Facts Hvdc Essay Example

Facts Hvdc Paper A Paper presented on â€Å"FACTS HVDC TRNAMISSION SYSTEMS† PRESENTED BY NITIN K. MOHURLE (T. E. ELECTRICAL) SHRAM SADHANA TRUST’S COLLEGE OF ENGINEERING TECHNOLOGY BAMBHORI, JALGAON E-mail:- nitin. [emailprotected] com FACTS AND HVDC TRNSMISSION SYSTEMS Nitin Mohurle Shram Sadhana Trust’s College of Engineering And Technology, Jalgaon nitin. [emailprotected] com Abstract — Development of electrical power supplies began more than one hundred years ago. At the beginning, there were only small DC networks within narrow local boundaries, which were able to cover the direct needs of industrial plants by means of hydro energy. With an increasing demand on energy and the construction of large generation units, typically built at remote locations from the load centres, the technology changed from DC to AC. Power to be transmitted, voltage levels and transmission distances increased. FACTS (Flexible AC Transmission System) and HVDC (High Voltage Direct Current) are controllable devices whose functions are to enhance the security, capacity and flexibility of power transmission systems. Application of these components in power systems implies an improvement of transient and voltage stability, power oscillation damping and optimal power flow. DC transmission and FACTS (Flexible AC Transmission Systems) has developed to a viable technique with high power ratings since the 60s. From the first small DC and AC mini networks, there are now systems transmitting 3 4 GW over large distances with only one bipolar DC transmission: 1. 000 2. 000 km or more are feasible with overhead lines. We will write a custom essay sample on Facts Hvdc specifically for you for only $16.38 $13.9/page Order now We will write a custom essay sample on Facts Hvdc specifically for you FOR ONLY $16.38 $13.9/page Hire Writer We will write a custom essay sample on Facts Hvdc specifically for you FOR ONLY $16.38 $13.9/page Hire Writer With submarine cables, transmission levels of up to 600 – 800 MW over distances of nearly 300 km have already been attained, and cable transmission lengths of up to 1. 300 km are in the planning stage. As a multiterminal system, HVDC can also be connected at several points with the surrounding three-phase network. FACTS is applicable in parallel connection or in series or in a combination of both. The rating of shunt connected FACTS controllers is up to 800 Mvar, series FACTS devices are implemented on 550 and 735 kV level to increase the line transmission capacity up to several GW. The fast development of power electronics based on new and powerful semiconductor devices has led to innovative technologies, such as high voltage dc transmission (HVDC) and flexible ac transmission system (FACTS), which can be applied in transmission and distribution systems. This paper has discussed the application of high voltage power electronics FACTS and HVDC controllers, needs of advance FACTS and HVDC based control for future power system and enhancing system stability and its development. HVDC and FACTS offer major advantages in meeting these requirements. Keywords—SVC Static Var Compensator SC Series Capacitor, VSC, STATCOM I. INTRODUCTION The development of Electric Power Industry follows closely the increase of the demand on electrical energy. Main driving factors for energy consumption are listed in Fig. 1. In the early years of power system developments this increase was extremely fast, also in industrialized countries, many decades with the doubling of energy consumption each 10 years. Such fast increase is nowadays still present in the emerging countries, especially in Far-East. In the industrialized countries the increase is, however, only about 1 to 2 % per year with an estimated doubling of the demand in 30 to 50 years. In next 20 years, power consumption in developing and emerging countries is expected to more than double, whereas in industrialized countries, it will increase only for about 40 %. Fast development and further extension of power systems can therefore be expected mainly in the areas of developing and emerging countries. However, because of a lack on available investments, the development of transmission systems in these countries does not follow the increase in power demand. Hence, there is a gap between transmission capacity and actual power demand, which leads to technical problems in the overloaded transmission systems. Interconnection of separated grids in the developed countries can solve some of these problems, however, when the interconnections are heavily loaded due to an increasing power exchange, the reliability and availability of the transmission will be reduced. A. POWER FACTOR To understand power factor, visualize a horse pulling a railroad car down a railroad track. Because the railroad ties are uneven, the horse must pull the car from the side of the track. The horse is pulling the railroad car at an angle to the direction of the car’s travel. The power required to move the car down the track is the working (real) power. The effort of the horse is the total (apparent) power. Because of the angle of the horse’s pull, not all of the horse’s effort is used to move the car down the track. The car will not move sideways; therefore, the sideways pull of the horse is wasted effort or nonworking (reactive) power. The angle of the horse’s pull is related to power factor, which is defined as the ratio of real (working) power to apparent (total) power. If the horse is led closer to the centre of the track, the angle of side pull decreases and the real power approaches the value of the apparent power. Therefore, the ratio of real power to apparent power (the power factor) approaches 1. As the power factor approaches 1, the reactive (nonworking) power approaches 0. Fig. 2 Reactive Power analogy B. REACTIVE POWER: Reactive power is a concept used by engineers to describe the background energy movement in an Alternating Current (AC) system arising from the production of electric and magnetic fields. These fields store energy which changes through each AC cycle. Devices which store energy by virtue of a magnetic field produced by a flow of current are said to absorb reactive power; those which store energy by virtue of electric fields are said to generate reactive power. Explanation for reactive power says that in an alternating current system, when the voltage and current go up and down at the same time, only real power is transmitted and when there is a time shift between voltage and current both active and reactive power are transmitted. But, when the average in time is calculated, he average active power exists causing a net flow of energy from one point to another, whereas average reactive power is zero, irrespective of the network or state of the system. In the case of reactive power, the amount of energy flowing in one direction is equal to the amount of energy flowing in the opposite direction (or different parts -capacitors, inductors, etc- of a network, exchange the rea ctive power). II. FACTS AND HVDC SYSTEM A. HVDC SYSTEM HVDC was first used commercially 50 years ago. Since then a growing number of transmission schemes have been constructed around the world. HVDC differs from high voltage alternating current, HVAC that the voltage is not alternating 50 or 60 cycles per second but is constant. The advantage of HVDC is that long distance transmission is more efficient as there is no need to charge the capacitance of a transmission line with the alternating voltage. The drawback of HVDC is that one needs more expensive terminals at the line ends. Fig. 3 Typical HVDC converter station using thyristor valves HVDC has a number of properties that makes it different from ac transmission. The most important are: The two stations can be connected to networks that are not synchronized or does not even have the same frequency * Power can be transmitted over very long distances without compensation for the reactive power. Reactive power is power that does not add to the transmitted power, but is a by-product at ac- transmission as the line or cable capacitances has to be charged 50 or 60 times per second. As HVDC has constant voltage it does not gene rate reactive power. See also figure 4. * Only two conductors are needed (or even one conductor if the ground or the sea is used as return) for HVDC compared to thre conductors for alternating current. Fig. 4 Shows the power that is possible to transmit as a function of the distance for ac- cables of various voltage stress compared to HVDC With ac high power can be transmitted short distances or low power long distances. HVDC cables can transmit high power over long distances. B. Flexible Alternating Current Transmission System The objective of incorporating FACTS is into the power system lines are similar to HVDC but greater flexibility are involved like improving real power transfer capability in the lines, prevention of sub synchronous resonance (SSR)oscillations and damping of power swings [1]. FACTS devices have four well-known types which are used in many power systems in the world. ‘Single’ type controller is the types of FACTS that installed in series or shunt in an AC transmission line, while ‘unified’ type controller are the combined converters type of FACTS controllers like UPFC and HVDC. The size of a controller is dependent on the requirements of the network and desired power transmission at loading point Voltage Source Controller (VSC) is sinusoidal voltage and is used in power system and other application. The quality of the sine wave is dependent on the size or amount of the power electronics installed. C. LIMITATIONS OF LARGE AC SYSTEMS In large AC Systems with long distance transmission and synchronous interconnections, technical problems can be expected, which are summarized in Fig. 2. Main problems occur regarding load flow, system oscillations and inter-area oscillations. If systems have a large geographic extension and have to transmit large power over long distances, additional voltage and stability problems can arise. Fig. Limitations of Large AC systems III. FACTS, HVDC AND REACTIVE POWER FACTS has a lot to do with reactive power compensation, and indeed, that used to be the term utilized for the technology in the old days. Reactive power appears in all electric power systems, due to the laws of nature. Contrary to active power, which is what we really want to transmit over our power system, and which performs real work, such as keeping a lamp lit or a motor running, reactive power does not perform any such work. Consequently, in a way one can say that the presence f reactive power in a grid makes it heavier for it to perform its task, i. e. transmit power from A to B (Figure 1), and consequently less efficient than would otherwise be possible. We can also refer to Lenz? law, formulated already in the nineteenth century: Every change in an electrical system induces a counter-reaction opposing its origin. So, as a consequence, if we can minimize the flow of reactive power over the transmission system, we can make the system more efficient and put it to better and more economical use. We cannot altogether do without reactive power, though, because it is intimately linked with grid voltage (500 kV, 400 kV, 220 kV, etc). To get the correct grid voltage, we need the right amount of reactive power in the system. If there is not enough reactive power, the voltage will sag. And vice versa, if there is too much of it, the voltage will be too high. So, to have it in the right amounts at all times, and in the right places of the grid, that is the task to be performed by means of Reactive Power Compensation. Reactive power balance is important also from another point of view: it ensures that valuable space in transmission lines and equipment such as transformers is not occupied by â€Å"idle† reactive power, but rather available for a maximum of useful, active power as in fig. This is particularly crucial in situations where some fault appears in the grid. In such a situation, it will often be a matter of milliseconds for the Reactive Power Compensator, i. e. the FACTS device, to go into action and help restore the stability, and the voltage of the grid, in order to prevent, or mitigate, a voltage collapse. IV. LOSSES Maintaining proper balance of reactive power in the grid is important also from another point of view: too much reactive power flowing in the grid also gives rise to losses, and losses cost money which is always, at the end, charged to the customer. To prevent such losses, it is important that reactive power is not permitted to flow over long distances, because losses grow with the distance that the reactive power is flowing over. Instead, reactive power should be inserted where it is needed, i. e. close to large cities and/or large industry enterprises. This, too, is a task for FACTS HVDC SYSTEM. V. NEED OF REACTIVE POWER COMPENSATION: In Figure , the representation of a capacitor inserted at the middle of the line for its compensation, is illustrated. Fig. 7 Distribution line with a capacitor for a given compensation The phasor diagram illustrated in Figure 8 it is illustrated the effect of the capacitor for a 50% of compensation. Once inserted the compensator, Figure 8, it can operate injecting a voltage in the line according to equation. VI. TECHNOLOGY UNDRRLYING FACTS AND HVDC A. VOLTAGE SOURCE CONVERTER: A new type of HVDC using transistors for the ac/dc conversion has been developed. By using components that can not only switch on the current but also switch it off, making it possible to build voltage source converters (VSC). This type of converters offers many advantages when it comes to transmission of power especially from sustainable energy systems. Fig. 9 VSC converter valve B. SERIES COMPENSATION: Fig. 10 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 causes a large voltage drop. To compensate, series capacitors are connected. C. SHUNT COMPENSATION: In shunt compensation, power system is connected in shunt (parallel) with the FACTS. It works as a controllable current source. Fig. 11 Shunt compensation is of two types: A] Shunt capacitive compensation: This method is used to 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. B] 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  Ã¢â‚¬â€œ 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 (generally in case of very long transmission lines). To compensate, shunt inductors are connected across the transmission line. Fig. 12 Use of Power Electronics foe Power Transmission VII. FACTS AND HVDC CONTROLLER A. SHUNT CONNECTED CONTROLLER FACTS controllers can be impedance type, based on thyristors without gate turn-off capability, which are called Static Var Compensator (SVC) for shunt-connected application. Another type of FACTS controllers is converter-based which is usually in the form of a Static Synchronous Compensator (STATCOM). B. STATIC VAR COMPENSATOR Static Var Compensator is â€Å"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 (typically bus voltage)†. SVC is based on thyristors without gate turn-off capability. The operating principal and characteristics of thyristors realize SVC variable reactive impedance. SVC includes two main components and their combination: (1) Thyristor-controlled and Thyristor-switched Reactor (TCR and TSR); and (2) Thyristor-switched capacitor (TSC). In Figure shows the diagram of SVC. C. CONVERTOR BASED COMPENSATOR Static Synchronous Compensator (STATCOM) is one of the key Converter-based Compensators which are usually based on the voltage source inverter (VSI) or current source inverter (CSI), as shown in Figure. Unlike SVC, STATCOM controls the output current independently of the AC system voltage, while the DC side voltage is automatically maintained to serve as a voltage source. Mostly, STATCOM is designed based on the VSI. D. SERIES CONNECTED CONTROLLLER As shunt-connected controllers, series-connected FACTS controllers can also be divided into either impedance type or converter type. The former includes Thyristor-Switched Series Capacitor (TSSC), Thyristor-Controlled Series Capacitor (TCSC), Thyristor-Switched Series Reactor, and Thyristor-Controlled Series Reactor. The latter, based on VSI, is usually in the form of a Static Synchronous Series Compensator (SSSC). The composition and operation of different types are similar to the operation of the shunt-connected peers. E. STATIC SYNCHRONOUS COMPENSATOR As discussed in the previous section, STATCOM is a very popular FACTS controller application effective in transmission system voltage control. Since 1980 when the first STATCOM (rated at 20 Mvar) using force-commutated thyristor inverters was put into operation in Japan , many examples have been installed and the ratings have been increased considerably. 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 * Provide better power flow control. ADVANTAGES OF FACTS AND HVDC SYSTEMS When implemented on a broad-scale basis, FACTS HVDC technologies deliver the following benefits. * A Rapidly 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. * 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. * 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. Improved System Controllability: FACTS allow improved system controllability by building â€Å"intelligence† into the transmission network via the ability to instantaneously respond to system disturbances and gridlock constraints and to enable redirection of power flows * Seamless System Interconnections: FACTS, in the form of BTB dc-link configurations, can establish â€Å"seamless† interconne ctions within and between regional and local networks, allowing controlled power transfer and an increase in grid stability. DISADVANTAGES OF FACTS * The amount of power that can be send over a transmission line is limited. Origin of the limits varies depending on the length of the line. For short line, the heating of the conductors due to line losses sets a â€Å"thermal† limit. * The power flowing over an AC line is proportional to the sine of the phase angle of the voltage at the receiving and transmitting ends and it never reaches 90 degrees. Hence series capacitors or phase-shifting transformers are used on long lines to improve stability. CONCLUSION Power supply industry is undergoing dramatic change as a result of deregulation and political and economical driving forces in many parts of the world. This new market environment puts growing demands for flexibility and power quality into focus. Also, trade of electric power between countries is gaining momentum, to the benefit of all involved. This calls for the right solutions as far as power transmission facilities between countries as well as between regions within countries are concerned. * As indicated by the acronym, FACTS stands for flexibility in AC power systems. Properly utilized, this offers benefits to users of a variety of kinds. Without the need to reinforce the grid by means of additional or upgraded existing lines and/or substations FACTS brings about: * An increase of synchronous stability of the grid; * Increased power transmission capability; * Increased voltage stability in the grid; * Decreased power wheeling between different power systems; * Improved load sharing between parallel circuits; * Decreased overall system transmission losses; * Improved power quality in grids. The choice of FACTS device in each given case may not be obvious but may need to be made the subject of system studies, taking all relevant requirements and prerequisites of the system into consideration, so as to arrive at the optimum technical and economical solution. In fact, the best solution may often be a combination of devices. Finally, a rough and quick guideline to the use of FACTS in various applications: REFRRENCE: [1] Zhang, B. M. ; Ding, Q. F â€Å"The development of FACTS and its control†, Advances in Power System Control, Operation and Management, APSCOM-97. Fourth International Conference, Vol. 1, Nov. 1997, pp: 48 – 53 [2] Paserba, J. J. ; â€Å"How FACTS controllers benefit AC transmission systems†, Power Engineering Society General Meeting, IEEE, Vol. 2, June 2004, pp:1257 1262 [3] Edris, A, â€Å"FACTS technology development: an update†, Power Engineering Review, IEEE, Vol. 20, Issue 3, March 2000, pp: 599 627 [4] L. Gyugyi, â€Å"Application characteristics of converter-based FACTS controllers†, IEEE Conference on Power System Technology, Vol. , pp. 391 396, Dec. 2000. [5] N. G. Hingorani, L. Gyugyi, Understanding FACTS, Concepts and Technology of Flexible AC Transmission systems, IEEE Press 2000 [6] N. G. Hingorani,â€Å"High power electronics and flexible AC transmission system†, IEEE Power Engineering Review, Vol. 8, No. 7, pp. 3-4, July 1988. [7] IEEE FACTS Terms Definitions Task Force of the FACTS Working Group of the DC and FACTS subcommittee, â€Å"Proposed terms and definitions for Flexible AC Transmission System (FACTS)†, IEEE Trans. on Power Delivery, Vol. 12, No. 4, pp. 1848-1853, Oct. 1997.