Series Compensation is a well established technology that primarily is used to reduce transfer reactances, most notably in bulk transmission corridors. The result is a signifcant increase in the transmission system transient and voltage stability. Series Compensation is self regulating in the sense that its reactive power output follows the variations in transmission line current, a fact that makes the series compensation concept extremely straightforward and cost effective.
Thyristor Controlled Series Capacitors adds another controllability dimension, as thyristors are used to dynamically modulate the ohms provided by the inserted capacitor. This is primarily used to provide inter-area damping of prospective low frequency electromechanical oscillations, but it also makes the whole Series Compensation scheme immune to Subsynchronous Resonance (SSR).