Abstract: The excitation transformer is an important part of the excitation system of the hydropower plant, that provides the excitation current to the rotor through the three-phase full bridge rectifier circuit. There are overvoltage exist during the using of the excitation transformer, such as atmospheric overvoltage, fault overvoltage, operation overvoltage, and it will also have commutation overvoltage produced by the rectifier circuit, so the work condition of the excitation transformer is worse than the normal transformers, the requirement of the insulation construction is stricter too. Three large hydropower plant accidents caused by HV winding failure of the excitation transformer with the type of DCB-9 is analyzed, the reason of the accident is researched, and it provide some experiences for the same type of transformer to prevent this kind of accident.