Abstract: To the longitudinal differential protection operation accident of 20 MW hydrogenerator at Longkou Hydropower Station, confirm the cause of the accident is the V-phase insulation breakdown of the cold-shrinkable terminal head on the high-voltage side of the hydrogenerator excitation transformer after the test of insulation resistance of the hydrogenerator electrical primary equipment and the analysis of accident action report. Replacing the cold-shrink terminal of the high-voltage side of the excitation transformer with a thermal shrink terminal, and increasing the SOE startup source in the PLC electrical shutdown process of the hydrogenerator monitoring system. After processing, the hydrogenerator runs in a good condition.