FACTS: Devices and applications

The concept of FACTS (Flexible Alternating Current Transmission System) refers to a family of power electronics-based devices able to enhance AC system controllability and stability and to increase power transfer capability. The design of the different schemes and configurations of FACTS devices is based on the combination of traditional power system components (such as transformers, reactors, switches, and capacitors) with power electronics elements (such as various types of transistors and thyristors). Over the last years, the current rating of thyristors has evolved into higher nominal values making power electronics capable of high power applications of tens, hundreds and thousands of MW. FACTS devices, thanks to their speed and flexibility, are able to provide the transmission system with several advantages such as: transmission capacity enhancement, power flow control, transient stability improvement, power oscillation damping, voltage stability and control. Depending on the type and rating of the selected device and on the specific voltage level and local network conditions, a transmission capacity enhancement of up to 40-50% may be achieved by installing a FACTS element. In comparison to traditional mechanically-driven devices, FACTS controllers are also not subject to wear and require a lower maintenance. Costs, complexity and reliability issues represent nowadays the main barriers to the integration of these promising technologies from the TSOs' perspective. Further FACTS penetration will depend on the technology providers' ability to overcome these barriers, thanks to more standardization, interoperability and economies of scale. In the present day scenario, transmission systems are becoming increasingly stressed, more difficult to operate, and more insecure with unscheduled power flows and greater losses because of growing demand for electricity and restriction on the construction of new lines. However, many high-voltage transmission systems are operating below their thermal ratings due to constraints, such as voltage and stability limits. Now, more advanced technology is used for reliable and operation of transmission and distribution in power system. To achieve both reliable and benefit economically, it has become clearer that more efficient utilization and control of the existing transmission system infrastructure is required. Improved utilization of the existing power system is provided through the application of advanced control technologies. Power electronics has developed the flexible AC transmission system (FACTS) devices. FACTS devices are effective and capable of increasing the power transfer capability of a line and support the power system to work with comfortable margins of stability. FACTS devices are used in transmission system to control and utilize the flexibility and system performance. To achieve all, the insertion of FACTS devices required in plant in order to control the main parameters namely voltage.

CLASSIFICATION OF FACTS DEVICES : The FACTS controllers are of different types. These are classified according to their connection like shunt connected controllers, series connected and combined series and shunt connected controllers. The main types of FACTS devices are described below: A. Static Var Compensators (SVC) The most important shunt FACTS device such as Static Var Compensator, it have been dynamic voltage problems. The accuracy, availability and fast response enable Static Var Compensators to provide high performance steady state and transient voltage control compared with classical shunt compensation. Static Var Compensators are also used improve the transient stability, damp power swings and reduce system losses by using reactive power control. B. Static Synchronous Compensator (STATCOM) STATCOM’s are GTO (gate turn-off type thyristor) based SVC’s. They do not require large inductive and capacitive components to provide inductive or capacitive reactive power to high voltage transmission systems as required in Static Var Compensators (SVC). STATCOM requires less area. Another advantage is the higher reactive output at low system voltages where a STATCOM can be considered as a current source independent from the system voltage. C. Static Synchronous Series Compensator (SSSC) Voltage sourced converter (VSC) based series onnected FACTS controller known as SSSC. It can inject a voltage with controllable magnitude and phase angle at the line frequency and found to be more capable of handling power flow control, improvement of transient. D. Thyristor Controlled Series Compensators (TCSC) TCSC is an extension of conventional series capacitors but only addition of thyristor-controlled reactor with it. Connecting a reactance in parallel with a series capacitor enables a continuous and rapidly variable series compensation system. The main advantages of TCSC’s are increased real transfer power, power oscillations damping, sub-synchronous resonances damping, and power flow line control E. Unified Power Flow Controller (UPFC) UPFC is combination of shunt connected device (STATCOM) and a series branch (SSSC) in the transmission line via its DC link. This device is most versatile FACTS device. It can not only perform the function of STATCOM ,TCSC and phase prevent faults, they can mitigate the effects of faults and make electricity supply more secure by reducing the number of line trips. The advantages of using FACTS devices in electrical transmission systems are described below. • More utilization of existing transmission system • Reliability of Transmission system increases. • More Increased transient and dynamic stability of the system. • Increased more quality ofsupply for large industries • Beneficial for Environment. A. More utilization of existing transmission system In all the countries, the power demand is increasing day by day to transfer the electrical power and controlling the load flow of the transmission system is very necessary .this can be achieved by more load centers which can change frequently. Addition of new transmission line is very costly to take the increased load on the system; in that case FACTS devices are much economical to meet the increased load on the same transmission lines. B. More Increased transient and dynamic stability of the system The Long transmission lines are inter-connected with grids to absorb the changing the loading of the transmission line and it is also seen that there should be no line fault creates in the line / transmission system. By doing this the power flow is reduced and transmission line can be trip. By the use of FACTS devices high power transfer capacity is increased at the same time line tripling faults are also reduces. C. Increased more quality of supply for large industries New industries wants good quality of electric supply , constant voltage with less fluctuation and desired frequency as mentioned by electricity department . Reduce voltage, variation in frequency or loss of electric power can reduce the manufacturing of the industry and cause to high economical loss .FACTS devices can helps to provide the required quality of supply. D. Beneficial for Environment FACTS devices are becoming environmentally friendly. FACTS devices does not produce any type of waste hazard material so they are pollution free. These devices help us to deliver the electrical power more economically with better use of existing transmission lines while reducing the cost of new transmission line and generating more power. E. Increased transmission system reliability and availability Transmission system reliability and availability is affected by many different factors. Although FACTS devices had ability to reduce such factors and improves the system reliability and availability.