The on-load tap changer (OLTC) was the only moving components in transformer. It was a precision electromechanical equipment with high reliability requirements. The main types of OLTC were oil arc extinguishing type OLTC and vacuum type OLTC. The vacuum type OLTC was more and more widely used because of its strong arc extinguishing ability. Especially for the converter transformer in High Voltage Direct Current (HVDC) project, due to its characteristics of large load current and high current change rate (di/dt), the vacuum type OLTCs were always adopted in the selection process of high-capacity converter transformer. Generally, the vacuum tap changer has compact structure and frequent switching action, which requires high reliability of vacuum interrupter. However, in actual operation, the faults of vacuum type OLTC often occur due to the failure of vacuum interrupter in vacuum type OLTC.
In this paper, the switching circuit principle of vacuum type OLTC and the service duty switching test platform of OLTC were introduced firstly. A defect model of vacuum interrupter of vacuum type OLTC was made according to OLTC fault analysis experience.  The service duty switching test was carried out on the vacuum type OLTC with defect vacuum interrupter. The evolution process of tap changer from normal switching to switching fault was studied, and the transient voltage and current changes during the switching process were analyzed. The results show that the switching failure of vacuum type OLTC caused by the defect of vacuum interrupter was not an emergency, but a gradual development process. Based on the research result of this paper, the switching state of OLTC can be monitored online in real time to improve its switching reliability.