Electromagnetic transient simulation analysis on electrical fault of vacuum type on-load tap changer in converter transformer

The on-load tap changer (OLTC) is the only moving part of the converter transformer, which plays an important role in compensating the fluctuation of AC voltage in HVDC transmission. However, various mechanical and electrical faults such as vacuum bubble vacuum drop, internal transfer switch burning and protection gap breakdown are easy to occur in the application process of vacuum OLTC. Taking the resistance-type vacuum OLTC which is widely used in engineering as an example, this paper studies the internal electrical characteristics based on the analysis of its structure and working principle, and deduces the calculation formula of recovery voltage and current. The simulation model of switch electrical circuit is established by using PSCAD software. The arc characteristics under normal condition are simulated and analyzed. On the basis of this, the variation of the electric parameter of the tap switch is analyzed when the common faults such as vacuum bubble arc extinguishing failure and mechanical structure loosening. The simulation results show that OLTC will have obvious electrical changes under fault conditions, which will lead to more serious electrical faults. The research in this paper provides reference for the design and fault diagnosis of OLTC in practical DC engineering.