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معلومات كهربائية مهمة جدا

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    bader269
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    معلومات كهربائية مهمة جدا

    Electric Power Systems
    Electric Power Systems, components that transform other types of energy into electrical energy and transmit this energy to a consumer. The production and transmission of electricity is relatively efficient and inexpensive, although unlike other forms of energy, electricity is not easily stored and thus must generally be used as it is being produced.

    11 components of electric system:
    A modern electric power system consists of six main components: 1) the power station, 2) a set of transformers to raise the generated power to the high voltages used on the transmission lines, 3) the transmission lines, 4) the substations at which the power is stepped down to the voltage on the distribution lines, 5) the distribution lines, and 6) the transformers that lower the distribution voltage to the level used by the consumer's equipment

    APower Station
    The power station of a power system consists of a prime mover, such as a turbine driven by water, steam, or combustion gases that operate a system of electric motors and generators. Most of the world's electric power is generated in steam plants driven by coal, oil, nuclear energy, or gas. A smaller percentage of the world’s electric power is generated by hydroelectric (waterpower), diesel, and internal-combustion plants

    B Transformers
    Modern electric power systems use transformers to convert electricity into different voltages. With transformers, each stage of the system can be operated at an appropriate voltage. In a typical system, the generators at the power station deliver a voltage of from 1,000 to 26,000 volts (V). Transformers step this voltage up to values ranging from 138,000 to 765,000 V for the long-distance primary transmission line because higher voltages can be transmitted more efficiently over long distances. At the substation the voltage may be transformed down to levels of 69,000 to 138,000 V for further transfer on the distribution system. Another set of transformers step the voltage down again to a distribution level such as 2,400 or 4,160 V or 15, 27, or 33 kilovolts (kV). Finally the voltage is transformed once again at the distribution transformer near the point of use to 240 or 120 V.
    C Transformers Lines
    The lines of high-voltage transmission systems are usually composed of wires of copper, aluminum, or copper-clad or aluminum-clad steel, 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 six towers to the kilometer. In some areas high-voltage lines are suspended from tall wooden poles spaced more closely together.
    For lower voltage distribution lines, wooden poles are generally used rather than steel towers. In cities and other areas where open lines create a safety hazard or are considered unattractive, insulated underground cables are used for distribution. Some of these cables have a hollow core through which oil circulates under low pressure. The oil provides temporary protection from water damage to the enclosed wires should the cable develop a leak. Pipe-type cables in which three cables are enclosed in a pipe filled with oil under high pressure (14 kg per sq cm/200 psi) are frequently used. These cables are used for transmission of current at voltages as high as 345,000 V (or 345 kV).

    D Supplementary Equipment
    Any electric-distribution system involves a large amount of supplementary equipment to protect the generators, transformers, and the transmission lines themselves. The system often includes devices designed to regulate the voltage or other characteristics of power delivered to consumers.
    To protect all elements of a power system from short circuits and overloads, and for normal switching operations, circuit breakers are employed. These breakers are large switches that are activated automatically in the event of a short circuit or other condition that produces a sudden rise of current. Because a current forms across the terminals of the circuit breaker at the moment when the current is interrupted, some large breakers (such as those used to protect a generator or a section of primary transmission line) are immersed in a liquid that is a poor conductor of electricity, such as oil, to quench the current (see dielectric). In large air-type circuit breakers, as well as in oil breakers, magnetic fields are used to break up the current. Small air-circuit breakers are used for protection in shops, factories, and in modern home installations. In residential electric wiring, fuses were once commonly employed for the same purpose. A fuse consists of a piece of alloy with a low melting point, inserted in the circuit, which melts, breaking the circuit if the current rises above a certain value. Most residences now use air-circuit breakers.

    111 Power Failures
    In most parts of the world, local or national electric utilities have joined in grid systems. The linking grids allow electricity generated in one area to be shared with others. Each utility that agrees to share gains an increased reserve capacity, use of larger, more efficient generators, and the ability to respond to local power failures by obtaining energy from a linking grid.
    These interconnected grids are large, complex systems that contain elements operated by different groups. These systems offer the opportunity for economic savings and improve overall reliability but can create a risk of widespread failure. For example, a major grid-system breakdown occurred on November 9, 1965, in eastern North America, when an automatic control device that regulates and directs current flow failed in Queenston, Ontario, causing a circuit breaker to remain open. A surge of excess current was transmitted through the northeastern United States. Generator safety switches from Rochester, New York, to Boston, Massachusetts, were automatically tripped, cutting generators out of the system to protect them from damage. Power generated by more southerly plants rushed to fill the vacuum and overloaded these plants, which automatically shut themselves off. The power failure enveloped an area of more than 200,000 sq km (80,000 sq mi), including the cities of Boston; Buffalo, New York; Rochester, New York; and New York City.
    Similar grid failures, usually on a smaller scale, have troubled systems in North America and elsewhere. On July 13, 1977, about 9 million people in the New York City area were once again without power when major transmission lines failed. In some areas the outage lasted 25 hours as restored high voltage burned out equipment. These major failures are termed blackouts.
    The worst blackout in the history of the United States and Canada occurred August 14, 2003, when 61,800 megawatts of electrical power was lost in an area covering 50 million people. (One megawatt of electricity is roughly the amount needed to power 750 residential homes.) The blackout affected such major cities as Cleveland, Detroit, New York, Ottawa, and Toronto. Parts of eight states—Connecticut, Massachusetts, Michigan, New Jersey, New York, Ohio, Pennsylvania, and Vermont—and the Canadian provinces of Ontario and Québec were affected. The blackout prompted calls to replace aging equipment and raised questions about the reliability of the national power grid.
    The term brownout is often used for partial shutdowns of power, usually deliberate, either to save electricity or as a wartime security measure. From November 2000 through May 2001 California experienced a series of planned brownouts to groups of customers, for a limited duration, in order to reduce total system load and avoid a blackout due to alleged electrical shortages. However, an investigation by the California Public Utilities Commission into the alleged shortages later revealed that five energy companies withheld electricity they could have produced. In 2002 the commission concluded that the withholding of electricity contributed to an “unconscionable, unjust, and unreasonable electricity price spike.” California state utilities paid billion more for energy in 2000 than in 1999 as a result, the head of the commission found.
    The commission also cited the role of the Enron Corporation in the California brownouts. In June 2003 the Federal Energy Regulatory Commission (FERC) barred Enron from selling electricity and natural gas in the United States after conducting a probe into charges that Enron manipulated electricity prices during California’s energy crisis. In the same month the Federal Bureau of Investigation arrested an Enron executive on charges of manipulating the price of electricity in California. Two other Enron employees, known as traders because they sold electricity, had pleaded guilty to similar charges. See also Enron Scandal.
    Despite the potential for rare widespread problems, the interconnected grid system provides necessary backup and alternate paths for power flow, resulting in much higher overall reliability than is possible with isolated systems. National or regional grids can also cope with unexpected outages such as those caused by storms, earthquakes, landslides, and forest fires, or due to human error or deliberate acts of sabotage.

    III Power Quality
    In recent years electricity has been used to power more sophisticated and technically complex manufacturing processes, computers and computer networks, and a variety of other high-technology consumer goods. These products and processes are sensitive not only to the continuity of power supply but also to the constancy of electrical frequency and voltage. Consequently, utilities are taking new measures to provide the necessary reliability and quality of electrical power, such as by providing additional electrical equipment to assure that the voltage and other characteristics of electrical power are constant

    A Voltage Regulation
    Long transmission lines have considerable inductance and capacitance. When a current flows through the line, inductance and capacitance have the effect of varying the voltage on the line as the current varies. Thus the supply voltage varies with the load. Several kinds of devices are used to overcome this undesirable variation in an operation called regulation of the voltage. The devices include induction regulators and three-phase synchronous motors (called synchronous condensers), both of which vary the effective amount of inductance and capacitance in the transmission circuit.
    Inductance and capacitance react with a tendency to nullify one another. When a load circuit has more inductive than capacitive reactance, as almost invariably occurs in large power systems, the amount of power delivered for a given voltage and current is less than when the two are equal. The ratio of these two amounts of power is called the power factor. Because transmission-line losses are proportional to current, capacitance is added to the circuit when possible, thus bringing the power factor as nearly as possible to 1. For this reason, large capacitors are frequently inserted as a part of power-transmission systems

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    لى استفسار ارجوا الرد عليه من الساده المهندسين الزملاء
    كيفيه عمل لاتش لاشارة (npn10volt)خارجه من حساس مغناطيسى تقاربى بسرعه30/1من الثانيه وذالك لتشغيل ريلاى
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    محمود خيرى عبده
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    سبحان الله وبحمده......سبحان الله العظيم

  
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