Mitigation of SSR in Series Compensated Power System Using a FACTS based Subsynchronous Damping Controller

Due to the continuous increase in the demand of electricity, we are left with two options; either to increase the installed generating capacity in proportion with the rise in demand or to use some other methods along with former; keeping the stability intact. In this regard series capacitive compensation comes at our rescue. Series capacitive compensation virtually reduces the length of transmission line by minimizing the line inductive reactance and enhances the power transmission capacity. However, in 1971 it was learned in one of the USA's Mohave thermal power plant that series capacitors can create an adverse interaction between the series compensated electrical system and the spring-mass mechanical system of the turbine-generators. This effect is called sub synchronous resonance (SSR). Due to this torsional oscillation, the rotor shaft faces severe fatigue and can be damaged if a proper solution is not adopted to mitigate SSR. There are many methods proposed in literature for SSR mitigation.However this paper proposes that SSR can be mitigated with a proper gating pulse to TCSC. A Subsynchronous damping controller(SSDC) is proposed to be modeled for detecting SSR phenomena. SSDC observes this SSR phenomenon and directs an appropriate signal to the TCSC automatically. Thyristor controlled series capacitor (TCSC) which improves transmission line capability, can also be used in damping out Sub synchronous resonance (SSR) efficiently.All results are validated using simulations in MATLAB.