Abstract: The slow-melting fault of the primary-side high-voltage fuse of voltage transformer (PT) is a relatively concealed abnormal working condition in the auxiliary power system of coal-fired power units. The faulty phase presents a slight and long-term drop in secondary voltage, which cannot be reliably identified by conventional PT disconnection criteria, and may easily cause mal-operation of low-voltage protection and abnormal action of the auxiliary power fast switching device. Aiming at the A-phase slow-melting fault of the high-voltage fuse for the PT on the 10kV B-section bus of Unit 1 in a coal-fired power unit, this paper analyzes the asymmetric characteristics of secondary voltage during the fault, and compares the electrical parameter differences among slow-melting fault, single-phase grounding fault and complete PT disconnection fault. The physical process and inducing factors of fuse slow-melting are elaborated. On this basis, an online fuse replacement scheme without unit shutdown is formulated and implemented, and the complete disposal procedures are proposed, including exiting relevant protection and automatic devices, measuring fuse insulation resistance, uniformly replacing three-phase fuses and restoring power supply. The comparison of conduction resistance and insulation resistance of fuses before and after replacement verifies the accuracy of fault location. Finally, preventive measures are put forward from four aspects including fuse type selection, regular detection, protection logic optimization and operational monitoring, which can provide a reference for the disposal of similar PT slow-melting faults of auxiliary power buses.