Abstract : Through a fault analysis of the bus differential protection at Wan'an Hydropower Plant, this paper identifies potential problems with the plant's bus differential protection system, clarifies some questions arising from previous operation of the system, and arrives at more accurate results. Keywords: Relay protection; Bus differential protection; Fault; Analysis 0 Introduction On February 20, 1998, the 220kV bus differential protection at Wan'an Hydropower Plant tripped. An accident analysis report was subsequently prepared, but some questions remained unanswered. On April 26, 2000, the 220kV bus differential protection tripped again, failing to generate an output due to voltage blocking, but exhibiting similar symptoms to the previous trip. Therefore, we conducted a thorough analysis of the fault situation of the bus differential protection on April 26, 2000. 1. Fault Situation On April 26, 2000, with the approval of the provincial dispatch center, the operators prepared to switch all incoming and outgoing lines connected to the 220kV II section busbar to the I section busbar for operation, so that the 231DL circuit breaker could be deactivated for maintenance personnel to handle oil leaks. After 9:00 AM, the operators began the operation. After switching the 220kV busbar differential protection panel switch handle QK to single busbar operation mode, they removed the 231DL operating fuse and switched the No. 2 main transformer, Wanhu II line, and Wanji II line to the I section busbar. Just as they were about to activate the 31DL operating fuse, the monitoring system reported an operation signal from the 220kV I section busbar differential protection. The operators checked the 220kV busbar differential protection panel and found an abnormality in 3CKJ. The 1XJ signal relay had been deactivated, so the operation was stopped. Finding that 3CY,J was still abnormal, the 220kV busbar differential protection was deactivated. [b][align=center]For details, please click: A Fault Analysis of 220kV Busbar Differential Protection[/align][/b]