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Nov. 21, 2005 I love to see pylons marching along on the countryside. These of course are the towers of steel or reinforced concrete carrying high-tension electrical conductors. Their magnificent procession over field and farm assures us that electrical power will be delivered to our factories and households, enabling us to enjoy the many devices that today make life so much easier than it was in bygone times. Have you ever wondered why there are usually many conductors, 10 or 20 sometimes, instead of just a single large conductor? Have you ever wondered why the outriggers and crossbeams supporting the conductors sometimes have what seems to be rather fanciful shapes? These features are not motivated entirely by esthetic ideals, but are dictated, in part, by practical considerations, with money being the decisive factor. Perhaps many people suppose that those conductors, which are also called cables or lines, are made of copper, which is reputed to be the best conductor. And so it is, on the basis of cross-sectional area. Aluminum is only 60% as conductive as copper, but weighs only 28% as much, with the result that aluminum is twice as conductive as copper on a weight basis. Unfortunately, aluminum does not have the tensile strength to span from pylon to pylon. The result is that stranded cables are used, with the majority of the strands being aluminum and the others being steel. Therefore, the conductors are called ACSR, aluminum cable steel reinforced. They come in diameters up to about three inches or perhaps a little more. Suppose we have a power plant generating 2000 megawatts, which is 2,000,000,000 watts, transmitted with a tension of 500,000 volts. This implies a current of 4000 amperes, which is gotten by dividing watts by volts. The size of conductors is limited by the fact that the heat generated in them is proportional to the square of the diameter, which means that, if they are too large, they will melt, or at least get hot enough to lose tensile strength. I don’t believe there is a conductor in existence that could safely carry 4000 amperes. Therefore, the current is distributed over the 10 or 20 conductors that you see on high. This multiplicity does not necessarily mean that the several conductors have different destinations. Generally, the electricity is transmitted as AC (alternating current), because voltage can be stepped down at transformer substations more readily than would be the case with DC (direct current). When a conductor is carrying AC, it produces a magnetic field around itself, and this magnetic field in turn induces a secondary current, opposite to the originally generated current. In other words, 100 amperes might be reduced to 95 amperes. This phenomenon is called self-inductance. The magnetic fields about each of the conductors in the whole configuration induce secondary back-currents in each of the other conductors too, and this is called mutual inductance. The resultant losses in current are calculated by using a quantity called inductive reactance. If adjacent conductors are out-of-phase, that is, at any given point at a given time, one is positive and the other is negative, there will be an attraction through the air, which is called capacitance. This also reduces the current being transmitted, which is gotten by figuring in the capacitive reactance. Adding up capacitive reactance and inductive reactance, we get total reactance. Some reactance arises also from the height of the conductors. If they are too close to the ground, there are additional losses. Another factor, which is determined by the distance that the power is being transmitted rather than the configuration of the conductors, is resistance. If the power goes 200 miles the resistance will be double what it would be for 100 miles. Resistance and reactance considered together constitute a property called impedance. From this, it should be obvious that the configuration of the conductors in a system of parallel conductors such as we see spanning from pylon to pylon is a very important consideration. Studying these configurations is called ‘line geometry’. Changing relative positions of conductors and distances between them can mean savings or losses of dollars. Power generated at the power station, therefore, undergoes line losses. And so we may speak of apparent power, that which was generated, and real power, that which is delivered, with impedance making the difference. To study line geometry, you should know complex numbers, matrices, calculus and differential equations. But it makes a fascinating pastime. I worked on power stations for years, but only on structural steel design. My knowledge of line geometry, as rudimentary as it is, is a labor of love. ------------ About the author Thomas Keyes: I have written two books: A SOJOURN IN ASIA (non-fiction) and A TALE OF UNG (fiction), neither published so far. I have studied languages for years and traveled extensively on five continents. Email: udikeyes@yahoo.com Tell a friend about this site! ------------ All articles are EXCLUSIVE to Useless-Knowledge.com and are not allowed to be posted on other websites. ARTICLE THIEVES WILL BE PROSECUTED! |
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