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Factors and condition effecting Corona

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    Corona

     

    A corona is a partial discharge and takes place at the surface of a transmission line conductor when the electrical stress, that is, the electric field intensity (or surface potential gradient), of a conductor exceeds the breakdown strength of the surrounding air. In such a no uniform field, various visual manifestations of locally confined ionization and excitation processes can be viewed. These local breakdowns (i.e., corona or partial discharges) can be either of a transient (non-self-sustaining) or steady-state (self-sustaining) nature. These manifestations are called coronas due to the similarity between them and the glow or corona surrounding the sun (which can only be observed during a total eclipse of the sun). In nature the corona phenomenon can also observed between and within electrically charged clouds. According to a theory of clouds electrification such a corona is not only the effect but also the cause of appearance of charged clouds and thus of lighting and thunder storms.[4]

     

     

     

     

     Factors and condition effecting Corona

     

    The phenomenon of corona is affected by the physical state of the atmosphere as well as by the conditions of the line. The following are the factors upon which corona depends:

     

    (i) Atmosphere:

     

    As corona is formed due to ionization of air surrounding the conductors, there-fore, it is affected by the physical state of atmosphere. In the stormy weather, the number of ions is more than normal and as such corona occurs at much less voltage as compared with fair weather.

     

    (ii) Conductor size:

     

    The corona effect depends upon the shape and conditions of the conductors. The rough and irregular surface will give rise to more coronas because unevenness of the surface decreases the value of breakdown voltage. Thus a stranded conductor has irregular surface and hence gives rise to more corona that a solid conductor.

     

    (iii) Spacing between conductors:

     

    If the spacing between the conductors is made very large as Compared to their diameters, there may not be any corona effect. It is because larger distance between conductors reduces the electro-static stresses at the conductor surface, thus avoiding corona formation.

     

    (iv) Line voltage:

     

    The line voltage greatly affects corona. If it is low, there is no change in the condition of air surrounding the conductors and hence no corona is formed.

     

    Corona effects on communication lines.

     

    Corona has many advantages and disadvantages. In the correct design of a high voltage overhead line, a balance should be struck between the advantages and disadvantages.

     

    Below are the Advantages and disadvantages of Corona.

     

    Advantages:

     

    Due to corona formation, the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased. The increased diameter reduces the electro-static stresses between the conductors.

     

    Corona reduces the effects of transients produced by surges.

     

    Disadvantages:

     

    Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line.

    Ozone is produced by corona and may cause corrosion of the conductor due to chemical action.

    The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal Voltage drop occurs in the line. This may cause inductive interference with neighboring Communication lines.

     

    Methods of reducing corona effect

     

    It has been seen that intense corona effects are observed at a working voltage of 33 kV or above.

    Therefore, careful design should be made to avoid corona on the sub-stations or bus-bars rated for 33kV and higher voltages otherwise highly ionised air may cause flash-over in the insulators or between the phases, causing considerable damage to the equipment. The corona effects can be reduced by the following methods:

     

    • By increasing conductor size. By increasing conductor size, the voltage at which corona occurs is raised and hence corona effects are considerably reduced. This is one of the reasons that ACSR conductors which have a larger cross-sectional area are used in transmission lines.

     

    • By increasing conductor spacing. By increasing the spacing between conductors, the voltage at which corona occurs is raised and hence corona effects can be eliminated. However, spacing cannot be increased too much otherwise the cost of supporting structure (e.g., bigger cross arms and supports) may increase to a considerable extent.

     

     

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