Flexible AC Transmission System (FACTS) 

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.

FACTS could be connected:

Series Compensation

In Series Compensation, the FACTS device 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 flows, it causes a large voltage drop. To compensate, series capacitors are connected.

Shunt Compensation

In Shunt Compensation, FACTS device is connected in shunt with the power system. 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 while 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.                   

 High Voltage Direct Current (HVDC) 

The advantage of HVDC is the ability to transmit large amounts of power over long distances with lower capital costs and with lower losses than AC. Depending on voltage level and construction details, losses are quoted as about 3% per 1000 km. High Voltage Direct Current transmission allows use of energy sources remote from load centers.

In a number of applications, HVDC is more effective than AC transmission. Examples include:

Long undersea cables have a high capacitance. While this has minimal effect for DC transmission, the current required to charge and discharge the capacitance of the cable causes additional I²R power losses when the cable is carrying AC. In addition, AC power is lost to dielectric losses.

HVDC can carry more power per conductor, because for a given power rating the constant voltage in a DC line is lower than the peak voltage in an AC line. This voltage determines the insulation thickness and conductor spacing. This allows existing transmission line corridors to be used to carry more power into an area of high power consumption, which can lower costs.

Because HVDC allows power transmission between unsynchronized AC distribution systems, it can help increase system stability, by preventing cascading failures from propagating from one part of a wider power transmission grid to another. Changes in load that would cause portions of an AC network to become unsynchronized and separate would not similarly affect a DC link, and the power flow through the DC link would tend to stabilize the AC network. The magnitude and direction of power flow through a DC link can be directly commanded, and changed as needed to support the AC networks at either end of the DC link. This has caused many power system operators to contemplate wider use of HVDC technology for its stability benefits alone.

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