Analysis of shaft torque amplification caused by interaction between AC and DC system

In general opinion, subsynchronous oscillations (SSO) of turbine-generators occurring at the rectifier side of HVDC is caused by the interaction between the rapid response of the HVDC control system and the electrical torque on the generator rotor. So the normal method for analysis of SSO induced by HVDC is focused only on the rectifier side and the adjacent ac systems. However in Suizhong plant, which is located nearby the rectifier side of Gaoling back-to-back converter station, shaft torque amplifications have occurred following single phase instantaneous faults at Shangdu busbar, 380 km away from the inverter side of the Gaoling converter station. In this paper, the mechanism of commutation failure caused by ac faults and SSO at rectifier side caused by commutation failure at inverter side is analyzed. A detailed electromagnetic transient model is established and the shaft torque amplification at Suizhong plant is duplicated. The field recorded data verifies that the analysis is correct and the simulation is credible. The innovative mechanism of SSO induced by interaction between ac and dc system is revealed, which contributes to SSO risks forecast.