Simulation and Experimental Study of Switching Transient Process of On-load Tap Changer of Converter Transformer

The on-load tap changer was the only moving part in converter transformer, which with complex structure and high reliability requirements. It was very important for the design and selection of on-load tap changer (OLTC) to investigate the switching transient process of on load tap changer. In this paper a diverter switch insert (DSI) with symmetrical arrangement and symmetrical switch circuit of vacuum type OLTC was development based on the characteristics of two columns of converter transformer. The operation condition of OLTC of converter transformer was first analyzed. A model of the developed OLTC operating under AC power frequency load was built by PSCAD, and the simulated results were proved by compared with the practically measured result. In order to simulate the actual operating condition of OLTC of converter transformer, a current waveform through AC side of converter transformer was generated by 6 pulsating valve in HVDC transmission system. The switching process of the developed OLTC in converter transformer was simulated, and the recovery voltage, di/dt and du/dt characteristics across the main contact and transition contact of OLTC were obtained. The simulated results indicate that the OLTC of converter transformer was more tougher than that of traditional power transformer.