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Chapter 1
Electric Energy Systems. An
Overview
Ignacio J. P´erez Arriaga, Hugh Rudnick and Michel Rivier
1.1 A first vision
1.1.1 The energy challenges in modern times
Energy is a fundamental ingredient of modern society and its supply impacts directly in the social and economic development of nations. Economic growth and energy consumption go hand to hand. The development and quality of our life and our work are totally dependent of a continuous, abundant and economic energy supply. This reality is being faced world- wide as basic energy resources become scarce and increasingly costly. While coal remains an abundant resource, oil and natural gas supply face restrictions, concerns arising on declining volumes on the long term. This reliance on energy for economic growth has historically im- plied dependence on third parties for energy supply, with geopolitical connotations arising, as energy resources have not been generally in places where high consumption has devel- oped, Energy has transformed itself in a new form of international political power, utilized by owners of energy resources (mainly oil and natural gas). Within that framework, electricity has become a favorite form of energy usage at the consumer end, with coal, oil, gas, uranium, and other basic resources used to generate the electricity. With its versatility and controllability, instant availability and consumer- end cleanliness, electricity has become an indispensable, multi-purpose form of energy. Its domestic use now extends far beyond the initial purpose, to which it owes its colloquial name (“light” or “lights”), and has become virtually irreplaceable in kitchens –for refrigerators, ovens and cookers or ranges and any number of other appliances– and in the rest of the house as well, for air conditioning, radio, television, computers, and the like. But electricity usage is even broader in the commercial and industrial domains: in addition to providing power for lighting and air conditioning, it drives motors with a host of applications: lifts, cranes, mills, pumps, compressors, lathes or other machine tools, and so on and so forth: it’s
2 CHAPTER 1. ELECTRIC ENERGY SYSTEMS. AN OVERVIEW
nearly impossible to imagine an industrial activity that doesn’t use some sort of electricity. Thus, modern societies have become totally dependent on an abundant electricity supply.
1.1.2 Characteristics of electricity
At first glance electricity must appear to be a commodity much like any other on consumers’ list of routine expenses. In fact, this may be the point of view that prompted the revolution that has rocked electric energy systems world-wide, as they have been engulfed in the wave of liberalization and de-regulation that has changed so many other sectors of the economy. And yet electricity is defined by a series of properties that distinguish it from other products, an argument often wielded in an attempt to prevent or at least limit the implementation of such changes in the electricity industry. The chief characteristic of electricity as a product that differentiates it from all others is that it is not susceptible, in practice, to being stored or inventoried. Electricity can, of course, be stored in batteries, but price, performance and inconvenience make this impractical for handling the amounts of energy usually needed in the developed world. Therefore, electricity must be generated and transmitted as it is consumed, which means that electric systems are dynamic and highly complex, as well as immense. At any given time, these vast dynamic systems must strike a balance between generation and demand and the disturbance caused by the failure of a single component may be transmitted across the entire system almost instantaneously. This sobering fact plays a decisive role in the structure, operation and planning of electric energy systems, as discussed below. Another peculiarity of electricity has to do with its transmission: this is not a product that can be shipped in “packages” from origin to destination by the most suitable medium at any given time. Electric power is transmitted over grids in which the pathway cannot be chosen at will, but is determined by Kirchhoff’s laws, whereby current distribution depends on impedance in the lines and other elements through which electricity flows. Except in very simple cases, all that can be said is that electric power flows into the system at one point and out of it at another, because ascribing the flow to any given path is extraordinarily complex and somewhat arbitrary. Moreover, according to these laws of physics, the alternative routes that form the grid are highly interdependent, so that any variation in a transmission facility may cause the instantaneous reconfiguration of power flows; and that, in turn, may have a substantial effect on other facilities. All this renders the dynamic balance referred to in the preceding paragraph even more complex.
1.1.3 Electrical energy systems: the biggest industrial system created by
mankind
Indeed, for all its apparent grandiloquence, the introductory sentence to this unit may be no exaggeration. The combination of the extreme convenience of use and countless appli- cations of electricity on the one hand and its particularities on the other has engendered these immense and sophisticated industrial systems. Their size has to do with their scope, as they are designed to carry electricity to practically any place inhabited by human beings from electric power stations located wherever a supply of primary energy –in the form of potential energy in moving water or any of several fuels– is most readily available. Carrying
4 CHAPTER 1. ELECTRIC ENERGY SYSTEMS. AN OVERVIEW
scale of the process by using a single generator to feed many more bulbs. In 1882, Edison’s first generator –driven by a steam turbine located on Pearl Street in lower Manhattan– successfully fed a direct current at a voltage of 100 V to around 400 80-W bulbs in office and residential buildings on Wall Street. Shortly thereafter London’s 60-kW Holborn Viaduct station was commissioned, which also generated 100-V direct current. This local generation and distribution scheme was quickly adopted, exclusively for lighting, in many urban and rural communities world-wide. The invention of the transformer in France in 1883-84 revealed –in a process not ex- empt from controversy– the advantages of alternating current, which made it possible to conveniently raise the voltage to reduce line losses and voltage drops over long transmis- sion distances. Alternating, single phase electric current was first transmitted in 1884, at a voltage of 18 kV. On 24 August 1891, three-phase current was first transmitted from the hydroelectric power station at Lauffen to the International Exposition at Frankfort, 175 km away. Swiss engineer Charles Brown –who with his colleague and fellow countryman Wal- ter Boveri founded the Brown-Boveri Company that very year– designed the three phase AC generator and the oil-immersed transformer used in the station. In 1990 the Institute of Electrical and Electronic Engineers, IEEE, agreed to take 24 August 1891 as the date marking the beginning of the industrial use and transmission of alternating current. The transmission capacity of alternating current lines increases in proportion to the square of the voltage, whereas the cost per unit of power transmitted declines in the same proportion. There was, then, an obvious motivation to surmount the technological barriers limiting the use of higher voltages. Voltages of up to 150 kV were in place by 1910 and the first 245-kV line was commissioned in 1922. The maximum voltage for alternating current has continued to climb ever since, as Figure 1.1 shows. And yet direct current has also always been used, since it has advantages over alternating current in certain applications, such as electrical traction and especially electricity transmission –in overhead, underground or submarine lines– when the distances are too long for alternating current. The upward trend in maximum direct current voltage throughout the twentieth century is also depicted in Figure 1.1. The alternating voltage frequency to be used in these systems was another of the basic design parameters that had to be determined. Higher frequencies can accommodate more compact generating and consumption units, an advantage offset, however, by the steeper voltage drops in transmission and distribution lines that their use involves. Some countries –the USA, Canada, the Central American countries and the northern-most South Ameri- can countries– adopted a frequency of 60 Hz, whilst countries in the rest of South America, Europe, Asia and Africa adopted a frequency of 50 Hz. The International Electrotechnical Commission was created in 1906 to standardize electrical facilities everywhere as far as pos- sible. It was, however, unable to standardize frequency, which continues to divide countries around the world into two different groups. The advantages of interconnecting small electric energy systems soon became obvious. The reliability of each system was enhanced by the support received from the others in the event of emergencies. Reserve capacity could also be reduced, since each system would be able to draw from the total grid reserve capacity. With such interconnections it was possible to deploy the generator units able to meet demand most economically at any given time;
1.1. A FIRST VISION 5
200
400
1600 1400 1200 1000 800 600
kV (Phase to phase voltage)
D.C.
A.C.
1900 1920 1940 1960 1980 2000
year
USA
Sweden
USSR
Canada
USA
Canada
Mozambique
Brazil
Russia
Sweden
England-France
New Zeland
USA
USSR
USA
Figure 1.1: Maximum AC and DC rated voltages. [Source: Tor´a [12]].
the advantage this affords is particularly relevant when peak demand time frames vary from one system to another and when the generation technology mix –hydroelectric and steam, for instance– likewise differs. In 1926 English Parliament created the Central Electricity Board and commissioned it to build a high voltage grid that would interconnect the 500 largest generation stations then in operation.
Organizational aspects
What sort of organizational structure is in place in the sector responsible for planning, operating and maintaining electric energy systems? Who makes the decisions in each case and under what criteria? The reply to these questions has evolved over time, largely to adapt to the conditioning factors imposed by technological development, but also depend- ing on prevailing economic theory. As mentioned above, the first industrial applications of electricity were strictly local, with a generator feeding a series of light bulbs in the surrounding area. Whole hosts of individual systems sprang up under private or public –usually municipal– initiative, primarily to provide urban lighting and, somewhat later, to drive electric motors for many different purposes. The vertically integrated electric utility –which generates, transmits, distributes and supplies electricity– arose naturally and was the predominant model in most countries until very recently. The enormous growth of elec- tricity consumption, the huge economies of scale in electricity generation and the increase in the transmission capacity of high voltage lines drove the development of transmission grids –often under State protection– to interconnect individual systems, giving rise to literally nationwide systems. Technical specialization and the huge volume of resources required to build large power stations led to the co-existence of local distribution companies –with scant or nil production capacity– and large vertically integrated utilities which also sold
1.1. A FIRST VISION 7
system, abiding by trade arrangements stipulated between the various countries, support in emergency situations, global analysis and control of certain grid stability phenomena, or management of grid restrictions deriving from international trade– that had been essentially solved or kept under control in the context of vertically integrated electric utilities, via well- established rules for support in emergencies in a climate of co-operation, scant competition and limited trade. These technical problems have become more acute and their complexity has grown with the need to accommodate economic and regulatory considerations in the recent context of open competition. The proliferation of international transactions conducted in a completely decentralized manner by individual players –buyers and sellers entitled to access the regional grid as a whole– has complicated matters even further. In addition to these technological problems, other issues must also be addressed, such as harmonizing different national reg- ulations, organizing and designing operating rules for regional markets, determining the transmission tolls to be applied in international transactions, pursuing economic efficiency in the allocation of limited grid capacity and solving technical restrictions or proposing suitable regulatory mechanisms to ensure efficient transmission grid expansion.
1.1.5 Environmental impact
In addition to ongoing technological development and the winds of change blowing in the global economy, a factor of increasing weight in the electricity industry, as in all other human activities, is the growing awareness of the importance of the natural environment. There is widespread belief that one of the major challenges facing humanity today is the design of a model for sustainable development, defined to be development that meets the needs of the present without compromising the ability of future generations to meet their own needs. Besides such weighty issues as the enormous social and economic inequalities between peoples or the existence of a growth model that can hardly be extended to the entire world population, other questions, such as the intense use of the known energy resources and their adverse impact on the environment, problems that relate directly to electric energy systems, also come under the umbrella of sustainable development. For these reasons environmental impact is a factor of increasing relevance and importance that conditions the present operation and development of these systems and will indisputably have an even more intense effect on the industry in the future. Generation is arguably the line of business in electric energy systems that produces the greatest environmental impact, in particular with regard to steam plant emissions and the production of moderately and highly radioactive waste. As far as combustion is concerned, coal- and oil-fired steam plants vie with the transport industry for first place in the emission of both carbon dioxide (CO2) –associated with greenhouse gas-induced climate change–, nitrous oxides (NOx) and sulphur dioxide (SO2) –the former related to the formation of tropospheric ozone and both responsible for acid rain. Carbon dioxide is an inevitable by-product of the combustion of organic material, NOx comes from the nitrogen in the air and SO2 from the sulphur in coal and oil. Other environmental effects of conventional steam power stations include the emission of particles and heavy metals, the generation of solid waste such as fly ash and slag, the heating of river, reservoir or sea water to cover
8 CHAPTER 1. ELECTRIC ENERGY SYSTEMS. AN OVERVIEW
refrigeration needs and, indirectly, the impact of mining. With respect to nuclear power stations, in turn, even assuming that the strict safety measures in place suffice to rule out the likelihood of an accidental catastrophe, the inevitable accumulation of radioactive waste is, irrefutably, an unsolved problem that conditions coming generations so severely that nuclear power as it is known today cannot be regarded to be a sustainable source of energy. In any event, it must be borne in mind that even generation facilities that use renewable energy and are considered to be the most environment-friendly technologies, have an adverse impact. The most numerous, namely hydroelectric power plants, which have existed ever since electric power was first industrialized, change the surroundings radically: alteration of hydrology, disturbance of habitats or even transformation of the microclimate, not to mention the risk of accidents that can spell vast ecological and human disaster. Other more recent technologies also have adverse consequences: wind, the disturbance of natural habitats and noise; solar, land occupancy and the pollution inherent in the manufacture of the components required for the cells, and more specifically the heavy metals present in their waste products; the use of biomass has the same drawbacks as conventional steam plants, although the effect is less intense, no SO2 is emitted and, if properly managed, it is neutral with respect to CO2 emissions. In fact, all electricity generation activities have one feature in common, namely the occupation of land and visual impact, but the area involved and the (not necessarily proportional) extent of social rejection vary considerably with technology and specific local conditions. In a similar vein, the huge overhead lines that carry electric power across plains, moun- tain ranges, valleys, and coasts and circle large cities have at least a visual impact on the environment, which is being taken more and more seriously. Less visible but indubitably present are the electromagnetic fields that go hand-in-hand with the physics of electricity, although their potential effects on people, fauna and flora are still under examination. Such considerations have important consequences, since environmental permits and rights of way constitute strong constraints on the expansion of the transmission grid. As a result, the grid is operating closer and closer to its maximum capacity, occasioning new technical problems –relating to its dynamic behavior, for instance– which logically have economic consequences. In some cases alternative solutions are available, albeit at a higher cost, such as running underground lines in densely populated areas. But the question is not solely one of establishing the magnitude of the environmental impact of the electricity industry or of the awareness that minimizing this impact gener- ally entails increased system costs. The question, rather, is whether or not this impact should be considered when deciding how to best allocate society’s scant resources. In a free market, the tool for resource allocation is product price –in this case, of the various power options. Nonetheless, the general opinion, among both the public at large and governmental authorities at the various levels, is that energy prices do not cover all the types of impact discussed above. This is what is known as a market failure or externality, defined to be the consequences of some productive or consumption processes for other economic agents that are not fully accounted for by production or consumption costs. The existence of such externalities –also called external costs– therefore leads to an undue allocation of resources in the economy, preventing the market from properly and efficiently allocating resources on
10 CHAPTER 1. ELECTRIC ENERGY SYSTEMS. AN OVERVIEW
Generation plants
Distributed Generation
Very large consumers
Large consumers
Medium consumers Small consumers
HV Substations
HV Substations
HV Transmission grid
Distribution grid
Interconnections
Generation plants
Distributed Generation
Very large consumers
Large consumers
Medium consumers Small consumers
HV Substations
HV Substations
HV Transmission grid
Distribution grid
Interconnections
Generation plants
Distributed Generation
Very large consumers
Large consumers
Medium consumers Small consumers
HV Substations
HV Substations
HV Transmission grid
Distribution grid
Interconnections
Figure 1.2: Electric power system configuration and structure.
turn feed local distribution networks, which bring electric power to consumers at less haz- ardous voltages, adapted to consumer needs –20 000, 15 000, 6 600, 380 or 220 V. Successive substations step the working voltage down in several phases and centralize the measuring and protection devices for the entire transmission grid. The configuration of these grids is usually radial, with tentacles stretching out to even the most remote consumption points. As the lines are split up at each step, the grids carry less and less power and consequently can operate at lower voltages. Consumers connect to the voltage level best suited to their power needs, in accordance with the basic principle that the lower the voltage, the smaller the power capacity. This means that highly energy-intensive businesses –iron and steel plants and mills, aluminium plants, railways, and the like– connect directly to the high voltage grid; other major consumers –large factories– receive power at a somewhat lower voltage and small consumers –households, retailers, small factories– are connected to the low voltage network. Based on a more or less reciprocal principle, generating stations with a very small output feed their electric power directly into the distribution network, instead of connecting to the high voltage grid. Such generators, which usually run small hydroelectric,
1.2. THE TECHNOLOGICAL ENVIRONMENT 11
Region 1980 1983 1986 1989 1992 1995 1998 2001 2004 North America 9,792 9,863 10,677 12,035 12,115 12,716 13,258 13,392 13, Latin America 796 860 1,005 1,048 1,090 1,209 1,331 1,356 1, Europe 4,111 4,240 4,711 5,005 4,983 5,192 5,520 5,854 6, Former Soviet Union 4,584 4,881 5,045 5,351 4,848 3,989 3,777 4,003 4, Asia & Oceania 476 514 587 697 793 926 1,009 1,112 1, Middle East 599 807 929 1,064 1,137 1,364 1,563 1,744 2, Africa 375 392 443 457 450 467 477 496 542 World 1,663 1,698 1,827 1,980 1,986 2,062 2,150 2,242 2,
Table 1.1: Electricity consumption per capita (kWh), 1980-2004 [Source: Energy Informa- tion Administration, U.S. Government]
photovoltaic, wind, CHP or other types of modular power stations engaging in distributed generation, are sometimes grouped under a single category for regulatory purposes; an ex- ample would be Spain’s Electricity Act, which deals with them collectively under the term “special regime generators”. The points below focus on these chief components of electric energy systems: consumption, production, transmission, distribution and protection and control.
1.2.2 Consumption
Demand growth
Electricity demand has undergone high, sustained growth since the outset. The creation of standards for the electricity “product” –voltage, frequency, current– paved the way for the enormous boom in electricity consumption. This in turn laid the grounds for the standardization of electrically powered fixtures and facilities –from light bulbs and motors to PCs–, dramatically lowering manufacturing costs and enhancing product versatility, making it possible to use a given electrically-powered item virtually anywhere. Electric power consumption is one of the clearest indicators of a country’s industrial development, and closely parallels GDP growth. As noted earlier, there are scarcely any production processes or sectors involved in creating wealth that do not require electricity. But electric power consumption has also been used as a measure of social development. Electricity consumption per capita and especially the degree of electrification in a country –i.e., the percentage of the population living in electrified homes– provide a clear indication of the standard of living. This is not surprising, since such basics as lighting, a supply of potable water, refrigerators and other household appliances depend on access to electricity. The curves in the figures shown below relate the growth in electricity consumption to other basic indicators, such as gross domestic product, population or energy consumption. The growth rate is obviously higher in countries with low baseline levels of electric power consumption and high economic growth. Electrification rates and electricity consumption per capita vary widely from one area of the world to another, as Table 1 below eloquently illustrates [13]. One third of the Earth’s six million inhabitants have no electricity. But the growth in electricity consumption is not limited to developing countries: it has definitely steadied, but is certainly not flat in developed countries. Whilst the industrial world’s consumer mentality may partly be driving such growth, it is nonetheless true that
1.2. THE TECHNOLOGICAL ENVIRONMENT 13
Transition Economies
Electricity
GDP
Primary Energy
Population 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004
Figure 1.4: Transition Economies Growth Rate referred to 1980 value [Source: Energy Information Administration, U.S. Government; U.S. Department of Agriculture].
It may be important in this regard for consumers, the final and key link in the electricity chain, to receive the sophisticated economic signals that deregulation is sending out to the various other players involved –producers, transmitters, distributors and suppliers. Pricing should be designed to make consumers aware of the real –economic and environmental– cost of meeting their power needs, taking account of their consumption patterns in terms of hourly profile and total load. In the medium term, this should accustom domestic, commercial and industrial users to monitor and actively control electric consumption, in much the same way that discriminatory hourly telephone rates encourage customers to make non-urgent long-distance calls at off-peak times. Similarly, customers will voluntarily reduce electricity consumption by foregoing the most superfluous applications at times when higher prices signal that expensive resources are being deployed or that the margin between the demand and supply of electric power is narrow. The capacity of demand to respond to pricing is generally characterized by a parameter termed price elasticity of demand. This is defined to be the percentage variation in consumption of electricity or any other product in response to a unit variation in the price. Electricity demand is characterized, generally speaking, by scant short-term elasticity; in other words, the reaction to changes in price are small, although this assertion is more accurate for some types of consumer than others. Such limited elasticity is arguably due to the mentality prevailing until very recently in the electricity industry: continuity of supply was regarded to be a nearly sacred duty, to be fulfilled at any price. Consumers –who were identified, indeed, as subscribers rather than customers– were merely passive recipients of the service provided. Advances in
14 CHAPTER 1. ELECTRIC ENERGY SYSTEMS. AN OVERVIEW
Developing Countries Electricity
GDP
Primary Energy
Population
1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004
Figure 1.5: Developing Countries Growth Rate referred to 1980 value [Source: Energy Information Administration, U.S. Government; U.S. Department of Agriculture].
communications technology, in conjunction with the liberalization of the electric and energy industries in much of the world, are going to change consumers’ role radically. Demand side mentality overall will not change readily or quickly. Nonetheless, the years to come will very likely witness the maturing and accentuation of the role played by demand in the electricity industry, which will become as relevant as other areas, such as generation. Elasticity will grow, although much of the demand will foreseeably remain impervious to price.
Demand profiles
Consumption is characterized by a variety of items, from the technical standpoint. The two most important are power and energy. Power, measured in watts (W) is the energy (Wh) required per unit of time. Power, therefore, is the instantaneous energy consumed. Since electric power is not stored, electric facilities must be designed to withstand the maximum instantaneous energy consumed, in other words, to withstand the maximum power load in the system throughout the consumption cycle. Therefore, not only the total electric capacity needed, but the demand profile over time is especially relevant to characterize consumption. Such profiles, known as load curves, represent power consumed as a function of time. It may be readily deduced that a given value of energy consumed may have a number of related load profiles. Some may be flat, indicating very constant electricity consumption over time, while others may have one or several very steep valleys or peaks, denoting very variable demand. An aluminium plant working around the clock 365 days a year and a factory operating at full capacity only during the daytime on weekdays would exemplify these two
16 CHAPTER 1. ELECTRIC ENERGY SYSTEMS. AN OVERVIEW
Thermal Dam Pass
Hours – 28/09/
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
100 0
200
300
400
500
600
700
MWh
Figure 1.6: Hourly Power Load for a South American system.
for instance, of the fact that while real power and energy are consumed in the system, reac- tive power is also either generated or consumed –usually the latter, since inductive motors, which consume reactive power, generally predominate. This gives rise to a power factor less than unity, which penalizes consumption as far as the tariff charged is concerned, because it entails the circulation of unproductive current and with it ohmic dissipation and line capacity saturation. Moreover, consumption may depend on supply conditions –voltage, frequency–, be static or dynamic, or vary with connection time due to heating or other effects. All of this must be taken into account in load modeling.
Service quality
Electric power consumption may be very sensitive to the technical properties of the supply of electricity. Many devices malfunction or simply do not operate at all unless the voltage wave is perfectly sinusoidal and its frequency and magnitude are constant and stable over time. The precision, quality, features and performance of electrical devices depend on the quality of the current that powers them. Problems may also arise in almost any type of electrical device when the supply voltage is too low or too high (overvoltage). Computer,
1.2. THE TECHNOLOGICAL ENVIRONMENT 17
Pass
Dam
Thermal
Days – September 2006
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
20000 0
40000
60000
80000
100000
120000
140000
MWh
25 26 27 2829 30
Figure 1.7: Monthly Power Load for a South American system.
motor and household appliance performance may suffer or these devices may even fail alto- gether when the supply voltage swings up or down. Most electrically powered equipment, especially particularly expensive equipment or any regarded to be vital for the proper and safe operation of all kinds of processes, is fitted with protection systems –fuses, circuit breakers and switches, protection relays– to prevent damage caused by voltage fluctuations outside an acceptable range. Thus, for instance, the motors that drive the cooling pumps in nuclear power plants are fitted with under- and over-voltage protection that may even trip systems that cause plant shutdown, given the vital role of these motors in safe plant operation. Finally, outages whether short or long are clearly detrimental to service quality. Who hasn’t lost unstored information representing hours of work on a PC because of an untimely power outage? But power failures can cause even greater harm in industries such as foundries or in chemical or mechanical processes whose interruption may entail huge losses. In developed countries, where the universal supply of electricity is guaranteed, attention increasingly focuses on quality, as in any other commercial product. Consumption and consumers have become more demanding in this regard and electricity industry regulation authorities assiduously include quality standards in laws and regulations. Designing the
1.2. THE TECHNOLOGICAL ENVIRONMENT 19
5000
10000
15000
20000
25000
MWh
Hours
(^24408792) 11761560194423282712309634803864424846325016540057846168655269367320770480888472
Figure 1.9: Monotonic load curve for a Canadian Utility. [Source: Grant County PUD, http://www.gcpud.org/energy.htm].
sine wave due to the saturation of ferromagnetic materials –in system transformers or generators, for instance– or to the loads themselves; these deviations may also have adverse effects on consumer appliances.
Flicker: Low frequency fluctuations in voltage amplitude normally due to certain types of loads. Arc furnaces and electronic devices with thyristors usually cause flicker, which is detrimental to the proper operation of devices connected to the network. The solution to this problem is complex, since it depends not on the supplier but on system loads.
Overvoltage: Voltage increases caused by short-circuits, faults, lightning or any other event, potentially causing severe damage to consumer appliances.
Finally, it should be added that electric power consumption may vary broadly with temperature or contingencies. What must be borne in mind in this regard is, as mentioned earlier, that this demand must be met instantaneously and therefore the electric power supply system –power stations, transmission, distribution– must be designed to be able to detect and respond immediately to such variations. The system must be fitted with sophis- ticated measurement, control and supervisory equipment and must have reserve generating capacity ready to go into production at all times. And yet most users flipping switches in their homes or workplaces to turn on the lights or start up an appliance or tool are blithely unaware of the host of systems, services and processes needed to provide that service.
20 CHAPTER 1. ELECTRIC ENERGY SYSTEMS. AN OVERVIEW
1.2.3 Generation
Different generation technologies
The electricity required to meet these consumption needs is generated in production centers commonly called power plants or stations, where a source of primary energy is converted into electric power with clearly defined characteristics. Specifically, these facilities generate a three-phase, sinusoidal voltage system, with a strictly standardized and controlled wave frequency and amplitude. There are many generation technologies, usually associated with the fuel used. Conventional power stations are divided into hydroelectric, steam and nuclear plants, as described below.
Water
Reservoir
T
G
Hydro turbine
Generator
Network
Hydro plant
a.c.
T
G
Steam turbine
Generator
Network
Boiler Water
Burner
Thermal plant
Fuel
Steam
Combustion gases
a.c.
Reactor
Steam
T
G
Steam turbine
Generator
Network
Boiler Water Heat
Nuclear plant
a.c.
Figure 1.10: Hydroelectric, thermal and nuclear plants.
The primary source of energy used in hydroelectric stations is water, which is expressed, energetically speaking, in terms of flow rate and height –or “head”. Hydroelectric energy is converted by a hydraulic turbine into mechanical energy, characterized by the torque and speed of the shaft coupled to the electric generator. In other words, hydraulic energy is con- verted into electrical energy in the generator, producing voltage and current in the machine terminals. Because of the source of primary energy used, hydroelectric stations produce less atmospheric pollution than other conventional generation technologies. Another advantage to this type of stations, in addition to the cost of the fuel and lack of pollution, is their connection and disconnection flexibility, making them highly suitable regulating stations to adjust production to demand needs. Nonetheless, they are costly to build, and ensuring a
