Abstract: In March 24, 2009, during connection into the power grid of the Unit No.2 of Hunan Huadian Changsha Power Generation Limited, the connected generator was disconnected due to the incorrect tripping of the overexcitation protection. According to the record diagram of the protection fault recorder, the protection acting report and the tracking records of the whole connection operation process, the reasons of the protection incorrect tripping were analyzed; and the reasons of the endless loop of the sequential control logic were analyzed too. The problems existed in generator deeply underexcitation operation were discussed. Some relevant preventive measures were adopted in accordance with site practical situation.

Key words: unit connection into grid, generator over-excitation protection, protection incorrect tripping, reason analysis, preventive measure