Hybrid series compensation using static synchronous series compensator (SSSC) and passive series capacitor can improve the stability of the system, increases the power transfer capability and is useful for the fast control of power flow. This paper analyzes the subsynchronous resonance (SSR) characteristics of the hybrid series compensated power system in detail and proposes a simple method for the extraction of subsynchronous components of line current using filter. The extracted subsynchronous frequency component of line current is used to inject a proportional subsynchronous voltage in series with the transmission line which suppresses subsynchronous current in the transmission network. This novel technique is termed as subsynchronous current suppressor. The design of subsynchronous current suppressor is based on damping torque analysis and using genetic algorithm. A novel graphical representation of series resonance condition when SSSC is incorporated in the system is presented. The detailed study of SSR is carried out based on eigenvalue analysis, transient simulation and damping torque analysis. The results of the case study on a system adapted from IEEE First Benchmark Model demonstrates the effectiveness and robust performance of subsynchronous current suppressor in damping of SSR under various system operating conditions.

To evaluate the stability of Subsynchronous Torsional Interactions (SSTI), we propose a simple and effective approach using admittance model. The proposed approach gives the subsynchronous conductance based criterion to find the stability of torsional interactions. The subsynchronous conductance based approach is utilized to evaluate the impact of Doubly-Fed Induction Generator based Wind Energy Conversion System (DFIG-WECS) on the torsional vibrations of nearby turbine-generator. The damping of \blue{Induction Generator Effect (IGE) and} SSTI is analyzed \blue{through admittance model} at different wind speed with various compensation level. The results of admittance analysis are validated through eigenvalue analysis and transient simulation. The results show that, the DFIG-WECS reduces the damping of torsional modes in the low frequency region of subsynchronous network mode. The damping of SSTI reduces with increase in wind speed. Hence, the DFIG-WECS significantly destabilizes the low frequency torsional modes at high wind speed with high compensation level. Further, it is found that, the network conductance at all critical torsional modes marginally reduces with Subsynchronous Damping Controller (SSDC) at the inner loop of Grid Side Converter (GSC) controller compared to other locations in DFIG converter controllers.

In a long transmission line the use of static series synchronous compensation (SSSC) with fixed series capacitor enables fast control of power flow. There is a potential risk of subsynchronous resonance (SSR) due to the series capacitor. In this paper, we propose to use the Kalman filter (KF) for state estimation of subsynchronous components present in series compensated line and the mitigation of SSR. This novel KF based SSR damping controller is referred to as KF-damping controller. The design of KF-damping controller is based on the magnitude of damping torque in the range of torsional mode frequencies. We apply the genetic algorithm to optimize the controller parameters. The proposed KF-damping controller is highly frequency selective and effectively suppresses subsynchronous components of line current in the presence of close frequency oscillations. Analytical and simulation results at various system operating conditions demonstrate the effectiveness and robustness of the proposed KF-damping controller in mitigating SSR.