transmission line towers

i have a presentation onthis topic the next wek and i really need to now this particular thing.... i need to know how we select a transmission towers? is it by the
voltage of the cabels or is it the place where it will be placed in . or what ? i think it is selected by the amount of voltage in the cable. but at this case if a (500kv) near a puplic area (area with humans live in) it will be very dangerous to put it there . or does the weather conditions affect this procces . and also i need to know if i choosed a special type of a tower what kind of cables i will choose with it and what kind of insulators . i think first i must know the amount of voltage i will transfer in this cables . then: know what cable i will choose (because the cable that will carry 500kv is not like the one that will carry 250kv). third: by knowing the cable i must choose the insulators. (but at this point i need to know first what kind of tower im working on becuse the susbention insulator i will choose will be affected by the dimension of the tower (its arm and its height )). last: i must choose the type of the tower. please can some one tell me is that right or wrong.... please its very important. note: im still styding .......
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Cost.
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wael hassan wrote:

Its all about economics. You start with the desired transmission voltage and line impedance. These determine the required conductor clearance to ground and other objects. The impedance (and voltage) are factors that affect the conductor spacing.
Then you do a trade study between shorter, cheaper towers with shorter spans or taller, more expensive towers with longer spans. You also need to consider conductor bundling (single vs 2, 3, or 4 conductors per phase) for its affect on current carrying, corona, and line impedance. EM fields at distances from the line (where such fields may be subject to regulations) will also be affected by conductor spacing. Ice and wind loading will affect conductor sag, lateral swing and insulator/tower loading. Longer spans will have more lateral conductor swing, requiring a wider transmission line right-of-way, as will the framing design.
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Paul Hovnanian snipped-for-privacy@hovnanian.com
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You can get a very rough idea of the transmission voltage required by assuming 1000V for every mile of transmission, so 500kV would be the voltage used on transmission lines of many hundreds of miles, or rather, carrying power many hundreds of miles (which may be built up of many shorter segments between switching stations).
In practice, each country has sets of preferred transmission voltages. In the UK, these are 11kV, 33kV, 132kV, 275kV, 400kV, so having worked out what the ideal transmission voltage would be, it is then rounded to the most appropriate one of these.
Once you have the voltage specified, you then have various regulations which come into play which are going to vary considerably from one country to another. In the UK, there are requirements on ground, road, and building clearences depending on the transmission voltage. This will govern the height of the towers, although this also depends on the number of 3-phase circuits carried. (In the UK, almost all of the high power transmission lines carry two 3-phase circuits, which is 7 conductors, stacked 4 high as 3 pairs plus a ground wire.) Other factors which increasingly come into play are how the towers might detract from the appearance of an area, and that can force the use of shorter towers with different cable arrangements in some cases. *
Then there are different types of towers. Some are designed to support conductors in a straight line. Others are designed to handle a change in direction of conductors whilst maintaining the correct conductor spacing on both directions. In the UK, all the national grid voltage towers (132kV upwards) are designed to cope with complete loss of conductor tension on one side (i.e. all conductors broken) without toppling. Think for a moment what would happen if this was not the case; if one tower suffered such a loss of tension, it would fall over towards the side which still had full tension. The next tower would see a loss of tension, and again topple over. The whole transmission line would collapse like a row of dominos. In some other countries, towers which can withstand such a failure are used only periodcially, such as every 5th or 10th tower, so if there is breakage of the conductors, you might lose up to 5 or 10 tower's worth of lines.
With regards to cable, you obviously need thicker cables for higher currents. However, you may need thicker cables than the current alone would require. Long spans may require thicker cables for strength, both due to their weight, and due to additional forces such as wind resistance and icing up. Higher voltage cables need to be much thicker to reduce the electric field strength and the resulting corona discharge from the cable surface. Actually, this requires the cable to be so thick, it's impossible to do just by making a cable thicker. What's done instead is to create a bundle of connected cables which are spaced apart, and for the purposes of electric field strength, this looks more like a single cable which is as thick as the outer spacing of the bundle. In the UK, we use a minimum bundle of two for 275kV and a minimum bundle of 4 for 400kV, although in both cases larger bundles can be used if the current carrying capacity demands it.
Insulator string lengths depend on the voltage carried, and the maintenance free period desired. The strings on 132kV and up have more elements than are required, to allow for some of the insulators failing without the whole string failing. Periodic inspection at night using infrared cameras shows which elements of a string have failed, and allow for planing maintenance downtime when there are sufficient failures accumulating.
* Interesting reflection...
The planning process for installing a new transmission line in the UK now, because of the public inquiries, environmental impact assessments, enevitable protest groups, etc, now means that it takes longer to install one new transmission line than it did to install the whole of the UK National Grid and supergrid in the 1950's, or the supergrid extentions in the 1960's.
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Andrew Gabriel
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