Torsional Oscillations of Series Capacitor Compensated AC/DC Systems

The phenomenon of subsynchronous resonance (SSR) as a result of interaction between a steam turbine-generator (T-G) unit and either a series capacitor compensated AC line or an HVDC converter controller, is well documented. In the former case, static VAr compensators (SVC´s) have been recognized as feasible devices for mitigating the SSR phenomenon. In the latter case, modification and addition of a supplemental subsynchronous damping control (SSDC) to the HVDC converter controller, can prevent the occurrence of the phenomenon and mitigate the oscillations when they are excited. When a T-G set is connected to a network comprising a series capacitor compensated AC line in parallel with an HVDC line (AC/DC system), both the series compensated AC line and the HVDC controller may contribute to the SSR of the T-G set. When an HVDC tie is in parallel with an AC line (with or without series capacitors) and the power rating of the DC line is noticeably higher than that of the AC line, the SSDC of the HVDC converter is technically a viable and economically the most feasible SSR countermeasure device. The effectiveness of the SSDC for damping SSR modes is noticeably reduced when the power rating of the series capacitor compensated AC line is comparable or larger than that of the DC tie. The reasons for ineffectiveness of the SSDC are: (1) The SSDC mitigates the SSR modes by modulating the power of the DC line.