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A thyristor-controlled series capacitor (TCSC) substantially improves transmission capacity. It also mitigates subsynchronous resonance (SSR) accompanying conventional series capacitors. With an appropriate angle of thyristor firing, electrical damping becomes almost zero, which is called SSR neutral. This quality comes from TCSC itself. However, negative damping still remains, and is large for firing angle 170∼180° where little current flows through thyristors. This paper deals with control of firing angle. First, we oscillate the firing angle at a given frequency, and present an analytical method of calculating electrical damping. Next, we show that the damping improves at all frequencies if the firing angle oscillation is in phase with that of rotor angle. Synchronizing torque decreases, however, so a limit must be put on the control gain. Lastly, we execute numerical simulations to verify our analytical results.