FACTS Devices

Flexible Alternating Current Transmission System Flexible Alternating Current Transmission System (FACTS) simply refers to a combi...

Flexible Alternating Current Transmission System

Flexible Alternating Current Transmission System (FACTS) simply refers to a combination of power electronics components with traditional power system components. They are intended to improve our power system reliability, power transfer capability, transient and dynamic stability improvements, voltage regulation etc…

With the advent of improved semiconductor technologies, these FACTS devices have been proven in their speed and flexibility. But there do exist some cost and complexity issues.

There can be series as well as shunt compensation for the transmission lines using these FACTS devices. In series compensation, line impedance is modified, that means net impedance is decreased and thereby increasing the transmittable active power. For shunt compensation, reactive current is injected into the line so as to regulate the voltage at the point of connection.

Thus, the active power transmission is increased. In both types of compensations (series and shunt), more reactive power must be provided.

The general classification of these FACTS devices is:

1. Series controllers
2. Shunt controllers
3. Combined series-series controllers
4. Combined series-shunt controllers

Series Controllers

Series controllers are being connected in series with the line as they are meant for injecting voltage in series with the line. These devices could be variable impedances like capacitor, reactor or power electronics based variable source of main frequency, sub synchronous or harmonic frequency, or can be a combination of these, to meet the requirements.

If the injected voltage is in phase quadrature with the line current, then only supply or consumption of variable reactive power is possible.

                 Series FACTS Controllers (figure credit: intechopen.com)

In order to handle real power also, any other phase relationship has to be involved. These type of controllers include:

• SSSC – Static synchronous series compensator,
• TCSC – Thyristor controlled series capacitor,
• TCSR – Thyristor controlled series reactor,
• TSSC – Thyristor switched series capacitor and
• TSSR – Thyristor switched series reactor.

Shunt controllers

Shunt controllers will be connected in shunt with the line so as to inject current into the system at the point of connection. They can also be variable impedance, variable source, or a combination of these.

If the injected line current is in quadrature with the line voltage, variable reactive power supply or consumption could be achieved. But any other phase relationship could involve real power handling as well.

Shunt FACTS Controller (figure credit: intechopen.com)

This category includes STATCOM (Static synchronous compensator) and SVC (Static VAR compensator). The common Static VAR compensators are:

• TCR – Thyristor controlled reactor,
• TSR – Thyristor switched reactor,
• TSC – Thyristor switched capacitor,

Combined Series-Series Controllers

This category comprises of separate series controllers controlled in a coordinated manner in the case of a multiline transmission system. It can also be a unified controller in which the series controllers perform the reactive power compensation in each line independently whereas they facilitates real power exchange between the lines via the common DC link.

Combined Series-Series Controllers

Because, in unified series-series controllers like Interline Power Flow Controller (IPFC), the DC terminals of the controller converters are all connected together.

Combined Series-Shunt Controllers

It is a combination of separate series and shunt controllers, being operated in a coordinated manner. Hence, they are capable of injecting current into the line using the shunt part and injecting series voltage with the series part of the respective controller.

If they are unified, there can be real power exchange between the shunt and series controllers via the common DC power link, as in the case of Unified Power Flow Controllers (UPFC).

Unified Power Flow Controllers – UPFC (figure credit: intechopen.com)

The important advantages of these FACTS controllers includes:

1. Improving power transfer capability
2. Confining power flow to designated routes
3. Transient and dynamic stability improvement
4. Damping of power system oscillations
5. Better voltage regulation
6. Flexible operation and control of the system
7. Secure loading of the transmission lines close to their thermal limits
8. Prevention of cascading outages by contributing to emergency control

Minimizing the cost

In short, FACTS technology enables the enhancement of power system performance. Researches are going on with the intension of minimizing the cost of the power electronics components.

But with respect to the losses incurred in a power system, incorporation of these FACTS devices to mitigate them could be considered as cost effective.

Source: EEP