreme environments is of great significance to improve the safe and stable operation level of power system. The fault characteristics of MMC sub-modules in extreme environments are analyzed, and a fault diagnosis method based on sub-module capacitance-voltage comparison is used to propose a bypass fault protection strategy with high reliability for MMC sub-modules in extreme environments where thyristors are installed between the output ports of sub-modules. The redundant energy supply circuit is configured to ensure the controllable state of sub-modules, and the overvoltage breakdown of thyristor is used as a backup passive protection. Combined with the multi-level protection of submodules and the input of hot and cold backup redundant submodules, it can realize the highly reliable bypass protection for faulty sub-modules in extreme environments. Taking the ±800 kV UHV HVDC flexible transmission system for example, the proposed strategy has good feasibility and can improve the reliability and quickness of sub-module fault protection in extreme environments. In order to meet the long-term bypass flow requirements, forced cooling must be carried out through the radiator after the breakdown of the installed thyristor.