Flexible AC transmission system
Encyclopedia
A flexible alternating current transmission system (FACTS) is a system composed 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 system.
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."
exists in all AC transmission lines. On long lines, when a large current flows, this causes a large voltage drop. To compensate, series capacitors are connected, decreasing the effect of the inductance.
(parallel) with the FACTS. It works as a controllable current source
. Shunt compensation is of two types:
Shunt capacitive compensation: This method is used to 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. The power transfer capability is thereby increased depending upon the power equation
power angle
Transmission results in a phase lag
that depends on line reactance X.
As it is a no-loss line, active power P is the same at any point of the line:
Reactive power at sending end is the opposite of reactive power at receiving end:
As
is very small, active power mainly depends on 
whereas reactive power mainly depends on voltage magnitude.
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
transmission of electrical energy
Electric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...
. It is meant to enhance controllability and increase power transfer capability of the network. It is generally a power electronics
Power electronics
Power electronics is the application of solid-state electronics for the control and conversion of electric power.-Introduction:Power electronic converters can be found wherever there is a need to modify a form of electrical energy...
-based system.
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."
Technology
History
The first FACTS installation was at the C. J. Slatt Substation in Northern Oregon. This is a 500 kV, 3-phase 60 Hz substation, and was developed by EPRI, the Bonneville Power Administration and General Electric Company.Series compensation
In series compensation, the FACTS is connected in series with the power system. It works as a controllable voltage source. Series inductanceInductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
exists in all AC transmission lines. On long lines, when a large current flows, this causes a large voltage drop. To compensate, series capacitors are connected, decreasing the effect of the inductance.
Shunt compensation
In shunt compensation, power system is connected in shuntShunt (electrical)
In electronics, a shunt is a device which allows electric current to pass around another point in the circuit. The term is also widely used in photovoltaics to describe an unwanted short circuit between the front and back surface contacts of a solar cell, usually caused by wafer damage.-Defective...
(parallel) with the FACTS. It works as a controllable current source
Current source
A current source is an electrical or electronic device that delivers or absorbs electric current. A current source is the dual of a voltage source. The term constant-current sink is sometimes used for sources fed from a negative voltage supply...
. Shunt compensation is of two types:
Shunt capacitive compensation: This method is used to improve the power factor
Power factor
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, and is a dimensionless number between 0 and 1 . Real power is the capacity of the circuit for performing work in a particular time...
. 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
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
. The net result is improvement in power factor.
Shunt inductive compensation: This method is used either when charging the transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
, 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
Ferranti effect
The Ferranti effect is a rise in voltage occurring at the receiving end of a long transmission line, relative to the voltage at the sending end, which occurs when the line is energized but there is a very light load or the load is disconnected....
). 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. The power transfer capability is thereby increased depending upon the power equation
power angleTheory
In the case of a no-loss line, voltage magnitude at the receiving end is the same as voltage magnitude at the sending end: Vs = Vr=V.Transmission results in a phase lag
that depends on line reactance X.As it is a no-loss line, active power P is the same at any point of the line:
Reactive power at sending end is the opposite of reactive power at receiving end:
As
is very small, active power mainly depends on 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.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.Examples of series compensation
- Static synchronous series compensator (SSSC)
- Thyristor-controlled series capacitor (TCSC): a series capacitorCapacitorA capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
bank is shunted by a thyristorThyristorA thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased .Some sources define silicon controlled rectifiers and...
-controlled reactorInductorAn inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries... - 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
- Static synchronous compensator (STATCOMSTATCOMA static synchronous compensator is a regulating device used on alternating current electricity transmission networks. It is based on a power electronics voltage-source converter and can act as either a source or sink of reactive AC power to an electricity network. If connected to a source of...
); previously known as a static condenser (STATCON) - Static VAR compensatorStatic VAr compensatorA static var compensator is an electrical device for providing fast-acting reactive power on high-voltage electricity transmission networks. SVCs are part of the Flexible AC transmission system device family, regulating voltage and stabilising the system...
(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.
External links
- ABB FACTS
- http://www.amsc.com/products/transmissiongrid/static-VAR-compensators-SVC.html
- http://www.amsc.com/products/transmissiongrid/reactive-power-AC-transmission.html