Flexible AC transmission system

A Flexible Alternating Current Transmission System (FACTS) is a system comprised of static equipment used for the 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 device.

FACTS is defined by the 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." ["Proposed terms and definitions for flexible AC transmission system(FACTS)", IEEE Transactions on Power Delivery, Volume 12, Issue 4, October 1997, Pages 1848-1853 [http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel4/61/13765/00634216.pdf?arnumber=634216] .]

FACTS technology

FACTS could be connected:
* in series with the power system (series compensation)
* in shunt with the power system (shunt compensation)
* both in series and in shunt with the power system

Series compensation

In series compensation, the FACTS is connected in series with the power system. It works as a controllable voltage source.

Series inductance occurs in long transmission lines, and when a large current flow it causes a large voltage drop. To compensate, series capacitors are connected.

Shunt compensation

In shunt compensation, power system is connected in shunt with the FACTS. It works as a controllable current source.

Shunt compensation is of two types:

Shunt capacitive compensation

This method is used improve the power factor. Whenever an inductive load is connected to the transmission line, power factor lags because of lagging load current. To compensate, a shunt capacitor is connected which draws current leading the source voltage. The net result is improvement in power factor.

Shunt inductive compensation

This method is used either when charging the transmission line, or, when there is very low load at the receiving end. Due to very low, or no load -- very low current flows through the transmission line. Shunt capacitance in the transmission line causes voltage amplification (Ferranti Effect). The receiving end voltage may become double the sending end voltage (generally in case of very long transmission lines). To compensate, shunt inductors are connected across the transmission line.

FACTS theory

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

$underline\{V\_s\}=V\; cos(frac\{delta\}\{2\})\; +jV\; sin(frac\{delta\}\{2\})$
$underline\{V\_r\}=V\; cos(frac\{delta\}\{2\})-jV\; sin(frac\{delta\}\{2\})$
$underline\{I\}=frac\{underline\{V\_s\}-underline\{V\_r\{jX\}=frac\{2Vsinfrac\{delta\}\{2\{X\}$

As it is a no-loss line, active power P is the same at any point of the line:
$P\_s=P\_r=P=V\; cos(frac\{delta\}\{2\})\; .\; frac\{2Vsinfrac\{delta\}\{2\{X\}=frac\{V^2\}\{X\}sin\; delta$
Reactive power at sending end is the opposite of reactive power at receiving end:$Q\_s=-Q\_r=Q=V\; sin(frac\{delta\}\{2\})\; .\; frac\{2Vsinfrac\{delta\}\{2\{X\}=frac\{V^2\}\{X\}(1-cos\; delta)$

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

Series compensation

FACTS for series compensation modify line impedance: X is decreased so as to increase the transmittable active power. However, more reactive power must be provided.
$P=frac\{V^2\}\{X-Xc\}sin\; delta$
$Q=frac\{V^2\}\{X-Xc\}(1-cos\; delta)$

Shunt compensation

Reactive current is injected into the line to maintain voltage magnitude. Transmittable active power is increased but more reactive power is to be provided.
$P=frac\{2V^2\}\{X\}sin\; (frac\{delta\}\{2\})$
$Q=frac\{2V^2\}\{X\}(1-cos(frac\{delta\}\{2\}))$

Examples of FACTS for series compensation

* Static Synchronous Series Compensator (SSSC)
* Thyristor Controlled Series Capacitor (TCSC): a series capacitor bank is shunted by a thyristor-controlled 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 FACTS for shunt compensation

* 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.

References

* Narain G. Hingorani, Laszlo Gyugyi "Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems", Wiley-IEEE Press, December 1999. ISBN 978-0-7803-3455-7

* Xiao-Ping Zhang, Christian Rehtanz, Bikash Pal, "Flexible AC Transmission Systems: Modelling and Control", Springer, March 2006. ISBN 978-3-540-30606-1. http://www.springer.com/3-540-30606-4

ee also

Internal links

*HVDC

External links

* [http://www.siemens.com/FACTS Siemens AG - FACTS] website.
* [http://www.abb.com/facts ABB FACTS]