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- - - - AC electrical energy can be transmitted at very high voltages over long distances (lowering I 2 R losses). Transformers raise or lower voltages as needed at generating stations or distribution points. Transformers cannot be used on dc systems. HERNAN, 19
        
        As the number of poles in an ac generator increases, the actual required driven speed in r/min decreases proportionally for a given frequency. When using a higher frequency, less iron and copper are required in transformers, motors, and other electrical equipment.
    - - Generation
        
        A simple alternating-voltage generator consisting of a single coil rotating in a uniform magnetic field is shown in Figure 1–11. The use of Fleming’s generator rule shows that an alternating voltage is generated in the coil as it rotates. If the ends of the coil are connected to two slip rings, the alternating voltage can be observed on an oscilloscope. This voltage pattern is a typical sine wave, as shown in Figure 1–12. HERNAN, 2011, p. 10 The amount of voltage induced in a conductor is proportional to three factors: The strength of the magnetic field (flux density). The length of the conductor (often expressed as the number of turns of wire). The speed of the cutting action.
        
        The generation of large amounts of ac energy in large central stations is a more efficient
        and economical operation than in smaller local units.
        
        The ac induction motor is simple and rugged in construction. It has excellent operating characteristics and is far more economical in initial costs, replacement, and mainte- nance than are dc motors. HERNAN, 19
      - monstrate the graphical method of generating sine and cosine waves (giving the for-mula for each). describe how a sine wave of voltage is obtained as a coil rotates in a uniform magnetic field (simple ac generator), giving the equation for instantaneous voltage. Herman, 2011
      - list the factors affecting the frequency of the voltage from ac generators and give the
        equation expressing the frequency, list the advantages of a 60-hertz (Hz) service over a 25-Hz service. Herman, 2011
        
        The advantage to using a higher-frequency service is that less iron and copper are required in the transformers. Therefore, they are lighter and lower in cost. Also, incandescent lamps operating at 60 Hz have no noticeable flicker. At 25 Hz, the flicker of incandescent lamps can be annoying. The speed of a generator and the number of poles determine the frequency of the generated voltage. If a generator has two poles (north and south), and the coils rotate at a speed
  - - - With the use of alternating current, the voltage can be stepped up or stepped down efficiently by means of transformers. A transformer has no moving parts, and its losses are relatively low. The efficiency of most transformers at the rated load is high, from 95% to more than 99%. Transformers cannot be used with direct current. DC voltage changes are obtained by using series resistors, which give rise to I 2 R losses, or motor– generator sets, which have relatively low overall efficiencies. However, the reduction or increase of dc voltages in dc systems is inefficient.
      - Large ac generators having very high power ratings (Figure 1–1), plus efficient transformers to step up or step down the alternating voltage, make it possible to conduct ac energy economically over long distances from generating stations to the various load centers by way of high-voltage transmission systems. Thus, huge amounts of electrical energy can be generated at one location. For example, a large hydroelectric generating station may be located near a waterfall. Here, the energy can be generated at a relatively low cost per kilowatt-hour. Large steam-generating stations are also located where fuel is easy to obtain and abundant water is available. Steam-generating stations use very large-capacity alternators having efficiency ratings as high as 97%. Large high-speed turbines operating at very high steam pressures are used to turn the ac generators. The efficiency of these steam turbines, operating at speeds of 1200, 1800, or 3600 revolutions per minute (r/min), is much greater than that of steam turbines used in smaller generating plants. Completely automated control systems are used in modern generating stations to increase the total operating efficiency even more. As a result, large steam-generating plants and hydroelectric stations operate efficiently to produce electrical energy at a low generating cost per kilowatt-hour.
      - AC generators can be built with much larger power and voltage ratings than dc generators. An ac generator does not require a commutator. The armature or output wind ing of the ac generator can be mounted on the stator (stationary part) of the machine. The output connections are made with cables or bus bars bolted directly to the stator windings (stationary armature windings). Armature voltages of 13,800 volts or more are common. Currents of any desired value can be obtained with the proper machine design. The rotating member of the alternator is the field. This field is supplied with direct current by means of slip rings or by means of a brushless exciter from an external dc source. The voltage of the source is in the range of 100 to 250 V. In contrast to the ac generator, the armature or output winding of a dc generator must be the rotating part of the machine. The connection of the armature to the external load is made through a commutator and brushes. These components restrict the maximum voltages and currents that can be obtained from dc machines to practical levels. Large dc machines rated at 600 to 750 V are common. Occasionally, a machine rated at 1500 V is required for certain applications. The commutators of dc generators are usually rated at less than 8000 amperes (A). These large current ratings are practical only on slow-speed machines. For these reasons, dc generator ratings are limited to relatively low voltage and power values as compared to ac generators.
      - The ac induction motor (Figure 1–2) has no commutator or brushes. This type of motor has a relatively constant speed. It is rugged and simple in construction. The initial pur-chase price and the maintenance and repair costs for the ac induction motor are con-siderably less than the costs for a dc motor of comparable horsepower, voltage, and speed. Further, the starting equipment used with a typical induction motor is also lower in cost initially when compared to the starting equipment used with dc motors having similar horsepower ratings. Because ac induction motors do not contain a commutator or brushes, they generally have a longer life span and require less maintenance than dc machines.
- - - - In recent years, the electric power delivery system has been changing dramatically. Concern for the environment, the economy, and power system security has placed stringent demands on power transmission. We must acknowledge the inevitable de-pletion of fossil fuels and the growing importance of sustainable and environmental awareness. Electricity derived from renewable energy sources offers advantages in terms of economical efficiency, especially when compared with oil, gas and coal. Existing rights-of-way for high voltage transmission lines are generally underuti-lized and some transmission circuits are nearing their end of life. New rights-of-way for overhead transmission lines are becoming difficult if not impossible to acquire. Voltages Sourced Converter (VSC) transmission is the way forward with its capabil-ity to operate with many terminals, allow for greater power density in its circuits, andreplace aging AC transmission lines or existing AC lines that may be converted to DC. DC overhead transmission lines can be compacted to share right-of-way with roads and railway lines. VSC transmission is more favorable for use with underground and undersea cable transmission where it is expedient to do so. In the future, issues of the combination of diversity of loads, wind and solar energy across wide geographical areas will become crucial. DC grids are critical to solving these major concerns. CHAUDHURI, 2014, p. 2
        
        During the early 1950s, there was renewed interest in the use of DC technology primarily driven by the need for long distance cable transmission. It was realized that the power capacity of an AC cable reduces drastically due to excessive charging current even for moderate distances and voltage levels necessitating the use of DC cables where no such limitation exists. This led to the first DC cable link between mainland Sweden and Gotland island in 1953. Although DC reappeared in the scene in the context of cable transmission, it was soon realized that DC could be a cost effective option even for overhead line transmission if the transmission distance is very high (beyond 1000 km) where AC transmission capacity is increasingly limited due to stability considerations. RESUMIR
      - POr qué no se utiliza actualmente
  - - - Direct current applied to variable speed motors results in stepless, precise speed adjustments. Such motors are used in metal rolling mills, papermaking machines, high-speed ments. Such motors are used in metal rolling mills, papermaking machines, high-speed gearless elevators, automated machine tools, and high-speed printing presses.
      - Traction motors require direct current. Such motors are used on locomotives, subway cars, trolley buses, and large construction machinery that will not be driven on highways. Using a dc motor in these applications eliminates the need for clutches, gear shift-ing transmissions, drive shafts, universal joints, and differential gearing. Thus, almost all large locomotives have diesel engines that drive direct-current generators to supply the power for dc traction motors installed in each locomotive truck.
      - Direct current is used to excite the field windings of alternating-current generators.
      - Direct current is used for various electrochemical processes, including electroplating, refining of copper and aluminum, electrotyping, production of industrial gases by electrolysis, and charging of storage batteries.
  - - - With increased penetration of intermittent renewable energy sources (e.g., wind power), balancing the supply and demand is likely to be a major problem. To ensure reliable operation of the system, there is a growing need for meshed interconnection to effectively share the diverse portfolio of renewable energy resources and thereby increase operational flexibility. For instance, in Europe, the hydropower from Nor-way and solar power in Spain and Portugal could be utilized when the wind is not blowing in the UK or mainland Europe and vice versa. To enable such sharing of power and also harness remote offshore wind, there is a business case for setting up an European Offshore Supergrid [2, 3, 4, 5]. There are several visions for an offshore grid in Europe, some of which are shown in Fig. 1.1. One aspect in common with all such visions is that several DC links are connected at a single point forming a DC grid. Because of the subsea transmission distances involved, the only viable option is to use DC, which essentially calls for an MTDC or meshed grid. In such a meshed DC grid, the power flow in the DC links would have to reverse frequently depending on the geographical distribution of renewable generation and the electricity price differential at a given point in time. The VSC technology allows such power flow reversals.... CHAUDHURI, 2014, p. 3-4
  - - - It was only after 1997 that semiconductor switches with both controlled turn-on and turn-off capability, like an insulated gate bipolar transistor (IGBT) became commercially available at high power ratings. This enabled the use of voltage-sourced converter (VSC)-based HVDC technology, which offered significant advantages over its LCC counterpart. These include but are not limited to reliable operation with weak AC systems, low cost and footprint of converter stations, use of lighter, and stronger cables that makes VSC particularly attractive for offshore transmission. Despite the obvious potential and promise, the uptake of VSC technology was initially hindered by its limited power ratings (few hundred MWs) compared to LCC (up to 8000 MW). Rapid development in the VSC technology since has resulted in availability of relatively higher ratings (up to 1000 MW is under development now) for VSC-based HVDC links, but it is yet to catch up with the ratings offered by LCC. For overhead lines, HVDC is cost-effective only at large transmission distances (e.g., above 600 km) at which level meshed interconnections are not economically justifi- able. For underground or subsea cable transmission, the distance beyond which DC technology is effective is much shorter. This has resulted in a number of point-to- point interconnections between AC systems separated by sea to allow exchange of cost-effective electricity.
- - - - “Green Building” means constructing buildings that are more energy efficient. This can
      - encompass many areas of construction, such as adding extra insulation and using “low E”
      - energy-efficient windows to elaborate systems such as solar collectors to heat water and
      - solar panels or wind generators to supplement utility-provided electrical service. For the
      - electrician, it may be installing larger wire than necessary to help overcome voltage drops
      - or energy-efficient appliances such as heat–pump type water heaters. These water heaters
      - use about one half the amount of power as a standard electric water heater.