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