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The corona effect can be minimized by increasing:


1. The length of conduct
2. Spacing between conductors
3. Diameter of conductors
4. Both spacing between conductors and diameter of the conductors

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Correct Answer - Option 4 : Both spacing between conductors and diameter of the conductors

Corona loss:

  • The power dissipated in the system due to corona discharges is called corona loss.
  • Accurate estimation of corona loss is difficult because of its variable nature.
  • It has been found that the corona loss under fair weather conditions is less than under foul weather conditions.

 

Methods by which the corona effect can be reduced:

It has been observed that intense corona effects are seen at a working voltage of 33 kV or above. So, design should be made to avoid corona on the sub-stations or bus bars rated for 33 kV and higher voltages otherwise highly ionized air may cause flash-over in the insulators or between the phases, causing considerable damage to the equipment. 
(i) 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.

 

(ii) 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.

 

∴ The corona effect can be minimized by increasing both spacing between conductors and diameter of the conductors


The corona loss under appropriate weather conditions is given below by the Peek’s formula,

\({P_C} = \frac{{244}}{δ }\left( {f + 25} \right){\left( {{E_n} - {E_0}} \right)^2}\sqrt {\frac{r}{D}} \times {10^{ - 5}}\frac{{kW}}{{km}}/\rm phase\)

Where Pc - corona power loss

f - Frequency of supply in Hz

δ - Air density factor

En - RMS phase voltage in kV

Eo - Disruptive critical voltage per phase in kV

\(E_0=m_0g_0 δ r\;log_e{(\frac{d}{r})}\;kV/phase\)

r - Radius of the conductor in meters

D - Spacing between conductors in meters

1. Only by seeing the power loss formula, we can say that only by increasing the diameter power-loss can be reduced.

2. But when we see the formula of disruptive critical voltage, by increasing the radius of the conductor E0 increases, which decreases the (En - E0), therefore result in decrease of power loss.

∴ We can say that the corona effect can be minimized by increasing both spacing between conductors and diameter of the conductors.

Corona loss is independent of the current in the conductor.

Important parameter:

1. Critical disruptive voltage:

  • It is the minimum amount of phase-neutral voltage at which corona occurs.
  •  If the two conductors of radius r cm and spaced d cm apart. If V is the phase-neutral potential,
    then potential gradient at the conductor surface is given by:

\(g=\frac{V}{r\;log_e\frac{d}{r}}\;volts/cm\)

  • In order that corona is formed, the value of g must be made equal to the breakdown strength of air.
  • The breakdown strength of air at 76 cm pressure and temperature of 25ºC is 30 kV/cm (max) or 21·2 kV/cm (r.m.s.) and is denoted by go.

\(V_c=m_0g_0 δ r\;log_e{(\frac{d}{r})}\;kV/phase\)

\(m_0=\frac{3.92b}{273+t}\)

Where δ = Air density factor

b = Barometric pressure in cm

t = Temperature in degree Celsius

m0 = 1 for polished conductors
     = 0·98 to 0·92 for dirty conductors
     = 0·87 to 0·8 for stranded conductors

2. Visual critical voltage:

  • It is the minimum amount of phase-neutral voltage at which corona glow appears all along the line conductors.
  • It has been seen that in case of parallel conductors, the corona glow does not begin at the disruptive voltage Vc but at a higher voltage Vv, called visual critical voltage.

\(V_v=m_vg_0 δ r{(1+\frac{0.3}{\sqrt{\delta r}})}\;log_e{(\frac{d}{r})}\;kV/phase\)

Advantages of corona effect:
(i) Due to corona formation, the air surrounding the conductor becomes conducting and hence the virtual diameter of the conductor is increased. The increased diameter reduces the electrostatic stresses between the conductors.
(ii) Corona reduces the effects of transients produced by surges.


Disadvantages of corona effect:
(i) Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line.
(ii) Ozone is produced by corona and may cause corrosion of the conductor due to chemical action.
(iii) 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. 

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