Application of controlled series capacitors with antiparallel thyristor combination across the capacitor segment is one of new and promising countermeasures for enhancement of power system stability. The line reactance can be directly controlled by the controlled series capacitors, hence, it is very effective to apply the capacitors for damping power system disturbances. However, the segmented series capacitors may cause subsynchronous resonance (SSR) that can lead to turbine-generator shaft failure and electrical instability at oscillation frequencies lower than normal system frequency as well as the conventional capacitor may cause. In this paper, we describe the following.
(1) The response of turbine-generator shaft torque are analytically discussed when the number of inserted series capacitor in power system is changed.
(2) It is proposed for SSR countermeasure that the control system will be designed to avoid the number of series capacitor segment which may cause SSR in addition to selecting the better on-off timing of series capacitors.
(3) The effects of the proposed countermeasures are confirmed by the results of EMTP simulation using both simplified two-mass shaft model and detailed five-mass shaft model of turbine-generator.