Power Loss Due to Corona and Mitigation Techniques

Demonstrative Video

Power loss due to Corona

  • Formation of corona is always accompanied by energy loss which is dissipated in the form of light, heat, sound and chemical action.

  • When disruptive voltage is exceeded, the power loss due to corona is given by Peek’s empirical formula in fair weather:

\[\boxed{P=242\displaystyle \cdot 2\left(\dfrac{f+25}{\delta}\right)\sqrt{\dfrac{r}{d}}\left(V-V_{c}\right)^{2}\times 10^{-5}}~\mathrm{kW/km/phase}\]
\[\begin{aligned} \mathrm{f} & = \text{supply frequency in Hz} \\ \mathrm{V} & = \text{phase-neutral voltage (r.m.s.)}\\ \mathrm{V_c} & = \text{disruptive voltage (r.m.s) per phase} \end{aligned}\]
where

  • At foul weather the value of critical disruption is taken as \(0.8V_c\)

  • The Peek formula is valid only if

    • Corona loss is predominant between \(25 < f < 120\) Hz

    • The ratio of phase voltage to the critical voltage \(>\) 0.8

    • Radius of the conductor \(>\) 0.25 cm.

Advantages and Disadvantages of Corona

  • 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.

  • 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.

    • It reduces the magnitude of high voltage steep fronted waves due to lighting or switching by partially dissipating as a corona loss. In this way it acts as a safety valve or control unit to some extent.

    • Corona reduces the effects of transients produced by surges.

  • Disadvantages:

    • Due to charge accumulation on the surface during corona discharge, the diameter of the conductor increases, which increases the effective capacitance and flow of charging current.

    • 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 neighbouring communication lines.

Why to Reduce Corona Effect?

  • Intense corona effects are observed at a working voltage of 33 \(\mathrm{k}\mathrm{V}\) or above.

  • Careful design should be made to avoid corona on the sub-stations or bus-bars rated for 33 \(\mathrm{k}\mathrm{V}\) and higher voltages.

  • Otherwise highly ionised air may cause flash-over in the insulators or between the phases, causing considerable damage to the equipment.

Methods of Reducing Corona Effect

  • 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 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.

  • Using Bundle Conductors

    • Bundled conductors increase the effective diameter of the conductor, hence reducing the corona effect

  • Using Corona Ring

    • The electric field is stronger at sharp conductor curvatures.

    • Due to which corona discharge first occurs first at those sharp points, edges, and corners.

    • So, Corona rings are used at the terminals of very high voltage equipment to reduce the corona effect by ‘rounding out’ conductors (i.e. making them less sharp).

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