A flexible alternating current transmission system (FACTS) is a system composed of static equipment used for the alternating current (AC) transmission of electrical energy. It is meant to enhance controllability and increase power transfer capability of the network. It is generally a power electronics-based system.

FACTS is defined by the Institute of Electrical and Electronics Engineers (IEEE) as "a power electronic based system and other static equipment that provide control of one or more AC transmission system parameters to enhance controllability and increase power transfer capability".^[1]

According to Siemens, "FACTS Increase the reliability of AC grids and reduce power delivery costs. They improve transmission quality and efficiency of power transmission by supplying inductive or reactive power to grid.^[2]

Technology

Transmission on a no-loss line.

Series compensation.

Shunt compensation.

Theory

In the case of a no-loss line, voltage magnitude at the receiving end is the same as voltage magnitude at the sending end: V_s = V_r = V. Transmission results in a phase lag ${\displaystyle \delta }$ that depends on line reactance X.

${\displaystyle {\begin{aligned}{\underline {V_{s}}}&=V\cos \left({\frac {\delta }{2}}\right)+jV\sin \left({\frac {\delta }{2}}\right)\\[3pt]{\underline {V_{r}}}&=V\cos \left({\frac {\delta }{2}}\right)-jV\sin \left({\frac {\delta }{2}}\right)\\[3pt]{\underline {I}}&={\frac {{\underline {V_{s}}}-{\underline {V_{r}}}}{jX}}={\frac {2V\sin {\left({\frac {\delta }{2}}\right)}}{X}}\end{aligned}}}$

As it is a no-loss line, active power P is the same at any point of the line:

${\displaystyle P_{s}=P_{r}=P=V\cos \left({\frac {\delta }{2}}\right)\cdot {\frac {2V\sin {\left({\frac {\delta }{2}}\right)}}{X}}={\frac {V^{2}}{X}}\sin(\delta )}$

Reactive power at sending end is the opposite of reactive power at receiving end:

${\displaystyle Q_{s}=-Q_{r}=Q=V\sin \left({\frac {\delta }{2}}\right)\cdot {\frac {2V\sin \left({\frac {\delta }{2}}\right)}{X}}={\frac {V^{2}}{X}}(1-\cos \delta )}$

As ${\displaystyle \delta }$ is very small, active power mainly depends on ${\displaystyle \delta }$ whereas reactive power mainly depends on voltage magnitude.

Examples of series compensation

Examples of FACTS for series compensation (schematic)

• Static synchronous series compensator (SSSC)
• Thyristor-controlled series capacitor (TCSC): a series capacitor bank is shunted by a thyristor-controlled inductor reactor
• Thyristor-controlled series reactor (TCSR): a series reactor bank is shunted by a thyristor-controlled reactor
• Thyristor-switched series capacitor (TSSC): a series capacitor bank is shunted by a thyristor-switched reactor
• Thyristor-switched series reactor (TSSR): a series reactor bank is shunted by a thyristor-switched reactor

Examples of shunt compensation

Examples of FACTS for shunt compensation (schematic)

• Static synchronous compensator (STATCOM); previously known as a static condenser (STATCON)
• Static VAR compensator (SVC). Most common SVCs are:
  • Thyristor-controlled reactor (TCR): reactor is connected in series with a bidirectional thyristor valve. The thyristor valve is phase-controlled. Equivalent reactance is varied continuously.
  • Thyristor-switched reactor (TSR): Same as TCR but thyristor is either in zero- or full- conduction. Equivalent reactance is varied in stepwise manner.
  • Thyristor-switched capacitor (TSC): capacitor is connected in series with a bidirectional thyristor valve. Thyristor is either in zero- or full- conduction. Equivalent reactance is varied in stepwise manner.
  • Mechanically-switched capacitor (MSC): capacitor is switched by circuit-breaker. It aims at compensating steady state reactive power. It is switched only a few times a day.

See also

• Static synchronous series compensator
• HVDC
• Dynamic line rating for electric utilities

References

In-line references

General references