A combined control scheme to mitigate SSR for steady state and transient state of power systems

Supplemental excitation damping control (SEDC) is one of the effective methods to mitigate subsynchronous resonance (SSR) caused by series compensation of long ac transmission lines. However, researches and field tests show SEDC does not provide enough damping to prevent torsional interaction occurring under large transient disturbances of the power systems. This paper proposed a new control scheme of a combination of an SEDC and an additional induction machine damping unit (IMDU) to solve the problem. For the syntheses of the controller, the SEDC is firstly analyzed. By phase shifting and amplifying of the feedback generator speed signal, the modulated output of the SEDC is added upon the excitation voltage, providing effective damping for steady state operations. Then the analysis of the IMDU is presented. The detail conformation of the IMDU is introduced. The principle of the IMDU to mitigate torque amplification during fault transient is explained based on the generator speed deviation. The mechanical and electrical parameters of the IMDU are also discussed. The IEEE first benchmark model for subsynchronous resonance investigation is adopted to perform the studies. Time-domain simulation based on PSCAD/EMTDC is used to verify the effectiveness of the proposed control scheme. The results indicate the combination control scheme can eliminate SSR instabilities in both steady and transient states of power systems.