The AC power supply system belongs to facility of the utility system of the hydropower plant. The automation elements, relay pro-tection device, emergency lightning power, etc, all are supplied by the AC power supply system. Therefore, liability of the AC power supply system is critical for the hydropower plant and distribution station to operate safely. It guarantees the safety operation of the plant and the sta-tion. Structure and principle of the AC power supply system in this paper are introduced. Furthermore, an incident and handling of the unit tripping by failure of the AC power supply system are analyzed.%交流电源系统属于水电厂的公用系统设备，机组各自动化元件、继电保护装置、事故照明用电等均取自交流电源系统，其可靠与否对发电厂和变电所的安全运行起着至关重要的作用，是安全运行的保证。文章介绍了某水电厂交流电源系统结构及工作原理，并分析了一起因交流电源系统故障导致机组跳闸的事故及其处理方法。
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