With the widespread application of series compensated capacitors

the power systems are facing the severe problem of subsynchronous resonance(SSR)in China. To solve the SSR problem

a method of supplementary damping control for unified power flow controller(UPFC)is proposed based on the UPFC with current control mode. The positions of supplementary subsynchronous damping signal accessing to UPFC controller are chosen at first. The UPFC supplementary subsynchronous damping controller(SSDC)is designed

and the principle how to damp SSR is deduced as well. Besides

the mode-splitting control is used in the designed SSDC to mitigate the multi-modal SSR. Finally

the complex torque coefficient and time domain simulation are both performed to analyze and verify the effectiveness of damping SSR with the designed SSDC. Meanwhile

the simulation results also suggest that the designed SSDCs have none negative effect on the performance of UPFC and the system. This work is supported by State Grid Corporation of China(No. SGRI-DL-71-14-002).