On feasibility of SSDC to improve the effect of HVDC on subsynchronous damping on several lower range torsional oscillation modes

Subsynchronous damping controllers (SSDC) has been implemented as a part of the HVDC control systems basically since it was first recognized that HVDC LCC converter has adverse effect on subsynchronous torsional damping of especially the lowest subsynchronous torsional oscillation modes of turbine-generator units. Typically SSDC is designed to improve the effect of HVDC on subsynchronous damping on one or two lower range subsynchronous torsional modes. In this paper feasibility of SSDC to improve the effect of HVDC on subsynchronous damping on five torsional frequencies within torsional frequency range 5-20 Hz is evaluated. The feasibility study presented targets to answer the question if the adverse effect of HVDC can be improved on all the five modes using one common SSDC structure that may become of interest for example at the pre-specification stage of a HVDC project. In the paper study approach is presented, the structure of the study described and factors affecting SSDC feasibility is discussed. Finally, the contribution of presented SSDC structures to subsynchronous damping of multimodal lower range subsynchronous torsional oscillations is illustrated using on results of dynamic frequency scanning studies conducted using EMT tool.