An overhead cable is an electrical power line suspended by towers or poles. Since most of the insulation is provided by air, overhead power lines are typically the lowest-cost technique of transmission for giant quantities of electrical power. Towers for support of the lines are made from wood, steel, concrete, aluminum, and sometimes bolstered plastics. The bar wire conductors on the road are typically made from atomic number 13, tho’ some copper wires are utilized in medium-voltage distribution and low-tension connections to client premises.
Towers for support of the lines square measure made from wood, steel or atomic number 13, concrete, and infrequently bolstered plastics. The blank wire conductors on the road square measure typically made from atomic number 13, although some copper wires square measure utilized in medium-voltage distribution and low-tension connections to client premises. a serious goal of overhead line style is to keep up adequate clearance between energized conductors and also the ground therefore on stop dangerous contact with the road and to supply reliable support for the conductors, resilience to storms, ice loads, earthquakes and different potential harm causes.
Since overhead transmission wires depend upon the air for insulation, the planning of those lines needs minimum clearances to be ascertained to take care of safety. Adverse atmospheric condition, like wind and low temperatures, will result in power outages. Wind speeds as low as twenty-three knots (43 km/h) will allow conductors to encroach operative clearances, leading to discharge and loss of offer. the oscillating motion of the physical line will be termed gallop or flutter reckoning on the frequency and amplitude of oscillation.
Overhead conductors area unit an artifact equipped by many corporations worldwide. Improved conductor material and shapes area unit are frequently wont to enable multiplied capability and modernize transmission circuits. Conductor sizes vary from twelve mm2 (#6 Yankee wire gauge) to 750 mm2, with variable resistance and current-carrying capability. For traditional AC lines, thicker wires would result in a comparatively little increase in capability because of the electrical phenomenon. owing to this current limitation, multiple parallel cables (called bundle conductors) area unit used once the higher capacity is required.
High-voltage overhead conductors don’t seem to be coated by insulation. The conductor material is sort of invariably Associate in Nursing metal alloy, created into many strands and probably bolstered with steel strands. Copper was generally used for overhead transmission, however, metal is lighter, yields solely marginally reduced performance and price a lot of less.
Electric power may also be transmitted by underground power cables rather than overhead power lines. Underground cables take up less right-of-way than overhead lines, have lower visibility, and square measure less plagued by inclementness. However, prices of insulated cable and excavation square measure a lot of beyond overhead construction. Faults in buried transmission lines take longer to find and repair.
Underground lines square measure strictly restricted by their thermal capability, which allows less overload or re-rating than overhead lines. Long underground AC cables have vital capacitance, which can cut back their ability to produce helpful power to masses on the far side fifty miles (80 kilometers). DC cables don’t seem to be restricted long by their capacitance, however, they are doing need HVDC device stations at each end of the road to convert from DC to AC before being interconnected with the transmission network.
In some metropolitan areas, underground transmission cables square measure fencelike by metal pipe associate degreed insulated with material fluid (usually an oil) that’s either static or circulated via pumps. If an electrical fault damages the pipe and produces a material leak into the encompassing soil, atomic number 7 trucks square measure mobilized to freeze parts of the pipe to modify the debilitating and repair of the broken pipe location. this sort of underground transmission cable will prolong the repair amount and increase repair prices. The temperature of the pipe and soil square measure sometimes monitored perpetually throughout the repair amount.
- Low voltage (LV) – less than 1000 volts,
- Medium voltage (MV; distribution) – between 1000 volts (1 kV) and 69 kV,
- High voltage (HV; sub transmission less than 100 kV,
- Extra high voltage (EHV; transmission) – from 345 kV, up to about 800 kV,
- Ultra high voltage (UHV) – higher than 800 kV.
The most common conductor in use for transmission nowadays is metal conductor steel bolstered (ACSR). additionally seeing abundant use is Associate in Nursing all-aluminum-alloy conductor (AAAC). metal is employed as a result of it’s concerning the burden and lower value of a comparable resistance copper cable. It does, however, need a bigger diameter than copper attributable to lower specific conductivity). Copper was a lot of standard within the past and continues to be in use, particularly at lower voltages and for grounding.
Since a conductor may be a versatile object with uniform weight per unit length, the form of a conductor hanging between 2 towers approximates that of a curved shape. The sag of the conductor (vertical distance between the best and lowest purpose of the curve) varies reckoning on the temperature and extra load like ice cowl. Minimum overhead clearance should be maintained for safety.
While larger conductors lose less energy attributable to their lower resistance, they value over smaller conductors. Associate in Nursing optimization rule known as Kelvin’s Law states that the optimum size of conductor for a line is found once the value|the value|the price of the energy wasted in a very smaller conductor is up to the annual interest paid thereon extra cost of the road construction for a bigger conductor.
In the role of steel core strands that are typically accustomed to increase overall conductor strength, the ACCC conductor uses a carbon and fiber core that gives a constant of thermal growth concerning 1/10 of that of steel. whereas the composite core is nonconducting, it’s considerably lighter and stronger than steel, that permits the incorporation of twenty-eighth a lot of metal (using compact trapezoidal-shaped strands) with none diameter or weight penalty.
Insulators area unit sometimes fabricated from wet-process ceramic ware or toughened glass, with the increasing use of glass-reinforced compound insulators. However, with rising voltage levels, compound insulators (silicone rubber based) area unit seeing increasing usage. China has already developed compound insulators having the very best system voltage of 1100 kilovolt and the Asian nation is presently developing a 1200 kilovolt (highest system voltage) line which is able to ab initio be charged with four hundred kilovolts to be upgraded to a 1200 kilovolt line.
At the top of the nineteenth century, the restricted electrical strength of telegraph-style pin insulators restricted the voltage to no over sixty-nine,000 volts. Up to concerning thirty-three kilovolts (69 kilovolts in North America), each sorts area unit usually used. At higher voltages, solely suspension-type insulators area unit common for overhead conductors.
Suspension insulators area unit fabricated from multiple units, with the number of unit nonconductor disks increasing at higher voltages. the amount of disks is chosen supported line voltage, lightning stand up to demand, altitude, and environmental factors like fog, pollution, or salt spray. In cases wherever these conditions area unit suboptimal, longer insulators should be used. Longer insulators with longer creepage distance for outflow current, area unit needed in these cases. Strain insulators should be sturdy enough automatically to support the complete weight of the span of the conductor, likewise as masses thanks to ice accumulation, and wind.
Porcelain insulators might have a semi-conductive glaze end, so a little current (a few milliamperes) passes through the nonconductor. This warms the surface slightly and reduces the result of fog and dirt accumulation. The conductive glaze additionally ensures an additional even distribution of voltage on the length of the chain of nonconductor units.
Electrical Design of Overhead Power Transmission Lines covers:
- AC power-flow analysis mistreatment repetitive strategies
- Symmetrical and unsymmetrical faults
- Control of voltage and power flow
- Stability in AC networks
- AC circuits and sequence circuits of power networks
- Matrix strategies in AC grid analysis
- Overhead conductor parameters
- Modeling of transmission lines
- High-voltage electrical energy (HVDC) transmission
- Corona and field of force effects of transmission lines
- Lightning performance of transmission lines
- Coordination of conductor insulation
- The ampacity of overhead line conductors
A double-circuit cable has 2 circuits. For three-phase systems, every tower supports and insulates six conductors. Single section AC-power lines as used for traction current have four conductors for 2 circuits. sometimes each circuit operates at identical voltage. In HVDC systems usually, 2 conductors are carried per line, however, in rare cases, only 1 pole of the system is carried on a collection of towers.
In some countries, like the Federal Republic of Germany, most power lines with voltages higher than a hundred kV are enforced as double, quadruple or in rare cases even hextuple cable as rights of the method are rare. generally, all conductors are put in with the erection of the pylons; typically some circuits are put in later. an obstacle of double circuit transmission lines is that maintenance is often tough, as either adds shut proximity of high voltage or switch-off of 2 circuits is needed. just in case of failure, each system is often affected.