Abstract: In a certain power plant,after an automatic air switch in secondary circuit of potential transformer of 6kV auxiliary working bus section of No.7 unit trip was discussioned. The distributed control system had mistake the fault for that the voltage of auxiliary working bus section is cut off, and sent out operation instruction to trip the 6kV power breaker. In the process of voltage interrupt and transfer of working bus section, the coal feed breaker tripped due to the momentary voltage interrupt, and this lead the main fuel tripping protection breaker to trip. At last, the Unit was disconnected with the system. The reason of this accident outage was analyzed, and the preventive measures of similar accident were proposed.

Key words: secondary circuit of potential transformer, automatic air switch, voltage interrupt, shut down, analysis, preventive measures