Technical English: Electric Power Systems
Electric Power Systems.
The production and transmission of energy in the form of electricity have important economic advantages in terms of cost per unit of power delivered. Electric power systems also make possible the utilization of hydroelectric power at a distance from the source. Alternating current (AC) is generally used in modern power systems, because it may be easily converted to higher or lower voltages by means of transformers. Thus, each stage of the system can be operated at an appropriate voltage. Such an electric power system consists of six main elements: the power station; a set of transformers to raise the generated power to the high voltages used on the transmission lines; the transmission lines; the substations at which the power is stepped down to the voltage on the sub transmission lines; the sub transmission lines; and the transformers that lower the sub transmission voltage to the level used by the consumer's equipment.
In a typical system the generators at the central station deliver a voltage of from 1000 to 26,000 volts (V); higher voltages are undesirable because of difficulties of insulation and the danger of electrical breakdown and damage. This voltage is stepped up by means of transformers to values ranging from 138,000 to 765,000 V for the primary transmission line. At the substation the voltage may be transformed down to levels of 69,000 to 138,000 V for further transfer on the sub transmission system. Transformers step down the voltage again to a distribution level. Finally the voltage is transformed once again at the distribution transformer near the point of use to 240 or 120 V.
The central station of a power system consists of a prime mover, such as a water or steam turbine, which operates an electric generator. Most of the world's electric power in the early 1990s was generated in steam plants driven by coal, oil, nuclear energy, or gas, with lesser percentages generated by hydroelectric, diesel, and internal-combustion plants.
The lines of high-voltage transmission systems are usually composed of wires of copper, aluminum, which are suspended from tall latticework towers of steel by strings of porcelain insulators. By the use of clad steel wires and high towers, the distance between towers can be increased, and the cost of the transmission line thus reduced. In modern installations with essentially straight paths, high-voltage lines may be built with as few as eight towers to the kilometer. In some areas high-voltage lines are suspended from tall wooden poles spaced more closely together. For lower voltage sub transmission and distribution lines, wooden poles are generally used rather than steel towers. In cities and other areas where open lines create a hazard, insulated underground cables are used for distribution. Any electric-distribution system involves a large amount of supplementary equipment for the protection of generators, transformers, and the transmission lines themselves. The system often includes devices designed to regulate the voltage delivered to consumers and to correct the power factor of the system.