Flexible AC Transmission Systems (FACTS)

Flexible AC Transmission Systems (FACTS)

In the modem power systems, the power flow in the transmission lines can be controlled with the use of power electronics. The Flexible AC Transmission System (FACTS) give solutions to the problems and limitations which were introduced in the power system with the introduction of power electronics based control for reactive power.

The FACTS technology making use of power electronics pormotes the control of transmission line. It also increases load on the line upto the thermal limits without having compromise with the reliability. The line capacity is thus increased which improves reliability of the system. Due to this, there is maximum utilisation of available equipments and additional bulk power transfers are possible. This also avoids the construction of new transmission lines which is time consuming process. 

The FACTS based controllers gives instantaneous control of transmission voltage and increases capacity providing larger flexibility in bulk power transmission. It also helps in damping out major grid oscillations. Tennessee Valley Authority (TVA) has installed the first Static Synchronous Compensator (STATCOM) in the year 1995. This has strengthen ties between Sulivan substation and the rest of the network.

1. Advantages of FACTS based Controllers

1. It controls line impedance angle and voltage which helps in controlling the power flow in transmission lines.

2. The power flow in the transmission lines can be made optimum.

3. It helps in damping out the oscillations and avoids damage of various equipments.

4. It supports the power system security by increasing the transient stability limit. It also limits overloads and short circuit currents.

5. The reserve requirements for generators are considerably reduced as these controllers provide secure and controllable tie line connections to neighbouring electric utilities.

6. The loading capacity of the line is greatly increased upto their thermal capabilities. Thus upgrading of lines is possible.

7. It limits the impacts of faults and equipment failures.

8. The reactive power flow in the lines can be decreased and the lines are made to carry more active power.

9. There is increase in utilisation of low cost generation due to cost effective enhancement of transmission line capacity.

3. Objectives of FACTS

The concept of FACTS was established in order to solve the problem which was emerging in power systems in the late 1980s as there are restrictions on the construction of transmission line and to promote power growth of import and export.

The main objectives behind the FACTS based controllers are

1. The power transfer capability of transmission systems is to be increased.

2. The power flow is to be kept over the designated routes.

The first objective indicates that the power flow in a given transmission line can be increased upto its thermal limit. This can be achieved by passing the required current through the series line impedance and maintaining the stability of the system through the proper real time control of power during and after system faults. 

The second objective indicates that the flow of power in the line can be restricted to select proper transmission corridors by controlling current in the line.

If these two objectives are fullfilled then there will be significant increase in the utilisation of new and existing transmission lines. It will promote the deregulation of power system and there will minimum requirements for new transmission lines. In order to implement these objectives, high power compensators and controllers are required.

The concept of Flexible AC Transmission Systems (FACTS) was first defined in 1988 by N.G. Hingorani. These controllers control all the interrelated parameters which are involved in power system operation such as series and shunt impedance, current, voltage and phase angle. Also it damps the oscillations at various frequencies below the rated frequency. Thus these controllers are advantageous to power systems in terms of its operation, control, planning of lines and finance.

3. Types of FACTS Controllers

There are large number of types of FACTS controllers with the recent advancement in power system. Some of these controllers are already installed and put in operation while some of them are still under construction. However more work is to be carried out in this area so that new characteristics of these controllers can be fully used.

Presently FACTS highlight on power flow control and modulation, operating devices, damping of oscillations and stability enhancement. The basic symbol of FACTS controller is shown in the Fig. 7.28.1.

There are four main categories under which FACTS controllers are divided.

a. Series Controllers

These type of controllers inject voltage in series with the line. The current flowing through the line multiplied by variable impedance represents injected series voltage in the line. Till the time the voltage is in phase quadrature with the line current, these controllers only supply variable reactive power. If there phase relationship between voltage and current is different then real power is also handled in addition to reactive power. A typical series FACTS controller is shown in the Fig. 7.28.2. 

The series controller is normally of variable impedance type such as capacitor, reactor or power electronics based variable source of main, subsynchronous and harmonic frequencies to satisfy the requirement.

b. Shunt Controllers

The shunt controllers inject current in the system at the point of connection. The shunt controllers are of variable impedance type, variable source type or a combination. The shunt type of FACTS controller is shown in the Fig. 7.28.3.

If a variable shunt impedance is connected to line voltage, variable current flows and current is injected in the line. So long as there is quadrature relationship between voltage and current, these controllers either supply or deal with reactive power. For other phase relationship, active power is also handled.

c. Combined Series Series Controllers

In these type of controllers, series controllers are controlled in co-ordinated manner in case of multiline transmission system. It is alternatively unified controller in which series controllers individually supply reactive power compensation independently for every line and also transfer real power among the lines through power link. The meaning of the term unified indicates that the dc terminals of all the controller converters are all connected together for transfering real power.

Such type of controller is shown in the Fig. 7.28.4.

The capacity of these controllers to transfer real power makes maximized utilisation of transmission system as it is possible to balance both real and reactive power flow in the lines. 

d. Combined Series-Shunt Controllers

It may be a combination of separate series and shunt controllers which are controlled in a co-ordinated manner as shown in the Fig. 7.28.5 (a) or a unified power flow controller with series and shunt elements as shown in the Fig. 7.28.5 (b).

In these type of controllers, current is injected in the system through shunt part of the controller while voltage is injected in the line through series part of the controller. In unified series-shunt controller, it is possible to exchange real power between series and shunt controllers through dc power link.

Review Questions

1. Explain in detail various types of FACTS controllers.

AU: May-07, 10, 16, Marks 16

2. State the advantages of FACTS based controllers.

3. What are the objectives of FACTS?

AU: Dec.-13, Marks 2