Torsional oscillations of series capacitor compensated AC/DC systems

Torsional oscillations in an AC/DC power system that result from series-capacitor compensation and HVDC converter controllers are investigated. A feasible approach for damping the torsional modes by coordinating the controllers of a static VAr compensator (SVC) unit with the supplemental subsynchronous damping control (SSDC) of the HVDC rectifier is presented. An eigenvalue method is used for the studies, and the analytical results are verified by detailed digital computer simulation, using the BPA´s Electromagnetic Transients Program (EMTP). The studies show that in a series-capacitor-compensated AC/DC system, the SSDC can be used to damp the subsynchronous resonance (SSR) modes only over a limited range of operating conditions. The coordinated SVC-SSDC approach provides adequate damping for all the SSR modes under all operating conditions, particularly at high series-compensation levels