Abstract: In a power plant, the water spurt from crack in the pipe caused an instant earthing fault in 110kV line, and the zero sequence differential protection of start and standby auxiliary transformer No.1 was tripped incorrectly and caused the accident scope enlarged. The phenomenon of the accident and the accident treatment process were expounded. The experiences of the accident treatment were summarized. The problems exposed in the accident were analyzed. According to the protection action report and the accident field situations, the accident developing process and the action behavior of the relay protection were analyzed; and some concrete preventive measures were given.

Key words: 110kV line earthing, zero sequence differential protection of start and stand-by auxiliary transformer, water spurt from crack in the pipe, protection incorrect tripping, unit power interruption