Introduction The stability and efficiency of modern power transmission networks are critical as electricity demand grows globally. Ensuring that power flows optimally across transmission lines while maintaining grid reliability is a major challenge for power system engineers. This is where Flexible AC Transmission Systems (FACTS) come into play, particularly Thyristor Controlled Series Capacitors (TCSC) and Gate Turn-Off Thyristor Controlled Series Capacitors (GCSC) . These FACTS controllers dynamically regulate transmission line reactance , optimize power flow , improve voltage stability , and minimize transmission losses . TCSC and GCSC are particularly beneficial for mitigating sub-synchronous resonance (SSR) and enhancing transient stability in high-voltage transmission networks. In this article, we will explore: ✅ What TCSC and GCSC are ✅ How they function and their technical differences ✅ The advantages and applications of TCSC and GCSC in power systems Keywords: Thyr...