Calculation of Damping Torque of Power System Compensated with TCSC

Abstract

Thyristor-controlled series capacitor (TCSC) is being developed to improve transmission capability of power systems. TCSC is thought to compensate transmission line reactance without causing subsynchronous resonance (SSR). However, in order to evaluate its effect quantitatively, we must calculate frequency responses of generator damping torque. Simulations need long computing time, and it is hard to choose frequency freely. In this paper, we propose a method of analytically calculating the damping torque. First, when a generator rotor oscillates at a frequency, two voltage components appear. We analytically calculate its damping torque from small current variations due to the voltage components. The damping characteristic changes depending on a method of firing thyristors. The best characteristic is obtained when we trigger referring to the fundamental wave of TCSC voltage or current. By choosing an appropriate firing angle, we can drastically reduce negative damping by TCSC. The damping characteristic is closely related with the system impedance. The fact that TCSC has large resistance in 0_??_60Hz, substantially contributes to improve the characteristic. Lastly, we execute numerical simulations of SSR, and examine the validity of our investigation.