Analysis on thyristor controlled series capacitor for subsynchronous resonance mitigation

The thyristor controlled series capacitor (TCSC) is expected to mitigate subsynchronous resonance (SSR) providing a continuously variable series capacitance. The computer simulation studies and tests in prototype installations in the literature showed that TCSC is neutral to SSR, but which has not been proven rigorously to date. In this paper, an algorithm to calculate the subsynchronous voltage across TCSC due to a subsynchronous current is developed under the assumption that charge reversals take place at the center of conduction angles instantaneously, and it is shown that TCSC would exhibit an impedance characteristic of a reactor at subsynchronous frequencies if the center of conduction angles is at the ideal zero crossing of capacitor voltage. Some significant observations for TCSC characteristics are obtained by theoretical analysis and computer simulation studies