Abstract: A number of inter-turn short-circuit accidents have occurred in recent years in the primary winding of voltage transformer (TV) at generator terminal, which often causes slow fuse melting due to the large short-circuit current. In this paper, a calculation formula of TV secondary winding voltage is derived respectively for TV’s primary winding inter-turn short-circuit and slow fuse melting, and it is concluded that the secondary winding voltage will decrease after a fault occurs. An analysis is made on the bugs in the TV disconnection logic in ABB Unitrol F excitation regulator, and a study is conducted on the mechanism of over-excitation protection and strong excitation of the excitation regulator caused by slow fuse melting. The theory is verified through a case study of an accident. Finally, the Matlab/Simulink simulation is used to verify the theory, and the simulation result proves the correctness of the theory.