Friday, January 25, 2013

Types of Transmission

       In general two types of systems are used for the transmission.
1. Overhead system            2. Underground system

1.1 Overhead System
        In this system, the transmission of electrical power is by using overhead transmission lines over long distances. In such system, the appropriate spacing is provided between the conductors, at the supports as well as at the intermediate points. This spacing provides insulation which avoids an electric discharge to occur between the conductors. The transmission by overhead system is much cheaper than the underground system. The overhead transmission lines are subjected to the faults occurring due to lightening, short circuits, breakage of line etc. but overhead lines can be easily repaired compared to underground system. It is also true that though such faults are rare, if occurred it is very difficult to find exact point of fault as transmission lines are very long. In the overhead system, the insulation must  be provided between the conductor and supporting structure. Hence the maximum stress exists between conductor and earth.

1.2 Underground System
        The cables are generally preferred in underground system. All the conductors must be insulated from each other in the underground system. As voltage level is high, insulation required is more. Hence due to insulation difficulties, the voltage level used in underground system is below 66 KV while the voltage level used in overhead transmission lines can be as high as 400 KV. The maintenance cost of the underground system is less compared to overhead system. In crowded areas, overhead system using bare conductors is not practicable where underground system using cables is preferred. The line surges are suppressed by using the cables hence cable must be used for the last part of the connection which can save transformers and generators from the damage due to line surges.
       In the underground system, the maximum stress exists on the insulation between the conductors.

7 التعليقات:

  1. In over head transmission lines, Paper is aimed at analyzing the lightning performance improvement of a shielded transmission line that can be achieved after installing metal oxide surge arresters as a function of the number of phases at which they are installed and at estimating the energy absorption capability of the selected arresters.

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  2. In fact, the rate can be reduced to zero by installing arresters at all phases and all towers of the over head transmission lines, but even if arresters are not installed at all phases, a significant improvement of the lightning performance can be achieved.

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  3. Foul weather conditions can lead to corona discharges on high voltage over head transmission lines which are perceivable as an audible broadband crackling and hissing noise. In such situations this noise is often accompanied by a humming noise at twice the mains frequency (2f).

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  4. over head transmission lines products should be designed and manufactured under strict quality control measures and should have international standards.

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  5. However, arresters have to be selected to withstand the energy stresses caused by lightning strokes; otherwise, failures could be caused by an excess of energy absorption in over head transmission.

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  6. In over head transmission lines Design, there are many factors that go into choosing the best conductor for your needs.

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  7. High quality polymers are used in designing and production of over head transmission lines spares to withstand extreme weather and temperature.

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