Description
Flexible AC Transmission System
A PAPER ON
FACTS (Flexible Transmission System) AC
Submitted by: Inderjeet Singh Mody 8th Semester Semester Electrical Engg. Dept. Dept. Electrical Engg. Karan 8th
B.I.T, Durg Email id: - [email protected] [email protected] Contact: - 09826369013
B.I.T, Durg
BHILAI INSTITUTE OF TECHNOLOGY, DURG
INDEX
• •
•
ABSTRACT INTRODUCTION ELECTRICAL TRANSMISSION NETWORKS CONVENTIONAL CONTROL MECHANISMS # Automatic Generation Control # Excitation Control # Transformer Tap Changer Control # Phase-Shifting Transformers
1. 1. 2.
•
3. 4. 5. 5. 6. 7. 9. 9. 10. 11. 11.
•
FACTS DEVICES # Static Var Compensators (SVC’s) #Thyristor Controlled or Thyristor Switched Reacters # Static Synchronous Compensator (STATCOM ) # Thyristor-Switched Series Capacitor (TSSC) # Thyristor-Controlled Series Capacitor (TCSC) # Phase Angle Regulator # Unified Power Flow Controller (UPFC)
• • • •
BENEFITS OF UTILIZING FACTS DEVICES FUTURE DEVELOPMENTS IN FACTS CONCLUSION BIBLIOGRAPHY
13. 14. 15. 15.
-iLIST OF FIGURES Figure 1: - Basic Speed Governing System Figure 2: - A conceptual block diagram of a modern excitation controller. Figure 3: - Phasor Diagram of Phase Shifting Transformer Figure 4: - Two machine system with SVC in the middle Figure 5: - Thyristor Controlled Reactor Figure 6: - Static Synchronous Compensator Figure 7: Course of capacitor voltage for a basic element in a TSSC Figure 8: Thyristor-Controlled Series Capacitor (TCSC) Figure 9: Phase Angle Regulator Figure 10: Concept of the UPFC in a two-machine power system
EQUATIONS USED Boost Factor: KB=(XTCSC/XC) Uq=-jXCI
Synchronous Voltage Source: -
NOTATIONS USED
KB
:-
Boost Factor Apparent Reactance Capacitive Reactance Output Voltage of Synchronous Voltage Source
XTCSC :XC Uq ::-
-ii-
ABSTRACT
Increased demands on transmission and absence of long-term planning have created tendencies towards less security and reduced quality of supply. The Flexible alternating current transmission systems or FACTS technology is essential to alleviate some but not all of these difficulties. The FACTS technology opens up new opportunities for controlling power and enhancing the usable capacity of present, as well as new and upgraded lines. The possibility that current and therefore power through a line can be controlled enables a large potential of increasing the capacity of existing lines. These opportunities arise through the ability of FACTS controllers to control the interrelated parameters that govern the operation of transmission systems including series impedance, shunt impedance, current, voltage and phase angle. FACTS devices provide strategic benefits for improved transmission system management through better utilization of existing transmission assets, increased transmission system reliability and availability, increased dynamic and transient grid stability, increased quality of supply for sensitive industries, and enabling environmental benefits. This paper starts by providing the IEEE definitions of the most common FACTS devices as well as enumerates their benefits. This chapter also introduces the basic operating principles of new FACTS devices.
INTRODUCTION
The need for more efficient electricity systems management has given rise to innovative technologies in power generation and transmission. FACTS as they are generally known, are new devices that improve transmission systems. Flexible alternating-current transmission systems (FACTS) are defined by the IEEE as “ac transmission systems incorporating power
electronics-based and other static controllers to enhance controllability and increase power transfer capability”. Worldwide transmission systems are undergoing continuous changes and restructuring. They are becoming more heavily loaded and are being operated in ways not originally envisioned. Transmission systems must be flexible to react to more diverse generation and load patterns. In addition, the economical utilization of transmission system assets is of vital importance to enable utilities in industrialized countries to remain competitive and to survive. In developing countries, the optimized use of transmission systems investments is also important to support industry, create employment and utilize efficiently scarce economic resources. -1Flexible AC Transmission Systems (FACTS) is a technology that responds to these needs. It significantly alters the way transmission systems are developed and controlled together with improvements in asset utilization, system flexibility and system performance.
ELECTRICAL TRANSMISSION NETWORKS
The rapid growth in electrical energy use, combined with the demand for low cost energy, has gradually led to the development of generation sites remotely located from the load centers. In particular, the remote generating stations include hydroelectric stations, which exploit sites with higher heads and significant water flows; fossil fuel stations, located close to coal mines; geothermal stations and tidal-power plants, which are site-bound; and, sometimes, nuclear power plants purposely built distant from urban centers. The generation of bulk power at remote locations necessitates the use of transmission lines to connect generation sites to load centers. Furthermore, to enhance system reliability, multiple lines that connect load centers to several sources, interlink neighboring utilities, and build the needed levels of redundancy have gradually led to the evolution of complex interconnected electrical transmission networks. These networks now exist on whole world. An electrical power transmission network comprises mostly 3-phase alternating current (ac) transmission lines operating at different transmission voltages (generally at 230 kV and higher). With increasing requirement of power-transmission capacity and/or longer transmission distances, the transmission voltages continue to increase; indeed, increases in transmission voltages are linked closely to decreasing transmission losses. An ac power transmission network comprises 3-phase overhead lines. However, in densely populated areas underground cable transmission is used. Increasing pressures arising from ecological and
aesthetic considerations, as well as improved reliability, favor underground transmission for future expansion. In a complex interconnected ac transmission network, the source-to-a-load power flow finds multiple transmission paths. For a system comprising multiple sources and numerous loads, a load-flow study must be performed to determine the levels of active- and reactive-power flows on all lines. Its impedance and the voltages at its terminals determine the flow of active and reactive powers on a line. The result is that whereas interconnected ac transmission networks provide reliability of power supply, no control exists on line loading except to modify them by changing line impedances by adding series and/or shunt-circuit -2elements (capacitors and reactors). The long-distance separation of a generating station from a load center requiring long transmission lines of high capacity and, in some cases in which a transmission line must cross a body of water, the use of ac/dc and dc/ac converters at the terminals of an HVDC line, became a viable alternative from many years. The most significant feature of an HVDC transmission network is its full controllability with respect to power transmission. Until recently, active- and reactive-power control in ac transmission networks was exercised by carefully adjusting transmission line impedances, as well as regulating terminal voltages by generator excitation control and by transformer tap changers. At times, series and shunt impedances were employed to effectively change line impedances.
CONVENTIONAL CONTROL MECHANISMS
In today’s world, a lack of control on active- and reactive-power flow on a given line, embedded in an interconnected ac transmission network, was stated. Also, to maintain steady-state voltages and, in selected cases, to alter the power-transmission capacity of lines, traditional use of shunt and series impedances was there. In a conventional ac power system, however, most of the controllability exists at generating stations. These generators, in fact, are purposely operated at reduced power. Also, to regulate the system frequency and for maintaining the system at the rated voltage, controls are exercised on selected generators.
AUTOMATIC GENERATION CONTROL (AGC)
The megawatt (MW) output of a generator is regulated by controlling the driving torque, Tm, provided by a prime-mover turbine. In a conventional electromechanical system, it could be a
steam or a hydraulic turbine. The needed change in the turbine-output torque is achieved by controlling the steam/water input into the turbine. Therefore, in situations where the output exceeds or falls below the input, a speed-governing system senses the deviation in the generator speed because of the load-generation mismatch, adjusts the mechanical driving torque to restore the power balance, and returns the operating speed to its rated value. The speed-governor output is invariably taken through several stages of mechanical amplification for controlling the inlet (steam/water) valve of the driving turbine. Figure 1 shows the basic speed-governing system of a generator supplying an isolated load. The operation of this basic feedback-control system is -3enhanced by adding further control inputs to help control the frequency of a large interconnection. In that role, the control system becomes an automatic generation control (AGC) with supplementary signals.
Figure 1: - Basic Speed Governing System
Excitation Control
The basic function of an exciter is to provide a dc source for field excitation of a synchronous generator. A control on exciter voltage results in controlling the field current, which, in turn, controls the generated voltage. When a synchronous generator is connected to a large system where the operating frequency and the terminal voltages are largely unaffected by a generator, its
excitation control causes its reactive power output to change. In older power plants, a dc generator, also called an exciter, was mounted on the main generator shaft. A control of the field excitation of the dc generator provided a controlled excitation source for the main generator. In contrast, modern stations employ either a brushless exciter or a static exciter. An excitationcontrol system employs a voltage controller to control the excitation voltage. This operation is typically recognized as an automatic voltage regulator (AVR). However, because an excitation control operates quickly, several stabilizing and protective signals are invariably added to the basic voltage regulator. A power-system stabilizer (PSS) is implemented by adding auxiliary damping signals derived from the shaft speed, or the terminal frequency, or the power—an effective and frequently used technique for enhancing small-signal stability of the connected system. Figure 2 shows the functionality of an excitation-control system. -4-
Figure 2: - A conceptual block diagram of a modern excitation controller.
Transformer Tap-Changer Control
Next to the generating units, transformers constitute the second family of major powertransmission-system apparatuses. In addition to increasing and decreasing nominal voltages, many transformers are equipped with tap-changers to realize a limited range of voltage control. This tap control can be used out manually or automatically. Two types of tap changers are usually available: offload tap changers, which perform adjustments when de-energized, and onload tap changers, which are equipped with current-commutation capacity and are operated under load. Tap changers may be provided on one of the two transformer windings as well as on autotransformers. Because tap-changing transformers vary voltages and, therefore, the reactive power flow, these transformers may be used as reactive-power-control devices. On-load tap-
changing transformers are usually employed to correct voltage profiles on an hourly or daily basis to accommodate load variations. Their speed of operation is generally slow, and frequent operations result in electrical and mechanical wear and tear.
Phase-Shifting Transformers
A special form of a 3-phase-regulating transformer is realized by combining a transformer that is connected in series with a line to a voltage transformer equipped with a tap changer. The windings of the voltage transformer are so connected that on its secondary side, phase-quadrature voltages are generated and fed into the secondary windings of the series transformer. Thus the addition of small, phase-quadrature voltage components to the phase voltages of the line creates -5phase-shifted output voltages without any appreciable change in magnitude. A phase-shifting transformer is therefore able to introduce a phase shift in a line. Figure 3 shows a phasor diagram. The phasor diagram shows the phase shift realized without an appreciable change in magnitude by the injection of phase-quadrature voltage components in a 3-phase system. When a phase-shifting transformer employs an on-load tap changer, controllable phase-shifting is achieved. The interesting aspect of such phase shifters is that despite their low MVA capacity, by controlling the phase shift they exercise a significant real-power control. A promising application of these devices is in creating active-power regulation on selected lines and securing activepower damping through the incorporation of auxiliary signals in their feedback controllers. From this description, it is easy to visualize that an incremental in-phase component can also be added in lines to alter only their voltage magnitudes, not their phase.
Figure 3: - Phasor Diagram of Phase Shifting Transformer
FACTS devices
FACTS devices are used for the dynamic control of voltage, impedance and phase angle of high voltage AC transmission lines. Below the different main types of FACTS devices are described:
Static Shunt Compensator or Static Var Compensators (SVC’s)
It is the most important FACTS device, have been used for a number of years to improve transmission line economics by resolving dynamic voltage problems. The IEEE definition is as follows: -6‘A shunt-connected static var generator or absorber whose output is adjusted to exchange capacitive or inductive current so as to maintain or control specific parameters of the electrical power system’. The accuracy, availability and fast response enable SVC’s to provide high performance steady state and transient voltage control compared with classical shunt compensation. SVC’s are also used to dampen power swings, improve transient stability, and reduce system losses by optimized reactive power control. SVC is an umbrella term for several devices. The characteristics of a SVC are described as • based on normal inductive and capacitive elements • not based on rotating machines • control function is through power electronics. If the shunt compensator is located at the end of a line in parallel to a load it is possible to regulate the voltage at this end and therefore to prevent voltage instability caused by load variations or generation or line outages. As shunt compensation is able to change the power flow in the system by varying the value of the applied shunt compensation during and following dynamic disturbances the transient stability limit can be increased and effective power oscillation damping is provided.
Figure 4: - Two machine system with SVC in the middle
-7-
Thyristor Controlled or Thyristor Switched Reacters (TCR or TSR)
The IEEE definition of TCR explains it as ‘A shunt-connected, thyristor-controlled inductor whose effective reactance is varied in a continuous manner by partial-conduction control of the thyristor value.’ An elementary single-phase thyristor-controlled reactor (TCR) is shown in Fig. 5.The current in the thyristor controlled reactor can be controlled from maximum to zero by the method of firing delay angle control. That is the duration of the current conduction intervals is controlled by delaying the closure of the thyristor valve with respect to the peak of the applied voltage in each half-cycle (Fig. 5). For ? = 0? the amplitude is at its maximum and for ? = 90? the amplitude is zero and no current is flowing during the corresponding half-cycle. Like this the same effect is provided as with an inductance of changing value.
Figure 5: - Thyristor Controlled Reactor
A thyristor switched reactor (TSR) has similar equipment to a TCR, but is used only at fixed angles of 90? and 180?, i.e. full conduction or no conduction. The reactive current will be proportional to the applied voltage. Several TSRs can provide a reactive admittance controllable in a step-like manner. IEEE defines TSR as ‘A shunt-connected, thyristor-switched inductor whose effective reactance is varied in a stepwise manner by full- or zero-conduction operation of the thyristor value.’ -8-
Static Synchronous Compensator (STATCOM )
IEEE defines it as ‘A static synchronous generator operated as a shunt-connected static varcompensator whose capacitive or inductive output current can be controlled independent of the AC system voltage.’ STATCOMs are GTO (gate turn-off type thyristor) based SVC’s. A STATCOM is a controlled reactive-power source. It provides voltage support by generating or absorbing reactive power at the point of common coupling without the need of large external reactors or capacitor banks. The basic voltage-source converter scheme is shown in Fig. 7. Compared with conventional SVC’s, they don’t require large inductive and capacitive components to provide inductive or capacitive reactive power to high voltage transmission systems. This results in smaller land requirements. An additional 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.
Figure 6: - Static Synchronous Compensator
Thyristor-Switched Series Capacitor (TSSC)
The basic element of a TSSC is a capacitor shunted by bypass valve shown in Fig. 8. The capacitor is inserted into the line if the corresponding thyristor valve is turned off, otherwise it is bypassed. A thyristor valve is turned off in an instance when the current crosses zero. Thus, the capacitor can be inserted into the line by the thyristor valve only at the zero crossings of the line current. On the other hand, the thyristor valve should be turned on for bypass only when the capacitor voltage is zero in order to minimize the initial surge current in the valve, and the corresponding circuit transient. This results in a possible delay up to one full cycle to turn the valve on. -9-
Figure 7: Course of capacitor voltage for a basic element in a TSSC
Therefore, if the capacitor is once inserted into the line, it will be charged by the line current from zero to maximum during the first half-cycle and discharged from maximum to zero during the successive half-cycle until it can be bypassed again.
Thyristor-Controlled Series Capacitor (TCSC)
IEEE defines it as ‘A capacitive reactance compensator which consists of a series capacitor bank shunted by a thyristor-controlled reactor in order to provide a smoothly variable series capacitive reactance.’ The scheme of a Thyristor-Controlled Series Capacitor is given in Fig. 9.
Figure 8: Thyristor-Controlled Series Capacitor (TCSC) The operating modes of a TCSC are characterized by the so-called boost factor (KB): KB=(XTCSC/XC) -------- 1
where XTCSC is the apparent reactance.
-10The modes of operationa of TCSC are: 1) Blocking Mode 2) By Pass Mode 3) Capacitive Boost Mode 4) Inductive Boost Mode
Phase Angle Regulator
Phase Angle Regulators are able to solve problems referred to the transmission angle which cannot be handled by the other series compensators. Even though these regulators, based on the classical arrangement of tap-changing transformers, are not able to supply or absorb reactive power they are capable of exchanging active power with the power system. Modern voltage and phase angle regulators are used to improve the transient stability, to provide power oscillation damping and to minimize the post-disturbance overloads and the corresponding voltage drops. In Fig. 11 the concept of a Phase Angle Regulator is shown. Theoretically, the Phase Angle Regulator can be considered a sinusoidal AC voltage source with controllable amplitude and phase angle. The angle of the voltage U? relative to U1 is stipulated such that the magnitudes of U1 and U1eff are equal.
Figure 9: Phase Angle Regulator
Unified Power Flow Controller (UPFC)
IEEE definition states as: ‘A combination of static synchronous compensator (STATCOM) and a static series compensator (SSSC) which are coupled via a common dc link, to allow bidirectional flow of active power between the series output terminals of the SSSC and the shunt output -11terminals of the STATCOM, and are controlled to provide concurrent active and reactive series line compensation without an external electric energy source. The UPFC, by means of angularly unconstrained series voltage injection, is able to control, concurrently or selectively, the transmission line voltage, impedance, and angle or, alternatively, the active and reactive power flow in the line. The UPFC may also provide independently controllable shunt reactive compensation.’ The UPFC was developed for the real-time control and dynamic compensation of AC transmission systems. It is able to control all the parameters affecting power flow in the transmission line. Alternatively, it can independently control both the active and reactive power
flow in the line. The UPFC is conceptually a synchronous voltage source with controllable magnitude Upq and angle ? placed in series with the line (refer Fig. 12). The voltage source exchanges both active and reactive power with the transmission system. But the voltage source can only produce reactive power, the active power has to be supplied to it by a power supply or a sink. This power supply is one of the end buses.
Figure 10: Concept of the UPFC in a two-machine power system
-12-
Benefits of utilizing FACTS devices
The benefits of utilizing FACTS devices in electrical transmission systems can be summarized as follows: • • • • • Better utilization of existing transmission system assets Increased transmission system reliability and availability Increased dynamic and transient grid stability and reduction of loop flows Increased quality of supply for sensitive industries Environmental benefits
Better utilization of existing transmission system assets
In many countries, increasing the energy transfer capacity and controlling the load flow of transmission lines are of vital importance, especially in de-regulated markets, where the locations of generation and the bulk load centers can change rapidly. Frequently, adding new transmission lines to meet increasing electricity demand is limited by economical and environmental constraints. FACTS devices help to meet these requirements with the existing transmission systems.
Increased transmission system reliability and availability
Transmission system reliability and availability is affected by many different factors. Although FACTS devices cannot prevent faults, they can mitigate the effects of faults and make electricity supply more secure by reducing the number of line trips. For example, a major load rejection results in an over voltage of the line which can lead to a line trip. SVC’s or STATCOMs counteract the over voltage and avoid line tripping.
Increased dynamic and transient grid stability
Long transmission lines, interconnected grids, impacts of changing loads and line faults can create instabilities in transmission systems. These can lead to reduced line power flow, loop flows or even to line trips. FACTS devices stabilize transmission systems with resulting higher energy transfer capability and reduced risk of line trips. -13-
Increased quality of supply for sensitive industries
Modern industries depend upon high quality electricity supply including constant voltage, and frequency and no supply interruptions. Voltage dips, frequency variations or the loss of supply can lead to interruptions in manufacturing processes with high resulting economic losses. FACTS devices can help provide the required quality of supply
Environmental benefits
FACTS devices are environmentally friendly. They contain no hazardous materials and produce no waste or pollutanse. FACTS help distribute the electrical energy more economically through better utilization of existing installations thereby reducing the need for additional transmission lines.
Future Developments in FACTS
Future developments will include the combination of existing devices, e.g. combining a STATCOM with a TSC (thyristor switched capacitor) to extend the operational range. In addition, more sophisticated control systems will improve the operation of FACTS devices. Improvements in semiconductor technology (e.g. higher current carrying capability, higher blocking voltages) could reduce the costs of FACTS devices and extend their operation ranges. Finally, developments in superconductor technology open the door to new devices like SCCL (Super Conducting Current Limiter) and SMES (Super Conducting Magnetic Energy Storage). There is a vision for a high voltage transmission system around the world – to generate electrical energy economically and environmentally friendly and provide electrical energy where it’s needed. FACTS are the key to make this vision live.
-14-
Conclusion
This paper has shown working of the conventional devices as well as the FACTS devices used to enhance controllability and increase power transfer capability. It can be easily seen that by using FACTS devices the existing transmission system assets are utilized in a better way. The other benefits like increased transmission system reliability, stability and last but obviously not the least the environmental benefits should also be undertaken. It has also been clearly shown that the future of these devices also depends upon the future of the semiconductors for extension
of operation ranges & economical benefits. In view of the importance of electricity grid reliability to national welfare, these factors now call for an increased governmental role in electric system reliability R&D.
BIBLIOGRAPHY
[1] Soni, Gupta, Bhatnagar & Chakravarthy, Electrical Power Generation. Dhanpat Rai Publishers, 2004. [2] P. Kundur, Power system stability and control. McGraw-Hill. New York 1994. [3] Ashfaq Hussain, Electrical Power Systems. CBS Publishers, 2005. [4] R.M. Mathur and R.K. Verma. Thyristor-based facts controllers for electrical transmission systems. IEEE Press, Piscataway, 2002. [5] www.ieee.org
-15-
doc_101842789.doc
Flexible AC Transmission System
A PAPER ON
FACTS (Flexible Transmission System) AC
Submitted by: Inderjeet Singh Mody 8th Semester Semester Electrical Engg. Dept. Dept. Electrical Engg. Karan 8th
B.I.T, Durg Email id: - [email protected] [email protected] Contact: - 09826369013
B.I.T, Durg
BHILAI INSTITUTE OF TECHNOLOGY, DURG
INDEX
• •
•
ABSTRACT INTRODUCTION ELECTRICAL TRANSMISSION NETWORKS CONVENTIONAL CONTROL MECHANISMS # Automatic Generation Control # Excitation Control # Transformer Tap Changer Control # Phase-Shifting Transformers
1. 1. 2.
•
3. 4. 5. 5. 6. 7. 9. 9. 10. 11. 11.
•
FACTS DEVICES # Static Var Compensators (SVC’s) #Thyristor Controlled or Thyristor Switched Reacters # Static Synchronous Compensator (STATCOM ) # Thyristor-Switched Series Capacitor (TSSC) # Thyristor-Controlled Series Capacitor (TCSC) # Phase Angle Regulator # Unified Power Flow Controller (UPFC)
• • • •
BENEFITS OF UTILIZING FACTS DEVICES FUTURE DEVELOPMENTS IN FACTS CONCLUSION BIBLIOGRAPHY
13. 14. 15. 15.
-iLIST OF FIGURES Figure 1: - Basic Speed Governing System Figure 2: - A conceptual block diagram of a modern excitation controller. Figure 3: - Phasor Diagram of Phase Shifting Transformer Figure 4: - Two machine system with SVC in the middle Figure 5: - Thyristor Controlled Reactor Figure 6: - Static Synchronous Compensator Figure 7: Course of capacitor voltage for a basic element in a TSSC Figure 8: Thyristor-Controlled Series Capacitor (TCSC) Figure 9: Phase Angle Regulator Figure 10: Concept of the UPFC in a two-machine power system
EQUATIONS USED Boost Factor: KB=(XTCSC/XC) Uq=-jXCI
Synchronous Voltage Source: -
NOTATIONS USED
KB
:-
Boost Factor Apparent Reactance Capacitive Reactance Output Voltage of Synchronous Voltage Source
XTCSC :XC Uq ::-
-ii-
ABSTRACT
Increased demands on transmission and absence of long-term planning have created tendencies towards less security and reduced quality of supply. The Flexible alternating current transmission systems or FACTS technology is essential to alleviate some but not all of these difficulties. The FACTS technology opens up new opportunities for controlling power and enhancing the usable capacity of present, as well as new and upgraded lines. The possibility that current and therefore power through a line can be controlled enables a large potential of increasing the capacity of existing lines. These opportunities arise through the ability of FACTS controllers to control the interrelated parameters that govern the operation of transmission systems including series impedance, shunt impedance, current, voltage and phase angle. FACTS devices provide strategic benefits for improved transmission system management through better utilization of existing transmission assets, increased transmission system reliability and availability, increased dynamic and transient grid stability, increased quality of supply for sensitive industries, and enabling environmental benefits. This paper starts by providing the IEEE definitions of the most common FACTS devices as well as enumerates their benefits. This chapter also introduces the basic operating principles of new FACTS devices.
INTRODUCTION
The need for more efficient electricity systems management has given rise to innovative technologies in power generation and transmission. FACTS as they are generally known, are new devices that improve transmission systems. Flexible alternating-current transmission systems (FACTS) are defined by the IEEE as “ac transmission systems incorporating power
electronics-based and other static controllers to enhance controllability and increase power transfer capability”. Worldwide transmission systems are undergoing continuous changes and restructuring. They are becoming more heavily loaded and are being operated in ways not originally envisioned. Transmission systems must be flexible to react to more diverse generation and load patterns. In addition, the economical utilization of transmission system assets is of vital importance to enable utilities in industrialized countries to remain competitive and to survive. In developing countries, the optimized use of transmission systems investments is also important to support industry, create employment and utilize efficiently scarce economic resources. -1Flexible AC Transmission Systems (FACTS) is a technology that responds to these needs. It significantly alters the way transmission systems are developed and controlled together with improvements in asset utilization, system flexibility and system performance.
ELECTRICAL TRANSMISSION NETWORKS
The rapid growth in electrical energy use, combined with the demand for low cost energy, has gradually led to the development of generation sites remotely located from the load centers. In particular, the remote generating stations include hydroelectric stations, which exploit sites with higher heads and significant water flows; fossil fuel stations, located close to coal mines; geothermal stations and tidal-power plants, which are site-bound; and, sometimes, nuclear power plants purposely built distant from urban centers. The generation of bulk power at remote locations necessitates the use of transmission lines to connect generation sites to load centers. Furthermore, to enhance system reliability, multiple lines that connect load centers to several sources, interlink neighboring utilities, and build the needed levels of redundancy have gradually led to the evolution of complex interconnected electrical transmission networks. These networks now exist on whole world. An electrical power transmission network comprises mostly 3-phase alternating current (ac) transmission lines operating at different transmission voltages (generally at 230 kV and higher). With increasing requirement of power-transmission capacity and/or longer transmission distances, the transmission voltages continue to increase; indeed, increases in transmission voltages are linked closely to decreasing transmission losses. An ac power transmission network comprises 3-phase overhead lines. However, in densely populated areas underground cable transmission is used. Increasing pressures arising from ecological and
aesthetic considerations, as well as improved reliability, favor underground transmission for future expansion. In a complex interconnected ac transmission network, the source-to-a-load power flow finds multiple transmission paths. For a system comprising multiple sources and numerous loads, a load-flow study must be performed to determine the levels of active- and reactive-power flows on all lines. Its impedance and the voltages at its terminals determine the flow of active and reactive powers on a line. The result is that whereas interconnected ac transmission networks provide reliability of power supply, no control exists on line loading except to modify them by changing line impedances by adding series and/or shunt-circuit -2elements (capacitors and reactors). The long-distance separation of a generating station from a load center requiring long transmission lines of high capacity and, in some cases in which a transmission line must cross a body of water, the use of ac/dc and dc/ac converters at the terminals of an HVDC line, became a viable alternative from many years. The most significant feature of an HVDC transmission network is its full controllability with respect to power transmission. Until recently, active- and reactive-power control in ac transmission networks was exercised by carefully adjusting transmission line impedances, as well as regulating terminal voltages by generator excitation control and by transformer tap changers. At times, series and shunt impedances were employed to effectively change line impedances.
CONVENTIONAL CONTROL MECHANISMS
In today’s world, a lack of control on active- and reactive-power flow on a given line, embedded in an interconnected ac transmission network, was stated. Also, to maintain steady-state voltages and, in selected cases, to alter the power-transmission capacity of lines, traditional use of shunt and series impedances was there. In a conventional ac power system, however, most of the controllability exists at generating stations. These generators, in fact, are purposely operated at reduced power. Also, to regulate the system frequency and for maintaining the system at the rated voltage, controls are exercised on selected generators.
AUTOMATIC GENERATION CONTROL (AGC)
The megawatt (MW) output of a generator is regulated by controlling the driving torque, Tm, provided by a prime-mover turbine. In a conventional electromechanical system, it could be a
steam or a hydraulic turbine. The needed change in the turbine-output torque is achieved by controlling the steam/water input into the turbine. Therefore, in situations where the output exceeds or falls below the input, a speed-governing system senses the deviation in the generator speed because of the load-generation mismatch, adjusts the mechanical driving torque to restore the power balance, and returns the operating speed to its rated value. The speed-governor output is invariably taken through several stages of mechanical amplification for controlling the inlet (steam/water) valve of the driving turbine. Figure 1 shows the basic speed-governing system of a generator supplying an isolated load. The operation of this basic feedback-control system is -3enhanced by adding further control inputs to help control the frequency of a large interconnection. In that role, the control system becomes an automatic generation control (AGC) with supplementary signals.
Figure 1: - Basic Speed Governing System
Excitation Control
The basic function of an exciter is to provide a dc source for field excitation of a synchronous generator. A control on exciter voltage results in controlling the field current, which, in turn, controls the generated voltage. When a synchronous generator is connected to a large system where the operating frequency and the terminal voltages are largely unaffected by a generator, its
excitation control causes its reactive power output to change. In older power plants, a dc generator, also called an exciter, was mounted on the main generator shaft. A control of the field excitation of the dc generator provided a controlled excitation source for the main generator. In contrast, modern stations employ either a brushless exciter or a static exciter. An excitationcontrol system employs a voltage controller to control the excitation voltage. This operation is typically recognized as an automatic voltage regulator (AVR). However, because an excitation control operates quickly, several stabilizing and protective signals are invariably added to the basic voltage regulator. A power-system stabilizer (PSS) is implemented by adding auxiliary damping signals derived from the shaft speed, or the terminal frequency, or the power—an effective and frequently used technique for enhancing small-signal stability of the connected system. Figure 2 shows the functionality of an excitation-control system. -4-
Figure 2: - A conceptual block diagram of a modern excitation controller.
Transformer Tap-Changer Control
Next to the generating units, transformers constitute the second family of major powertransmission-system apparatuses. In addition to increasing and decreasing nominal voltages, many transformers are equipped with tap-changers to realize a limited range of voltage control. This tap control can be used out manually or automatically. Two types of tap changers are usually available: offload tap changers, which perform adjustments when de-energized, and onload tap changers, which are equipped with current-commutation capacity and are operated under load. Tap changers may be provided on one of the two transformer windings as well as on autotransformers. Because tap-changing transformers vary voltages and, therefore, the reactive power flow, these transformers may be used as reactive-power-control devices. On-load tap-
changing transformers are usually employed to correct voltage profiles on an hourly or daily basis to accommodate load variations. Their speed of operation is generally slow, and frequent operations result in electrical and mechanical wear and tear.
Phase-Shifting Transformers
A special form of a 3-phase-regulating transformer is realized by combining a transformer that is connected in series with a line to a voltage transformer equipped with a tap changer. The windings of the voltage transformer are so connected that on its secondary side, phase-quadrature voltages are generated and fed into the secondary windings of the series transformer. Thus the addition of small, phase-quadrature voltage components to the phase voltages of the line creates -5phase-shifted output voltages without any appreciable change in magnitude. A phase-shifting transformer is therefore able to introduce a phase shift in a line. Figure 3 shows a phasor diagram. The phasor diagram shows the phase shift realized without an appreciable change in magnitude by the injection of phase-quadrature voltage components in a 3-phase system. When a phase-shifting transformer employs an on-load tap changer, controllable phase-shifting is achieved. The interesting aspect of such phase shifters is that despite their low MVA capacity, by controlling the phase shift they exercise a significant real-power control. A promising application of these devices is in creating active-power regulation on selected lines and securing activepower damping through the incorporation of auxiliary signals in their feedback controllers. From this description, it is easy to visualize that an incremental in-phase component can also be added in lines to alter only their voltage magnitudes, not their phase.
Figure 3: - Phasor Diagram of Phase Shifting Transformer
FACTS devices
FACTS devices are used for the dynamic control of voltage, impedance and phase angle of high voltage AC transmission lines. Below the different main types of FACTS devices are described:
Static Shunt Compensator or Static Var Compensators (SVC’s)
It is the most important FACTS device, have been used for a number of years to improve transmission line economics by resolving dynamic voltage problems. The IEEE definition is as follows: -6‘A shunt-connected static var generator or absorber whose output is adjusted to exchange capacitive or inductive current so as to maintain or control specific parameters of the electrical power system’. The accuracy, availability and fast response enable SVC’s to provide high performance steady state and transient voltage control compared with classical shunt compensation. SVC’s are also used to dampen power swings, improve transient stability, and reduce system losses by optimized reactive power control. SVC is an umbrella term for several devices. The characteristics of a SVC are described as • based on normal inductive and capacitive elements • not based on rotating machines • control function is through power electronics. If the shunt compensator is located at the end of a line in parallel to a load it is possible to regulate the voltage at this end and therefore to prevent voltage instability caused by load variations or generation or line outages. As shunt compensation is able to change the power flow in the system by varying the value of the applied shunt compensation during and following dynamic disturbances the transient stability limit can be increased and effective power oscillation damping is provided.
Figure 4: - Two machine system with SVC in the middle
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Thyristor Controlled or Thyristor Switched Reacters (TCR or TSR)
The IEEE definition of TCR explains it as ‘A shunt-connected, thyristor-controlled inductor whose effective reactance is varied in a continuous manner by partial-conduction control of the thyristor value.’ An elementary single-phase thyristor-controlled reactor (TCR) is shown in Fig. 5.The current in the thyristor controlled reactor can be controlled from maximum to zero by the method of firing delay angle control. That is the duration of the current conduction intervals is controlled by delaying the closure of the thyristor valve with respect to the peak of the applied voltage in each half-cycle (Fig. 5). For ? = 0? the amplitude is at its maximum and for ? = 90? the amplitude is zero and no current is flowing during the corresponding half-cycle. Like this the same effect is provided as with an inductance of changing value.
Figure 5: - Thyristor Controlled Reactor
A thyristor switched reactor (TSR) has similar equipment to a TCR, but is used only at fixed angles of 90? and 180?, i.e. full conduction or no conduction. The reactive current will be proportional to the applied voltage. Several TSRs can provide a reactive admittance controllable in a step-like manner. IEEE defines TSR as ‘A shunt-connected, thyristor-switched inductor whose effective reactance is varied in a stepwise manner by full- or zero-conduction operation of the thyristor value.’ -8-
Static Synchronous Compensator (STATCOM )
IEEE defines it as ‘A static synchronous generator operated as a shunt-connected static varcompensator whose capacitive or inductive output current can be controlled independent of the AC system voltage.’ STATCOMs are GTO (gate turn-off type thyristor) based SVC’s. A STATCOM is a controlled reactive-power source. It provides voltage support by generating or absorbing reactive power at the point of common coupling without the need of large external reactors or capacitor banks. The basic voltage-source converter scheme is shown in Fig. 7. Compared with conventional SVC’s, they don’t require large inductive and capacitive components to provide inductive or capacitive reactive power to high voltage transmission systems. This results in smaller land requirements. An additional 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.
Figure 6: - Static Synchronous Compensator
Thyristor-Switched Series Capacitor (TSSC)
The basic element of a TSSC is a capacitor shunted by bypass valve shown in Fig. 8. The capacitor is inserted into the line if the corresponding thyristor valve is turned off, otherwise it is bypassed. A thyristor valve is turned off in an instance when the current crosses zero. Thus, the capacitor can be inserted into the line by the thyristor valve only at the zero crossings of the line current. On the other hand, the thyristor valve should be turned on for bypass only when the capacitor voltage is zero in order to minimize the initial surge current in the valve, and the corresponding circuit transient. This results in a possible delay up to one full cycle to turn the valve on. -9-
Figure 7: Course of capacitor voltage for a basic element in a TSSC
Therefore, if the capacitor is once inserted into the line, it will be charged by the line current from zero to maximum during the first half-cycle and discharged from maximum to zero during the successive half-cycle until it can be bypassed again.
Thyristor-Controlled Series Capacitor (TCSC)
IEEE defines it as ‘A capacitive reactance compensator which consists of a series capacitor bank shunted by a thyristor-controlled reactor in order to provide a smoothly variable series capacitive reactance.’ The scheme of a Thyristor-Controlled Series Capacitor is given in Fig. 9.
Figure 8: Thyristor-Controlled Series Capacitor (TCSC) The operating modes of a TCSC are characterized by the so-called boost factor (KB): KB=(XTCSC/XC) -------- 1
where XTCSC is the apparent reactance.
-10The modes of operationa of TCSC are: 1) Blocking Mode 2) By Pass Mode 3) Capacitive Boost Mode 4) Inductive Boost Mode
Phase Angle Regulator
Phase Angle Regulators are able to solve problems referred to the transmission angle which cannot be handled by the other series compensators. Even though these regulators, based on the classical arrangement of tap-changing transformers, are not able to supply or absorb reactive power they are capable of exchanging active power with the power system. Modern voltage and phase angle regulators are used to improve the transient stability, to provide power oscillation damping and to minimize the post-disturbance overloads and the corresponding voltage drops. In Fig. 11 the concept of a Phase Angle Regulator is shown. Theoretically, the Phase Angle Regulator can be considered a sinusoidal AC voltage source with controllable amplitude and phase angle. The angle of the voltage U? relative to U1 is stipulated such that the magnitudes of U1 and U1eff are equal.
Figure 9: Phase Angle Regulator
Unified Power Flow Controller (UPFC)
IEEE definition states as: ‘A combination of static synchronous compensator (STATCOM) and a static series compensator (SSSC) which are coupled via a common dc link, to allow bidirectional flow of active power between the series output terminals of the SSSC and the shunt output -11terminals of the STATCOM, and are controlled to provide concurrent active and reactive series line compensation without an external electric energy source. The UPFC, by means of angularly unconstrained series voltage injection, is able to control, concurrently or selectively, the transmission line voltage, impedance, and angle or, alternatively, the active and reactive power flow in the line. The UPFC may also provide independently controllable shunt reactive compensation.’ The UPFC was developed for the real-time control and dynamic compensation of AC transmission systems. It is able to control all the parameters affecting power flow in the transmission line. Alternatively, it can independently control both the active and reactive power
flow in the line. The UPFC is conceptually a synchronous voltage source with controllable magnitude Upq and angle ? placed in series with the line (refer Fig. 12). The voltage source exchanges both active and reactive power with the transmission system. But the voltage source can only produce reactive power, the active power has to be supplied to it by a power supply or a sink. This power supply is one of the end buses.
Figure 10: Concept of the UPFC in a two-machine power system
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Benefits of utilizing FACTS devices
The benefits of utilizing FACTS devices in electrical transmission systems can be summarized as follows: • • • • • Better utilization of existing transmission system assets Increased transmission system reliability and availability Increased dynamic and transient grid stability and reduction of loop flows Increased quality of supply for sensitive industries Environmental benefits
Better utilization of existing transmission system assets
In many countries, increasing the energy transfer capacity and controlling the load flow of transmission lines are of vital importance, especially in de-regulated markets, where the locations of generation and the bulk load centers can change rapidly. Frequently, adding new transmission lines to meet increasing electricity demand is limited by economical and environmental constraints. FACTS devices help to meet these requirements with the existing transmission systems.
Increased transmission system reliability and availability
Transmission system reliability and availability is affected by many different factors. Although FACTS devices cannot prevent faults, they can mitigate the effects of faults and make electricity supply more secure by reducing the number of line trips. For example, a major load rejection results in an over voltage of the line which can lead to a line trip. SVC’s or STATCOMs counteract the over voltage and avoid line tripping.
Increased dynamic and transient grid stability
Long transmission lines, interconnected grids, impacts of changing loads and line faults can create instabilities in transmission systems. These can lead to reduced line power flow, loop flows or even to line trips. FACTS devices stabilize transmission systems with resulting higher energy transfer capability and reduced risk of line trips. -13-
Increased quality of supply for sensitive industries
Modern industries depend upon high quality electricity supply including constant voltage, and frequency and no supply interruptions. Voltage dips, frequency variations or the loss of supply can lead to interruptions in manufacturing processes with high resulting economic losses. FACTS devices can help provide the required quality of supply
Environmental benefits
FACTS devices are environmentally friendly. They contain no hazardous materials and produce no waste or pollutanse. FACTS help distribute the electrical energy more economically through better utilization of existing installations thereby reducing the need for additional transmission lines.
Future Developments in FACTS
Future developments will include the combination of existing devices, e.g. combining a STATCOM with a TSC (thyristor switched capacitor) to extend the operational range. In addition, more sophisticated control systems will improve the operation of FACTS devices. Improvements in semiconductor technology (e.g. higher current carrying capability, higher blocking voltages) could reduce the costs of FACTS devices and extend their operation ranges. Finally, developments in superconductor technology open the door to new devices like SCCL (Super Conducting Current Limiter) and SMES (Super Conducting Magnetic Energy Storage). There is a vision for a high voltage transmission system around the world – to generate electrical energy economically and environmentally friendly and provide electrical energy where it’s needed. FACTS are the key to make this vision live.
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Conclusion
This paper has shown working of the conventional devices as well as the FACTS devices used to enhance controllability and increase power transfer capability. It can be easily seen that by using FACTS devices the existing transmission system assets are utilized in a better way. The other benefits like increased transmission system reliability, stability and last but obviously not the least the environmental benefits should also be undertaken. It has also been clearly shown that the future of these devices also depends upon the future of the semiconductors for extension
of operation ranges & economical benefits. In view of the importance of electricity grid reliability to national welfare, these factors now call for an increased governmental role in electric system reliability R&D.
BIBLIOGRAPHY
[1] Soni, Gupta, Bhatnagar & Chakravarthy, Electrical Power Generation. Dhanpat Rai Publishers, 2004. [2] P. Kundur, Power system stability and control. McGraw-Hill. New York 1994. [3] Ashfaq Hussain, Electrical Power Systems. CBS Publishers, 2005. [4] R.M. Mathur and R.K. Verma. Thyristor-based facts controllers for electrical transmission systems. IEEE Press, Piscataway, 2002. [5] www.ieee.org
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