An N-2 fault in Sichuan grid is described. The related devices including protective relay, security and stability control system and under frequency load shedding device operated correctly. The master station of the security and stability control system identified fault pattern correctly with the emergency situation which has over 50 percent power gap and isolated island. The system sent load cutting instructions correctly according to the pre-defined strategy table. And then the under frequency load shedding device cut down additional required load. The three lines of defense coordinated correctly and clearly to avoid power system collapse. It is proved that coordination of three lines of defenses is very important and has the ability to handle multiple faults, prevent collapse of power system, which is the foundation for system operating safety. The effective measures must be taken to strengthen the construction of three defense lines for increasing the operating safety level.%分析了四川电网发生的一次N-2故障.相关继电保护装置快速正确动作切除故障,在受端电刚成为孤网、功率缺额接近地区负荷的50％紧急情况下,安全稳定控制系统主站正确判断出故障形态,根据策略表发出切负荷指令,各切负荷子站正确切除负荷,低频减载装置又补充切出部分负荷.电网的三道防线在故障的各阶段分工明确.成功地避免了一次电网瓦解事故:证明了三道防线协同配合对电网承受多重故障的考验、防止系统崩溃具有十分重要的作用,是电网安全稳定运行的重要基础.加强三道防线建没,以提高电网的安全稳定运行水平.
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