Switching Mechanism and Simulation Study of Vacuum Type On-load Tap Changer in Converter Transformer

The on-load tap changer (OLTC) is an important component in converter transformer, which is responsible for maintaining the stability of DC voltage and adjusting the trigger angle. With the complex operating conditions and the increase of switching capacity, the resistor type on-load tap changer with vacuum interrupters are widely used. The structure and working principle of vacuum type on-load tap changer are analyzed in this paper. In order to analyze the internal electrical characteristics of the on-load tap changer, a simulation model is built by PSCAD. The recovery voltage of interrupters and circulating current formulas are deduced. The simulation results are consistent with the theoretical analysis, which verifies the correctness of the simulation model. On this basis, it is pointed out that a large transition resistor leads to high recovery voltage of vacuum interrupter in the main branch, while a small transition resistor leads to large switched current of vacuum interrupter in the transition branch. Since the transition resistor is related to the breaking capacity of vacuum interrupter, the selection of the value of transition resistance should consider the match between the step voltage and the load current.