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Importance and History of electrical power systems
The Electricity is a global network that is fulfilled by the powers systems .The Electrical power system which connects Electrical Generation, Transmission and Distribution over a wide range of all electrical consumers' premises. The Electricity extensively used in many applications from nano sized motors to manmade satellites.
The electric utility industry can trace its beginnings to the early 1880s. During that period several companies were formed and installed water-power driven generation for the operation of arc lights for street lighting; the first real application for electricity in the United States. In 1882 Thomas Edison placed into operation the historic Pearl Street steam-electric plant and the pioneer direct current distribution system, by which electricity was supplied to the business offices of downtown New York. By the end of 1882, Edison’s company was serving 500 customers that were using more than 10,000 electriclamps. Satisfied with the financial and technical results of the New York City operation, licenses were issued by Edison to local businessmen in various communities to organize and operate electric lighting companies.1 By 1884 twenty companies were scattered in communities in Massachusetts, Pennsylvania, and Ohio; in 1885, 31; in 1886 48; and in 1887 62.These companies furnished energy for lighting incandescent lamps, and all operated under Edison patents. Two other achievements occurred in 1882: a water-wheel-driven generator was installed in Appleton, Wisconsin; the first transmission line was built in Germany to operate at 2400 volts direct current over a distance of 37 miles (59km).2 Motors were introduced and the use of incandescent lamps continued to increase. By 1886, the dc systems were experiencing limitations because they could deliver energy only a short distance from their stations since their voltage could not be increased or decreased as necessary. In 1885 a commercially practical transformer was developed that allowed the development of an ac system. A 4000 volt ac transmission line was installed between Oregon City and Portland, 13 miles away. A 112-mile, 12,000 volt three-phase line went into operation in 1891 in Germany. The first three-phase line in the United States (2300 volts and 7.5 miles) was installed in 1893 in California.3 In 1897 a 44,000-volt transmission line was built in Utah. In 1903, a 60,000-volt transmission line was energized in Mexico.4
In this early ac period, frequency had not been standardized. In 1891 the desirability of a standard frequency was recognized and 60Hz (cycles per second) was proposed. For many years 25, 50, and 60 Hz were standard frequencies in the United States. Much of the 25 Hz was railway electrification and has been retired over the years. The City of Los Angeles Department of Water and Power and the Southern California Edison Company both operated at 50Hz, but converted to 60Hz at the time that Hoover Dam power became available, with conversion completed in 1949. The Salt River Project was originally a 25 Hz system, but most of it was converted to 60 Hz by the end of 1954 and the balance by the end of 1973.5Over the first 90 years of its existence, until about 1970, the utility industry doubled about every ten years, a growth of about 7% per year. In the mid-1970s, due to increasing costs and serious national attention to energy conservation, the growth in the use of electricity dropped to almost zero. Today growth is forecasted at about 2% per year. The growth in the utility industry has been related to technological improvements that have permitted larger generating units and larger transmission facilities to be built. In 1900 the largest turbine was rated at 1.5MW.By 1930 the maximum size unit was 208MW. This remained the largest size during the depression and war years. By 1958 a unit as large as 335 MW was installed, and two years later in 1960, a unit of 450 MW was installed. In 1963the maximum size unit was 650MW and in 1965, the first 1,000MW unit was under construction. Improved manufacturing techniques, better engineering, and improved materials allowed for an increase in transmission voltages in the United States to accompany the increases in generator size. The highest voltage operating in1900 was 60kV. In 1923 the first 220 kV facilities were installed. The industry started the construction of facilities at 345 kV in 1954, in 1964 500 kV was introduced, and 765kV was put in operation in 1969. Larger generator stations
Required higher transmission voltages; higher transmission voltages made possible larger generators. These technological improvements increased transmission and generation capacity at decreasing unit costs, accelerating the high degree of use of electricity in the United States. At the same time, the concentration of more capacity in single generating units, plants, and transmission lines had considerably increased the total investment required for such large projects, even though the cost per unit of electricity had come down. Not all of the pioneering units at the next level of size and efficiency were successful. Sometimes modifications had to be made after they were placed in operation; units had to be dated because the technology was not adequate to provide reliable service at the level intended. Each of these steps involved a risk of considerable magnitude to the utility first to install a facility of a new type or a larger size or a higher transmission voltage. Creating the new technology required the investment of considerable capital that in some cases ended up being a penalty to the utility involved. To diversify these risks companies began to jointly own power plants and transmission lines so that each company would have a smaller share, and thus a smaller risk, in any one project. The sizes of generators and transmission voltages evolved together as shown in Figure 1.1.6 The need for improved technology continues. New materials are being sought in order that new facilities are more reliable and less costly. New technologies are required in order to minimize land use, water use, and impact on the environment. The manufacturers of electrical equipment continue to expend considerable sums to improve the quality and cost of their products. Unfortunately, funding for such research by electric utilities through the Electric Power Research Institute continues to decline.
Electrical power system over view
Electric power systems are real-time energy delivery systems. Real time means that power is generated, transported, and supplied the moment you turn on the light switch. Electric power systems are not storage systems like water systems and gas systems. Instead, generators produce the energy as the demand calls for it. Figure 1-1 shows the basic building blocks of an electric power system.
The system starts with generation, by which electrical energy is produced in the power plant and then transformed in the power station to high-voltage electrical energy that is more suitable for efficient long-distance transportation
The power plants transform other sources of energy in the process of producing electrical energy. For example, heat, mechanical, hydraulic, chemical, solar, wind, geothermal, nuclear, and other energy sources are0 used in the production of electrical energy. High-voltage (HV) power lines in the transmission portion of the electric power system efficiently transport electrical energy over long distances to the consumption locations. Finally, substations transform this HV electrical energy into lower-voltage energy that is transmitted over distribution power lines that are more suitable for the distribution of electrical energy to its destination, where it is again transformed for residential, commercial, and industrial consumption. A full-scale actual interconnected electric power system is much more complex than that shown in Figure 1-1; however the basic principles, concepts, theories, and terminologies are all the same. We will start with the basics and add complexity as we progress through the material.
TERMINOLOGY AND BASIC CONCEPTS
Let us start with building a good understanding of the basic terms and conceptsmost often used by industry professionals and experts to describe and discuss electrical issues in small-to-large power systems. Please take the time necessary to grasp these basic terms and concepts. We will use them
GENERATION
There are basically two physical laws that describe how electric power systems work. (Gravity is an example of a physical law.) One law has to do with generating a voltage from a changing magnetic field and the other haste do with a current flowing through a wire creating a magnetic field. Both physical laws are used throughout the entire electric power system from generation through transmission, distribution, and consumption. The combination of these two laws makes our electric power systems work. Understanding these two physical laws will enable the reader to fully understand and appreciate how electric power systems work.
Physical Law #1
Ac voltage is generated in electric power systems by a very fundamental physical law called Faraday’s Law. Faraday’s Law represents the phenomena behind how electric motors turn and how electric generators produce electricity. Faraday’s Law is the foundation for electric power systems. Faraday’s Law states, “A voltage is produced on any conductor in a changing magnetic field.” It may be difficult to grasp the full meaning of that statement at first. It is, however, easier to understand the meaning and significance of this statement through graphs, pictures, and animations. In essence, this statement is saying that if one takes a coil of wire and puts it next to a moving or rotating magnet, a measurable voltage will be produced in that coil. Generators, for example, use a spinning magnet (i.e., rotor) next to a coil of wire to produce voltage. This voltage is then distributed throughout the electric power system.
We will now study how a generator works. Keep in mind that virtually all generators in service today have coils of wire mounted on stationary housings, called stators, where voltage is produced due to the magnetic field provided on the spinning rotor. The rotor is sometimes called the field because it is responsible for the magnetic field portion of the generator. The rotor’s strong magnetic field passes the stator windings (coils), thus producing or generating an alternating voltage (ac) that is based on
Faraday’s Law. This principle will be shown and described in the followingsections.The amplitude of the generator’s output voltage can be changed by changing the strength of rotor’s magnetic field. Thus, the generator’s output voltage can be lowered by reducing the rotor’s magnetic field strength. The means by which the magnetic field in the rotor is actually changed
Single-Phase ac Voltage Generation
Placing a coil of wire (i.e., conductor) in the presence of a moving magnetic field produces a voltage, as discovered by Faraday. This principle is graphically presented in Figure 2-1. While reviewing the drawing, note that changing the rotor’s speed changes the frequency of the sine wave. Also recognize the fact that increasing the number of turns (loops) of conductor or wire in the coil increases the resulting output voltage.
Three-Phase ac Voltage Generation
When three coils are placed in the presence of a changing magnetic field, three voltages are produced. When the coils are spaced 120 degrees apart in a 360 degree circle, three-phase ac voltage is produced. As shown in Figure 2-2, three-phase generation can be viewed as three separate single-phase generators, each of which are displaced by 120 degrees.
THE THREE-PHASE ac GENERATOR
Large and small generators that are connected to the power system have three basic components: stator, rotor, and exciter. This section discusses these three basic components.
The Stator
A three-phase ac generator has three single-phase windings. These three windings are mounted on the stationary part of the generator, called the stator. The windings are physically spaced so that the changing magnetic field present on each winding is 120° out of phase with the other windings.
The Rotor
The rotor is the centre component that when turned moves the magneticfield. A rotor could have a permanent magnet or an electromagnet and stillfunction as a generator. Large power plant generators use electromagnets so that the magnetic field can be varied. Varying the magnetic field strength ofthe rotor enables generation control systems to adjust the output voltage according to load demand and system losses. A drawing of an electromagnetism shown in Figure 2-4.The operation of electromagnets is described by Physical Law #2.
Electromagnets
Applying a voltage (e.g., battery) to a coil of wire produces a magnetic field. The coil’s magnetic field will have a north and a south pole as shown in Figure2-4. Increasing the voltage or the number of turns in the winding increases the magnetic field. Conversely, decreasing the voltage or number of turns in the winding decreases the magnetic field. Slip rings are electrical contacts that are used to connect the stationary battery to the rotating rotor, as shown in Figure 2-4 and Figure 2-5.
The Exciter
The voltage source for the rotor, which eventually creates the rotor’s magnetic field, is called the exciter, and the coil on the rotor is called the field. Figure 2-5 shows the three main components of a three-phase ac generator: the stator, rotor, and exciter. Most generators use slip rings to complete the circuit between the stationary exciter voltage source and the rotating coil on the rotor where the electromagnet produces the north and south poles.
Note: Adding load to a generator’s stator windings reduces rotor speed because of the repelling forces between the stator’s magnetic field, and the
Rotor’s magnetic field since both windings has electrical current flowing through them. Conversely, removing load from a generator increases rotor speed. Therefore, the mechanical energy of the prime mover that is responsible for spinning the rotor must be adjusted to maintain rotor speed or frequency under varying load conditions.
Rotor Poles
Increasing the number of magnetic poles on the rotor enables rotor speeds to be slower and still maintain the same electrical output frequency. Generators that require slower rotor speeds to operate properly use multiple-pole rotors. For example, hydropower plants use generators with multiple-pole rotors because the prime mover (i.e., water) is very dense and harder to control than light-weight steam.
The relationship between the number of poles on the rotor and the speed of the shaft is determined using the following mathematical formula:
Revolutions per minute = 7200/ Number of poles
Figure 2-6 shows the concept of multiple poles in a generator rotor. Since these poles are derived from electromagnets, having multiple windings on arotor can provide multiple poles.
TRANSMISSION LINES
Why use high-voltage transmission lines? The best answer to that question is that high-voltage transmission lines transport power over long distances much more efficiently than lower-voltage distribution lines for two main reasons. First, high-voltage transmission lines take advantage of the power equation, that is, power is equal to the voltage times current. Therefore, increasing the voltage allows one to decrease the current for the same amount of power. Second, since transport losses are a function of the square of the current flowing in the conductors, increasing the voltage to lower the current drastically reduces transportation losses. Plus, reducing the current allows one to use smaller conductor sizes. Figure 3-1 shows a three-phase 500 kV transmission line with two conductors per phase.
The two-conductors-per-phase option is called bundling. Power companies bundle multiple conductors—double, triple, or more to increase the power transport capability of a power line. The type of insulation used in this line is referred to as V-string insulation. V-string insulation, compared to I-string insulation, provides stability in wind conditions. This line also has two static wires on the very top to shield itself from lightning. The static wires in this case do not have insulators; instead, they are directly connected to the metal towers so that lightning strikes are immediately grounded to earth. Hopefully, this shielding will keep the main power conductors from experiencing a direct lightning strike.
Raising Voltage to Reduce Current
Raising the voltage to reduce current reduces conductor size and increases insulation requirements. Let us look at the power equation again:
Power = Voltage × Current
Voltage In × Current In = Voltage Out × Current Out
From the power equation above, raising the voltage means that the current can be reduced for the same amount of power. The purpose of step-up transformer sat power plants, for example, is to increase the voltage to lower the current for power transport over long distances. Then at the receiving end of the transmission line, step-down transformers are used to reduce the voltage for easier distribution. For example, the amount of current needed to transport 100 MW of power at 230 kV is half the amount of current needed to transport 100 MW of power at 115 kV. In other words, doubling the voltage cuts the required current in half. The higher-voltage transmission lines require larger structures with longer insulator strings in order to have greater air gaps and needed insulation. However, it is usually much cheaper to build larger structures and wider right of ways for high-voltage transmission lines than it is to pay the continuous cost of high losses associated with lower-voltage power lines.
Also, to transport a given amount of power from point “a” to point “b,” a higher-voltage line can require much less right of way land than multiple lower-voltage lines that are side by side.
Raising Voltage to Reduce Losses
The cost due to losses decreases dramatically when the current is lowered. The power losses in conductors are calculated by the formula I2R. If the current (I) is doubled, the power losses quadruple for the same amount of conductor resistance ®! Again, it is much more cost effective to transport large quantities of electrical power over long distances using high-voltage transmission lines because the current is less and the losses are much less.
Bundled Conductors
Bundling conductors significantly increases the power transfer capability of the line. The extra relatively small cost when building a transmission line to add bundled conductors is easily justified since bundling the conductors actually doubles, triples, quadruples, and so on the power transfer capability of the line. For example, assume that a right of way for a particular new transmission line has been secured. Designing transmission lines to have multiple conductors per phase significantly increases the power transport capability of that line for a minimal extra overall cost.
CONDUCTORS
Conductor material (all wires), type, size, and current rating are key factors in determining the power handling capability of transmission lines, distribution lines, transformers, service wires, and so on. A conductor heats up when current flows through it due to its resistance. The resistance per mile is constant for a conductor. The larger the diameter of the conductor, the less resistance there is to current flow. Conductors are rated by how much current causes them to heat up to predetermined amount of degrees above ambient temperature. The amount of temperature rise above ambient (i.e., when no current flows) determines the current rating of a conductor. For example, when a conductor reaches70°C above ambient, the conductor is said to be at full load rating. The power company selects the temperature rise above ambient to determine acceptable conductor ratings. The power company might adopt a different current rating (i.e., temperature rating) for emergency conditions.
The amount of current that causes the temperature to rise depends on the conductor material and size. The conductor type determines its strength and application in electric power systems.
Conductor Material
Utility companies use different conductor materials for different applications. Copper, aluminium, and steel are the primary types of conductor materials used in electrical power systems. Other types of conductors, such as silver and gold, are actually better conductors of electricity; however, cost prohibits wide use of these materials.
Copper
Copper is an excellent conductor and is very popular. Copper is very durable and is not affected significantly by weather.
Aluminium
Aluminium is a good conductor but not as good or as durable as copper. However, aluminium costs less. Aluminium is rust resistant and weighs much less than copper.
Steel
Steel is a poor conductor when compared to copper and aluminium; however, it is very strong. Steel strands are often used as the core in aluminium conductors to increase the tensile strength of the conductor.
Conductor Types
Power line conductors are either solid or stranded. Rigid conductors such as hollow aluminium tubes are used as conductors in substations because of the added strength against sag in low-profile substations when the conductor is only supported at both ends. Rigid copper bus bars are commonly used in low-voltage switch gear because of their high current rating and relatively short lengths. The most common power line conductor types are shown below:
Solid. Solid conductors (Figure 3-2) are typically smaller and stronger than stranded conductors. Solid conductors are usually more difficult to bend and are easily damaged.
Stranded. As shown in Figure 3-3, stranded conductors have three or more strands of conductor material twisted together to form a single conductor. Stranded conductors can carry high currents and are usually more flexible than solid conductors. Aluminium Conductor, Steel-Reinforced (ACSR). To add strength to aluminium Conductors, Figure 3-4 shows steel strands that are used as the core of aluminium stranded conductors. These high-strength conductors are normally used on long span distances, for minimum sag applications.