Flexible AC Transmission Systems (FACTS)
2: Department of Electronic and Electrical Engineering, University of Bath, Bath, UK
Provides a comprehensive guide to FACTS, covering all the major aspects in research and development of FACTS technology.
Inspec keywords: flexible AC transmission systems; power apparatus; power electronics
Other keywords: flexible AC transmission systems technology; FACTS; power electronics technology; power system equipment
Subjects: a.c. transmission
- Book DOI: 10.1049/PBPO030E
- Chapter DOI: 10.1049/PBPO030E
- ISBN: 9780852967713
- e-ISBN: 9781849194419
- Page count: 610
- Format: PDF
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Front Matter
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1 Power transmission control: basic theory; problems and needs; FACTS solutions
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This chapter discusses power transmission control and FACTS.
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2 Power electronics: fundamentals
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Advantages of incorporating power electronics in FACTS controllers include unlimited life, fast response, and continuous control and repeatability of the output. In addition, unlike systems based on passive components, static compensators do not introduce potentially damaging resonant frequencies. Power electronic converters are therefore becoming essential to the implementation of most FACTS devices. The main drawbacks are losses, resulting in additional initial and operating costs, harmonic distortion, and capital costs associated with power semiconductor devices. Although many devices at this time use structures incorporating thyristors, force-commutated structures offer greater flexibility, in terms of output control, speed of response, harmonic minimisation, and implementation of complex control features. A UPFC is an example of the flexibility provided by back to back voltage sources: in addition to the degrees of freedom available for the control of system performance, it can process real power. Similarly, shunt devices have the capability of injecting real power to support the power system under fault conditions, if equipped with energy storage devices. Implementation of static power compensators based on force-commutated semiconductor switches should become more attractive as the power handling capability of power devices increases, and methods are developed to enhance reliability, reduce losses, minimize harmonic injection and improve control algorithms. A number of prototype units of the newer static compensator systems have demonstrated their potential.
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3 High voltage dc transmission technology
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The invention of the high voltage mercury valve half a century ago paved the way for the development of the HVdc transmission technology. By 1954, the first commercial dc link came successfully into operation and was soon followed by several other schemes orders of magnitude larger. The success of the new technology immediately triggered research and development into an alternative solid state valve, which by the mid-60s had already displaced the use of mercury arc valves in new schemes. Substantial progress made in the ratings and reliability of thyristor valves has increased the competitiveness of dc schemes by reducing converter costs and break-even transmission distances.The inclusion of dc transmission in this book seems to be a contradiction in terms, as often HVdc and FACTS are perceived as competing technologies. The problem arises from a rather restrictive interpretation of the word 'transmission', which often implies long distance, whereas a large proportion of the existing dc links are 'zero distance' interconnectors. This chapter has highlighted the basic components of the HVdc transmission technology as well as the main simulation techniques presently used for their integration in power system analysis. Some of the new concepts under consideration for future HVdc links have also been introduced.
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4 Shunt compensation: SVC and STATCOM
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Both SVCs and STATCOM employ static equipment for inherently or automatically varying shunt reactive power in transmission, distribution, and industrial power networks. The principal equipment may be based on special (saturated) reactors, thyristors, or gate turn-off thyristor (GTO) in a variety of configurations and control modes. The equipment responds rapidly to changes in the network and/or loads to supply or absorb reactive power so as to achieve enhanced system performance more economically than otherwise possible. When required the equipment has also been designed for relocatability at different points in networks which require more flexibility as system generation and load patterns vary over the years.
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5 Series compensation
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Series capacitors have been successfully used for many years in order to enhance the stability and loadability of high-voltage transmission networks. The principle is to compensate the inductive voltage drop in the line by an inserted capacitive voltage or in other words to reduce the effective reactance of the transmission line.
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6 Phase shifter
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This chapter briefly describes the concept of phase shifting in electric power systems and highlights the technical merits of semiconductor-controlled (static) phase shifters over conventional (mechanical) phase shifters. Based upon the steady-state principles of operation of a Static Phase Shifter (SPS), a steady-state SPS model is developed. Although simplifying assumptions are used, the model provides adequate insight to explore the impact of various parameters and operating conditions/constraints on the SPS system. Based on currently feasible semiconductor switches and converter topologies, five SPS configurations are identified. The first group is based on substituting a mechanical tap-changer by semiconductor switches. The second group uses ac controllers. The third group is based on a single-phase ac-ac bridge converter topology. The fourth and the fifth group use PWM voltage source converters and PWM current source converters. The basic power circuits, principles of operations and salient features of each group are briefly described. The chapter is concluded by a brief description of SPS applications in power systems.
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7 The unified power flow controller
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This chapter discusses unified power flow controllers.
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8 Electromagnetic transient simulation studies
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This chapter describes some research results of EMTP-based digital studies of SPWM UPFC. Useful insights into UPFC performance have been attained through detailed analysis and simulation of the UPFC internal structure and regulation method. Voltage and power control simulation results have demonstrated the functions of the UPFC. Studies have also indicated that PWM is a potentially promising method for the effective regulation of the UPFC.
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9 Steady-state analysis and control
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This chapter focuses on the steady state analysis and control of systems with FACTS devices with particular reference to the UPFC. A steady-state UPFC model is proposed and its power injection transformation is derived in rectangular form. The optimal multiplier power flow method for ill-conditioned system is applied to implement the UPFC model. Then a novel method for the steady state control of UPFC for power flow control and voltage support is presented. Essentially, it is based on a power injection model and optimal multiplier power flow. The proposed power injection power flow control can be effectively used to derive UPFC control parameters to achieve the required control objectives. A number of internal limits imposed on the UPFC have been considered, including series injection voltage magnitude, line current through the series inverter, real power transfer between the shunt inverter and series inverter, shunt side current and shunt injection voltage magnitude. The proposed constrained control strategies can co-ordinate the available control freedom to achieve an efficient usage of the UPFC when constrained by the internal limits. The comparison of rating and control flexibility of three major FACTS devices is also included in the numerical examples. The proposed UPFC model and power flow control algorithms have been vigorously tested in a number of systems. The results on two practical systems clearly illustrate the effectiveness of the proposed algorithms.
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10 Oscillation stability analysis and control
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This chapter introduces oscillation stability analysis and control of power systems installed with FACTS-based stabilizers. The discussions cover the subjects of modelling, analysis, and design of FACTS-based stabilizers installed in single machine infinite-bus and multi-machine power systems. Examples are given when it is appropriate to demonstrate the analytical studies. For a good design of FACTS-based stabilizers in a power system, three factors must be considered: the effectiveness, the robustness and the interactions among stabilizers. In this chapter, some aspects of these factors are explored. However, a great effort needs to be directed to investigate the following problems in future: a.) Design of a single FACTS-based stabilizer in the power system, which is effective, robust, and imposes non-negative interactions on other stabilizers in the system; and b.) Co-ordinated design of a group of FACTS-based stabilizers, which are effective, robust, and impose non-negative interactions on each other and other stabilizers in the power system excluded in the co-ordination. The reason is that when a FACTS-based stabilizer or a group of FACTS-based stabilizers are to be put into a power system, often other stabilizers may not be adjustable or it may not be desirable to readjust them.
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11 Transient stability control
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In this chapter the reader has been introduced to the basic theory of electromechanical transients in power systems. The relative generator rotor motion has been described by the swing equation. The problem of transient stability has been explained on the basis of the well known but, especially from the tutorial point of view, still valuable equal-area criteria. According to this criteria, transient stability of a power system is maintained if the accelerating area (which is in proportion to the additional kinetic energy stored in rotating masses during the fault) equals the decelerating area (which is in proportion to the excess of generator electric power over the mechanical power supplied by turbine) during the 1st rotor swing following the fault clearance. It can be concluded that the most effective device is the UPFC. This has been anticipated, since it combines properties of controllable parallel and series compensation as well as properties of controllable phase shifting transformers. However, a comparison has been carried out on the basis of device ratings which does not necessarily match with the economic comparison. In real systems there are many factors influencing transient stability therefore the presented study should be considered as a demonstration the example results of which should not be generalized.
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12 Protection for EHV transmission lines with FACTS devices
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In this chapter, the results of a digital simulation for a controllable series compensation transmission system are presented, and the feature extraction and topology of suitable artificial neural networks is discussed. The chapter concludes with a presentation of the overall performance of the protection scheme.
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13 FACTS development and applications
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This chapter reviews the status of power electronics technologies including the development of semiconductor devices for power application, the development of high performance SC converters, and research on FACTS equipment for power system enhancement. Remarkable progress in this field will be useful not only for improving the transmission capability of trunk power systems but also to serve for upgrading the distribution systems through the improvement of power quality. Amidst the business environments such as the deregulation of electric power utilities, global environmental restrictions and energy security for electric power resources, electric power utilities in Japan are facing many social and technical problems. The utilization of power electronics technologies is indispensable for reducing the cost of electric power, for enhancement of transmission capability to adapt to free power wheeling, and for preventing wide area black-out due to inadequate coordination of power system planning and operation. It is also indispensable for the improvement of power quality for meeting higher demand as imposed by the expanded use of upgraded information equipment. The authors expect that the new power electronics devices such as the GCT and IEGT will be used for more advanced SC converters, and that the results from R&D joint projects for interconnection reinforcement in Japan will be useful for developing better power systems in the future.
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14 Application of power electronics to the distribution system
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To date, the main applications for power electronics on the distribution system have been to improve two aspects of power quality: voltage dips or sags and harmonic distortion. These applications are of commercial interest when it is possible to identify a customer who will benefit from improved power quality either through a reduction in the sags experienced or by limiting the harmonic currents injected and thus complying with the utility requirements.
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Back Matter
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