Power transmission

Chapter Power transmission

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • Therefore, R717 and X−451.Using the phase-angle method to solve this (the numbers are more manageablethat way than they are with the R/Z method), calculatePhase angle arctan (X/R)arctan (−451/717)arctan (−0.629)−32.2 degreesThen the power factor isPFcos−32.20.84684.6 percentThe VA power, PVA, is given as 88.0 watts, and PFPT/PVA. Therefore, the true poweris found this way:PT/PVA0.846PT/88.00.846PT0.846× 88.074.4 wattsThis is a good example of a practical problem. Although there are several steps,each requiring careful calculation, none of the steps individually is very hard. It’s just amatter of using the right equations in the right order, and plugging the numbers in. Youdo have to be somewhat careful in manipulating plus/minus signs, and also in placingdecimal points.Power transmissionOne of the most multifaceted, and important, problems facing engineers is power:transmission.Generators produce large voltages and currents at a power plant, say from turbinesdriven by falling water. The problem: getting the electricity from the plant to the homes,businesses, and other facilities that need it. This process involves the use of long wiretransmission lines. Also needed are transformers to change the voltages to higher orlower values.A radio transmitter produces a high-frequency alternating current. The problem:getting the power to be radiated by the antenna, located some distance from the trans-mitter. This involves the use of a radio-frequency transmission line. The most commontype is coaxial cable. Two-wire line is also sometimes used. At ultra-high and microwavefrequencies, another kind of transmission line, known as a waveguide, is often em-ployed.The overriding concern in any power-transmission system is minimizing the loss.Power wastage occurs almost entirely as heat in the line conductors and dielectric, andin objects near the line. Some loss can also take the form of unwanted electromagneticradiation from a transmission line.In an ideal transmission line, all of the power is VA power; that is, it is in the form ofan alternating current in the conductors and an alternating voltage between them.Power transmission315