Role of FACTS devices

FACTS (Flexible AC Transmission Systems) are power-electronic devices that control voltage, line reactance and phase angle of AC transmission lines to improve controllability and power transfer capability. Since

$$P = \frac{V_sV_r}{X}\sin\delta,$$

controlling $V$, $X$ or $\delta$ directly controls the power flow.

Role of FACTS devices

1. Increase power transfer capability: lines can be loaded near their thermal limit; series devices (TCSC, SSSC) reduce $X$.
2. Power flow control: UPFC, TCSC and phase shifters send power along desired paths and remove loop flows and overloads.
3. Voltage control: shunt devices (SVC, STATCOM) supply or absorb reactive power to keep bus voltage within limits and prevent voltage collapse.
4. Stability improvement: fast response increases transient stability margin and damps power oscillations; also reduces subsynchronous resonance.
5. Reduce losses and reactive flow in the network.
6. Delay new lines: better use of existing lines saves cost and right-of-way.
7. Renewable integration and power quality: smooth voltage fluctuations from wind and solar; custom power devices reduce sags and harmonics.
8. Support deregulated operation: secure, controllable transfer for power trading.

Main devices: SVC and STATCOM (shunt), TCSC and SSSC (series), UPFC and IPFC (combined).