Control of Thyristor-Controlled Series Capacitor to Suppress Subsynchronous Resonance

Abstract

Thyristor-controlled series capacitor (TCSC) compensates transmission line reactance, and substantially improves transmission capability. It also does not cause 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 nature comes from TCSC itself but not from firing angle control. However, negative damping increases for firing angle 170_??_180° where little current flows thyristors. In this paper, we investigate improvement of damping by control of firing angle. First, we oscillate the firing angle at a frequency, and analytically calculate variation of the damping. Then, we show that the damping improves at all frequency if the oscillation is in phase with that of rotor angle. Synchronizing torque decreases however, so a limit must be put on the control gain. On the other hand, if their phases are 90° apart, the damping may improve in one frequency range, but it deteriorates in another range at the same time. This case happens when we feedback rotor speed deviation to control the firing angle. Lastly, we execute numerical simulation of SSR, and verify our investigation.