Electric power is transmitted or distributed either by suggests that of underground cables or by overhead lines. The underground cables square measure seldom used for power transmission because of 2 main reasons. Firstly, power is usually transmitted over long distances to load centers. Obviously, the installation prices for underground transmission are terribly significant. Secondly, power has got to be transmitted at high voltages for economic reasons. it’s terribly tough to supply correct insulation to the cables to resist such higher pressures. Therefore, as a rule, power transmission over long distances is disbursed by exploitation overhead lines.
An overhead line is subjected to ensure climatic conditions and different external interferences. This requires the utilization of correct mechanical factors of safety so as to make sure the continuity of operation within the line. In general, the strength of the road ought to be such therefore on give against the worst probable climatic conditions. during this chapter, we have a tendency to shall focus our attention on the varied aspects of the mechanical style of overhead lines.
Main Components of Overhead Lines:
While constructing AN overhead line, it ought to be ensured that the mechanical strength of the road is such therefore on offer against the foremost probable atmospheric condition. In general, most parts of AN overhead line are:
- Conductors that carry power from the causation finish station to the receiving finish station.
- Supports which can be poles or towers and keep the conductors at an acceptable level on top of the bottom.
- Insulators that are hooked up to support and insulate the conductors from the bottom.
- Cross arms which offer support to the insulators.
- Miscellaneous things like part plates, danger plates, lightning arrestors, anti-climbing wires, etc.
The continuity of operation within the overhead line depends upon the even-handed selection of the on top of parts. Therefore, it’s profitable to possess a close discussion on them.
The conductor material used for transmission and distribution of electrical power ought to have the subsequent properties :
- high electrical physical phenomenon.
- high strength so as to resist mechanical stresses.
- the low value so it is used for long distances.
- low relative density so weight per unit volume is tiny.
All on top of needs don’t seem to be found during a single material. Therefore, whereas choosing a conductor material for a specific case, a compromise is formed between the value and therefore the needed electrical and mechanical properties.
Commonly used conductor materials:
The most unremarkably used conductor materials for overhead lines are copper, aluminum, steel-cored metallic element, galvanized steel, and metal copper. the selection of a selected material can depend on the value, the desired electrical and mechanical properties and therefore the native conditions.
Copper has high current density i.e., the present carrying capability of copper per unit of X-sectional space is kind of giant. This results in 2 benefits. Firstly, smaller X-sectional space of the conductor is needed and second, the realm offered by the conductor to wind masses is reduced.
Moreover, this metal is kind of homogenous, sturdy and includes a high scrap worth. there’s hardly any doubt that copper is a perfect material for transmission and distribution of electrical power. However, thanks to its higher price and non-availability, it’s seldom used for these functions. Nowadays the trend is to use metallic element in situ of copper.
Aluminum is affordable and lightweight as compared to copper however it’s a lot of smaller physical phenomenon and strength. The relative comparison of the 2 materials is briefed below :
- The physical phenomenon of atomic number 13 is hr that of copper. The smaller physical phenomenon of atomic number 13 implies that for any specific transmission potency, the X-sectional space of conductor should be larger in atomic number 13 than in copper.
- For a similar resistance, the diameter of the atomic number 13 conductor is concerning 1·26 times the diameter of the copper conductor.
- The precise gravity of atomic number 13 (2·71 gm/cc) is less than that of copper (8·9 gm/cc).
- Therefore, associate atomic number 13 conductor has virtually simple fraction the burden of the equivalent copper conductor. For this reason, the supporting structures for atomic number 13 needn’t be created thus sturdy as that of copper conductor.
- Atomic number 13 conductor being light-weight, is prone to bigger swings and therefore larger cross-arms square measure needed.
- Because of lower strength and better constant of linear growth of atomic number 13, the sag is larger in atomic number 13 conductors.
Steel cored aluminum:
The steel cored metallic element conductors have the subsequent blessings :
- The reinforcement with steel will increase the strength however at an equivalent time keeps the composite conductor lightweight.
- Therefore, secured metallic element conductors can manufacture smaller sag and thence longer spans will be used.
- Thanks to smaller sag with steel cored metallic element conductors, towers of smaller heights will be used.
Steel contains a terribly high enduringness. Therefore, galvanized steel conductors are often used for terribly long spans or for brief line sections exposed to abnormally high stresses thanks to the climate. they need to be been found terribly appropriate in rural areas wherever cheapness is that the main thought. thanks to the poor physical phenomenon and high resistance of steel, such conductors don’t seem to be appropriate for sending giant power over an extended distance.
The conductor material currently being utilized inbound cases is copper alloyed with a metallic element. associate degree addition of 1 Chronicle or two metallic elements to copper will increase the enduringness by concerning five hundredths and also the physical phenomenon is merely reduced by 15 August 1945 below that of pure copper. Therefore, the metallic element copper conductor is often helpful for exceptionally long spans.
The supporting structures for overhead line conductors square measure numerous sorts of poles and towers known as line supports. In general, the road supports ought to have subsequent properties :
- High mechanical strength to face up to the burden of conductors and wind masses etc.
- light-weight in weight while not the loss of mechanical strength.
- price low-cost in cost and economical to keep up.
- Longer life.
- straightforward accessibility of conductors for maintenance.
The line supports used for transmission and distribution of electrical power square measure of varied varieties including wood poles, steel poles, R.C.C. poles, and lattice steel towers.
The wood poles tend to rot below the bottom level, inflicting foundation failure. so as to stop this, the portion of the pole below the bottom level is fertilized with preservative compounds like creosote oil. Double pole structures of the ‘A’ or ‘H’ kind area unit usually used to get higher thwartwise strength that might be economically provided by suggests that of single poles. the most objections to wood support area unit :
- the tendency to rot below the bottom level
- relatively smaller life (20-25 years)
- can’t be used for voltages above twenty kV
- less mechanical strength and
- need periodical scrutiny.
The steel poles square measure typically used as a substitute for picket poles. They possess bigger mechanical strength, longer life and allow longer spans to be used. Such poles square measure typically used for distribution functions within the cities. this sort of support has to be galvanized or painted so as to prolong its life. The steel poles square measure of 3 sorts viz.
- rail poles
- hollow poles and
- rolled steel joints.
The ferroconcrete poles became extremely popular as line supports in recent years. they need larger mechanical strength, longer life and allow longer spans than steel poles. Moreover, they provide a decent outlook, need very little maintenance and have sensible insulating properties. R.C.C. poles for one and double circuit. The holes within the poles facilitate the climb of poles and at a similar time scale back a load of line supports. the most issue with the employment of those poles is that the high price of transport thanks to their heavyweight. Therefore, such poles are typically factory-made at the positioning so as to avoid the significant price of transportation.
I follow wooden, steel and ferroconcrete poles are used for distribution functions at low voltages, say up to eleven kilovolts. However, for long-distance transmission at higher voltage, steel towers are invariably used. Steel towers have larger mechanical strength, longer life, will stand up to the most severe climate and allow the utilization of longer spans. the danger of interrupting service because of broken or cut insulation is significantly reduced attributable to longer spans.
Tower footings are sometimes grounded by driving rods into the planet. This minimizes the lightning troubles as every tower acts as a lightning rod.
The overhead line conductors ought to be supported on the poles or towers in such some way that currents from conductors don’t flow to earth through supports i.e., line conductors should be properly insulated from supports. this can be achieved by securing line conductors to supports with the assistance of insulators. [ Read More … ]