Because of China's energy and load imbalance, respectively, large-scale long-distance power transmission becomes inevitable. Under this background , HVDC develops rapidly, SSO (Sub-synchronous oscillations) caused by HVDC is becoming increasingly apparent. The sub-synchronous electrical damping within the frequency range is difficult to calculate. Based on small-signal test method-complex torque coefficient method in the time-domain simulation , the paper calculated the sub-synchronous electrical damping characteristics of the system and analyzed the impact of the unit interaction, DC power and the generator output on sub-synchronous electrical damping characteristics.

SSDC(Supplementary sub-synchronous oscillation damping controller) is one of effective measures to suppress the SSO(sub-synchronous oscillations) caused by HVDC. The article researched the phase compensation method of SSDC on the basis of studying the SSO and the SSDC mechanism in depth. We verified the validity of the method used by designing SSDC in an large domestic HVDC simulation model. And we find that the SSDC designed can effectively increase the positive electrical damping in the sub-synchronous frequency range with the sub-synchronous frequency scanning to suppress the SSO.

As the main types of parallel FACTS devices, the basic function of SVC and STATCOM is to realize the dynamic compensation of reactive power and maintain system voltage stability. However, when new control strategies are added to them .Both of them can suppress SSO effectively. Based on IEEE first benchmark model, this paper established simulation model by PSCAD/EMTDC. The effects of suppressing SSO by SVC/STATCOM are verified with time domain simulation, and the comparative analyses are carried out on the effects of suppressing SSO by SVC and STATCOM.

Supplementary subsynchronous damping controller (SSDC) is an effective countermeasure to damp the subsynchronous oscillation (SSO) caused by HVDC. On the basis of analyzing the mechanism of inducing SSO by HVDC, the principle of damping SSO by SSDC and SSDC design methods are expounded. A SSDC is designed for a practical power plant in China, and the correctness and validity of the SSDC control strategies are proved with time domain electromagnetic simulation results.