Facts

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FLEXIBLE AC TRANSMISSION SYSTEM (FACTS) PRESENTED BY P.SUNIL KUMAR (EEE 3rd yr) & P.MADHU BABU (EEE 3rd yr)

ST.ANN’S COLLEGE OF ENGINEERING&TECHNOLOGY CHIRALA Address for Communication: P.SUNIL KUMAR

E-mail ([email protected])

P.MADHU BABU E-mail ([email protected]) Phone no: 9290570252 & 9885696380

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ABSTRACT FACTS or "flexible AC transmission systems" is a term that has been suggested for the use of solid state devices to control bulk power flow in transmission systems. The Electric Power Research Institute supported this idea, and many researchers have invested efforts on the value and potential of FACTS. At this time, it appears that the main value of FACTS lies in improving transmission capability; increasing the flexibility of power flow control (e.g., for wheeling or for economic dispatch); for controlling voltage(and var flow); and possibly additional advantages in lower voltage systems (e.g., distributional systems). Flexible AC Transmission Systems (FACTS) controllers have been used in power systems since the 1970s with the objective of improving system dynamic performance. Due to the environmental, right-of-way, and cost problems in both bundled and unbundled power systems, many transmission lines have been forced to operate at almost their full capacities worldwide. FACTS controllers enhance the static performance viz. increased loading, congestion management, reduced system loss, economic operation, etc., and dynamic performance viz. increased stability limits, damping of power system oscillation, etc. In this paper, an overview of FACTS controllers is explained. Various FACTS controllers and several devices in FACTS family are also discussed.

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Flexible AC transmission system Flexible Alternating Current Transmission System (FACTS) is a static equipment used for the AC transmission of electrical energy. It is meant to enhance controllability and increase power the 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."

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.

Fig a

DocumentToPDF trial version, to remove this mark, please register this software. Shunt compensation In shunt compensation, power system is connected in shunt with the FACTS. It works as a controllable source. Shunt compensation is of two types

Fig b

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.To compensate, shunt inductors are connected across the transmission line.

Static VAR Compensator . It helps in keeping steady state and dynamic voltages within their limits, but it does not provide any control of active power. The var output can be varied continuously between the capacitive and inductive ratings of the equipment. The necessary reactive power required for the compensation is generated or absorbed by capacitor or reactor banks. Thyristor switches are only used to control the combined reactive impedancethe banks present to the system. An advanced static compensator has been recently presented . This device provide control for real and reactive power.

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FACTS theory In the case of a no-loss line, voltage magnitude at receiving end is the same as voltage magnitude at sending end: Vs = Vr=V. Transmission results in a phase lag d4that depends on line reactance X. This is shown in fig

As it is a no-loss line, active power P is the same at any point of the

Fig c

Reactive power at sending end is the opposite of reactive power at receiving end:

As dHis very small, active power mainly depends on dHwhereas reactive power mainly depends on voltage magnitude

What are FACTS devices? FACTS stand for Flexible AC Transmission Systems. The term “FACTS” covers several power electronics based systems used for AC power transmission. FACTS solutions are particularly suitable in applications, which require one or more of the following qualities: • Rapid dynamic response • Ability for frequent variations in output • Smoothly adjustable output •

Fast implementation to achieve considerable increase in transmission capacity.

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FACTS involve devices such as: • Static Var Compensators (SVC) which are a well know workhorses that have been around for the last 20 years and well utilized within Australia. • STATCOM are new innovative devices based on utilization of high tech power electronic components, which result in a small installation footprint and very fast response times. • Thyristor Controlled Series Compensation is a very cost-effective way to improve power transfer and counter power swings and SSR.

FACTS CONCEPTS SIMILAR TO HVDC While some of the relevant technology i.e., Static VAR Compensation is already in wide use, the FACTS concept has brought to the table a tremendous potential for thyristor based controllers which will surely revolutionize the power system. The technology offers the utilities the ability to: 1. Control power flows on their transmission routes; 2. Allow secure loading of transmission lines to their full thermal capacity. FACTS technology, while allowing use of transmission to its thermal capacity, does not do away with the need for additional transmission lines or the upgrading of existing lines where thermal limits have been reached or when evaluation of losses added to the cost of FACTS technology shows that new lines or upgrading of existing lines is the most optimum answer. Often, ac transmission systems are thought of as being "inflexible". Power

flow in ac

networks simply follows Ohm's law and ordinarily cannot be made to flow along specific desired paths. As a result, ac networks suffer from parallel-path, or "loop" flows. The power flows from source to load in inverse proportion to the relative impedances of the transmission paths. Low

DocumentToPDF trial version, to remove this mark, please register this software. impedance paths take the largest fraction of flow, but all lines in the interconnection are a part of the flow path. Thus, utilities not involved in an interchange power transaction can be affected.

A fundamental notion behind FACTS is that it is possible to continuously vary the apparent impedance of specific transmission lines so as to force power to flow along a "contract path". This is a brand-new concept for many system planners. As illustrated in Figure 1.3, with precise control of the impedance of transmission lines using FACTS devices, it is possible to maintain constant power flow along a desired path in the presence of continuous changes of load levels in the external ac network, and to react in a planned way to contingencies. Just as in HVDC applications, FACTS controls could be designed to enhance the behavior of the uncontrolled systems.. .

The flexible system owes its tighter transmission control to its ability to manage the interrelated parameters that constrain today's systems ,including series impedance, shunt impedance, phase angle, and the occurrence of oscillations at various frequencies below the rated frequency. By adding to in this way, the controllers enable a transmission line to function nearer its thermal rating. For example, a 500-kV line may have a loading limit of 1000-2000 MW for safe operation, but a thermal limit of 3000 MW. It is often not possible both to overcome these constraints and maintain the required system reliability by conventional mechanical means alone, such as tap changers, phase shifters, and switched capacitors and reactors (inductors). Granted, mechanical controllers are on the whole

DocumentToPDF trial version, to remove this mark, please register this software. less expensive, but they increasingly need to be supplemented by rapidly responding power electronics controllers.

The new technology is not a single, high-power electronic controller, but rather a collection of controllers, which can be applied individually or collectively in a specific power system to control the five interrelated functions already mentioned. The thyristor is their basic element, just as the transistor is the basic element for a whole variety of microelectronic circuit. Because all controllers for the flexible transmission system are applications of similar technology, their use will eventually benefit from volume production and further development of high-power electronics. Electric power networks integrate generation and load centers within each utility system and through interconnections among neighboring systems, share power with vast regional grids. The purpose of this is to take advantage of the diversity of loads, changes in peak demand due to weather and time differences, the availability of different generation reserves in various geographic regions, power sharing arrangements among utilities , shifts in fuel prices, regulatory changes, and other discrepancies.

Applying Flexibility to the Electric Power System The power industry term FACTS (Flexible AC Transmission Systems) covers a number of technologies that enhance the security, capacity and flexibility of power transmission systems. FACTS solutions enable power grid owners to increase existing transmission network capacity while maintaining or improving the operating margins necessary for grid stability. As a result, more power can reach consumers with a minimum impact on the environment, after substantially shorter project implementation times, and at lower investment costs - all compared to the alternative of building new transmission lines or power generation facilities. The two main reasons for incorporating FACTS devices in electric power systems are: • Raising dynamic stability limits

DocumentToPDF trial version, to remove this mark, please register this software. • Provide better power flow control

Neuro-Control Approach for Flexible AC Transmission Systems A neuro-control approach for flexible AC transmission systems (FACTS) based on radial basis function neural network (RBFNN) is presented in this paper. The proposed scheme consists of a single neuron network whose input is derived from the active or reactive power or voltage derivation at the power system bus, where the FACTS device (in this case an unified power flow controller) is located. The performance and usefulness of this approach is tested and evaluated using both single machine infinite-bus and two-machine power system subjected to various transient disturbances. It was found that the new intelligent controller for FACTS exhibits a superior dynamic performance in compensation to the existing classical control schemes. Its simple architecture reduces the computational overhead, thereby real-time implementation.

Benefits of FACTS When implemented on a broad-scale basis, FACTS technologies deliver the following benefits. ?ÄRapidly Implemented Installations: FACTS projects are installed at existing substations and avoid the taking of public or private lands. They can be completed in less than 12 to 18 months—a substantially shorter timeframe than the process required for constructing new transmission lines. ?ì Increased System Capacity: FACTS provide increased capacity on the existing electrical transmission system infrastructure by allowing maximum operational efficiency of existing transmission lines and other equipment.

DocumentToPDF trial version, to remove this mark, please register this software. ?‘ Enhanced System Reliability: FACTS strengthen the operational integrity of transmission networks, allowing greater voltage stability and power flow control, which leads to enhanced system reliability and security.

?ö Improved System Controllability: FACTS allow improved system controllability by building “intelligence” into the transmission network via the ability to instantaneously respond to system disturbances and gridlock constraints and to enable redirection of power flows. ?h Seamless System Interconnections: FACTS, in the form of BTB dc-link configurations, can establish “seamless” interconnections within and between regional and local networks, allowing controlled power transfer and an increase in grid stability.

Conclusions Over the past several years, FACTS applications have increased significantly as compared to the previous decade. There are now numerous FACTS applications, including recent installations in California and Texas, as well as in the New England region of the United States and in some areas of Canada. As more electricity stakeholders recognize the technical and public policy advantages that these technologies confer, additional applications will emerge. Advancements in the state of the art of FACTS technologies will continue and will further advance the case for breaking transmission gridlock with these innovative and proven systems. REFERENCES: Ø

Narain G. Hingorani, Laszlo Gyugyi Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems, Wiley-IEEE Press, December 1999.

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An Overview of Flexible AC Transmission System-- docs.lib.purdue.edu/ecetr/205/ -

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A Neuro-Control Approach for Flexible AC Transmission www.actapress.com/PaperInfo.aspx?PaperID=18012-

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Flexible AC Transmission system- Wikipedia, the free encyclopedia-http://en.wikepidia.org/wiki/Flexible_AC_transmission_system

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FACTS (Flexible AC Transmission Systems) www02.abb.com/GLOBAL/GAD/GAD02181.NSF/