FACTS devices changing the power system

FACTS (Flexible AC Transmission Systems) are power-electronic controllers (SVC, STATCOM, TCSC, SSSC, UPFC, phase-angle regulators) that control the three parameters that decide AC power flow: voltage, line reactance and phase angle. Because they bring fast, electronic control to a network that was earlier controlled only slowly and mechanically, they are changing how power systems are planned and operated.

$$P = \frac{V_s V_r}{X}\sin\delta, \qquad Q \approx \frac{V_s(V_s - V_r\cos\delta)}{X}$$

Significant changes initiated by FACTS

1. Higher utilisation of existing lines: traditional lines are loaded well below their thermal limit because of stability and voltage limits. Series compensation (TCSC, SSSC) reduces $X$ and shunt devices (STATCOM) hold $V$, so the same line carries much more power. This delays new lines and saves right-of-way.
2. Controllable power flow: earlier, power flowed by Kirchhoff's laws according to impedance; loop flows and overloading of parallel paths were common. UPFC and TCSC let operators direct power along chosen paths, which is essential for deregulated markets and wheeling contracts.
3. Improved stability: fast control (a few cycles) increases transient stability margin, damps inter-area oscillations, and mitigates subsynchronous resonance.
4. Voltage control and reactive support: STATCOM/SVC give dynamic VAr support, preventing voltage collapse and correcting the Ferranti effect on light load.
5. Renewable energy integration: wind and solar output varies; FACTS smooth the voltage fluctuations and help meet grid-code fault ride-through requirements.
6. Power quality: custom-power devices (D-STATCOM, DVR, active filters) at distribution level reduce sags, flicker and harmonics.
7. Interconnection of grids: FACTS allow secure interconnection of neighbouring utilities and regions with controlled exchange.
8. Deregulation and markets: independent power producers need guaranteed transfer paths; FACTS provide the control that open access needs.

Acceleration of change

• Mechanical switching (tap changers, switched capacitors) works in seconds and wears out; thyristor/IGBT switching works in milliseconds with no wear.
• With wide-area monitoring (PMUs) and digital control, FACTS make the grid "smart" and adaptive.
• Lower cost of power semiconductors and modular converters (MMC) make FACTS cheaper and more common.

Example

A 400 kV line with $X = 100\ \Omega$ and $\delta = 30^\circ$ carries $P = \frac{400^2}{100}\sin 30^\circ = 800$ MW. With 40% series compensation, $X = 60\ \Omega$ and $P = 1333$ MW at the same angle, a 67% increase without a new line.

Limitations

High cost, harmonics, need for skilled operation and coordination among devices.

Thus FACTS convert the passive AC network into an actively controlled, flexible system, and so they initiate and accelerate major changes in power system planning, operation and markets.