04-10-2012, 01:28 PM
High Voltage Power Transmission and Distribution
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Advantages of High Voltage Power Transmission and Distribution
Generating stations are usually located at places far away from the power consuming centers. As for example, thermal power stations are located at places where coal and water is available in abundance. Similarly hydro electric power stations are also situated at places where water head is available for running the turbine.
On the other hand, power consuming center are usually cities and industrial towns that are located at a significant distance away from the power generating stations. So power transmission is necessary over a long distance from the generation to the power consuming and distribution centers.
Also, three phase transmission is advantageous because though it consists of three single phase systems, the numbers of conductors required are four and not six.
D.C High Voltage Transmission
Advantages of HVDC transmission over HVAC transmission:
1) For insulation in an AC transmission system, the peak value has to be taken into account, whereas the real power transmitted is dependent upon the rms value. In the case of DC transmission, peak and rms values are identical. So in DC transmission there is a saving in insulation cost. In other words, AC transmission requires more insulation material compared to the DC transmission for the same voltage level.
2) In both cases of transmission, for the same peak stress (kVpeak), there is more corona loss on the conductor surface including radio interference in case of AC transmission than in case of DC transmission.
3) When power is transmitted through cables, DC transmission is always preferred, since there is no continuous charging current involved to feed the cable capacitances in case of DC.
4) In DC there is no skin effect involved.
5) There is a limit of power transmission through AC. This is due to the fact that the angle between sending end and receiving end voltages (‘’in Fig. 3) can not be allowed to exceed a critical value because of he stability consideration, considering the role of line inductances and capacitances in AC. For DC transmission, there is no question of phase differences; the only voltage drop in transmission is the resistive drop.
Insulating Material
Overhead line insulations are produced from high quality wet process porcelain or toughened glass. Porcelain insulations are usually glazed in brown colour over all exposed surfaces, but sometimes cream coloured glazed insulations are also used. Porcelain has been used from the very beginning as an insulator material and is still being used.
Pre-stressed or toughened glass has also been used in constructing line insulators. The material is placed in a furnace that heats it above its annealing point of about 720°C. The glass is then rapidly cooled with forced air drafts while the inside of the material remains at the higher temperature for a short time. As the inner material gets some time to cool, the inside surface of the glass contracts more than its outer surface during this process. This induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass. Toughened glass insulators have their surface layers in a state of high compression due to which their resistance to mechanical and thermal stresses is greater.
Type of insulators
There are four type of insulations in construction with overhead transmission lines e.g. i) Pin insulators, ii) Disc insulators, iii) Suspension insulators and iv) Strain insulators. Other than these four types of insulators, Post insulators are often used in high voltage systems.
Pin insulators
Pin insulator is supported on forged steel pin or bolt, which is secured to the supporting structure. This structure is generally earthed. The straight line conductor is tied to the top groove by binding wires. Binding wires get their support through the side groove that is provided. Single piece type pin insulator are used for lower voltage, but for higher voltages two or more pieces are connected together to provide sufficient thickness of porcelain and adequate creepage path. Pin type insulator are used for system voltages less than or equal to 33kV.Beyond 33kV pin type insulations become uneconomical. Pin insulators are so designed that the surface becomes equipotential so as to have less leakage current. The main body or stem is made along the field lines for more mechanical strength.