The electric discharge that occurs in a transmission line is called corona and this effect ionizes the air around the conductor.Voltage of the line, shape,radius(diameter), dust and mist are the factors that determine corona.It generally causes loss of power.
During Corona formation, there comes a sound from the transmission line. A hissing sound is generated by small electrical discharges, often caused by snow or mist or in times of rain.
During corona,there will be pulses of voltage and current on the transmission line, there can also be radio frequency noise. This interferes with radio and television reception. This interference is not significant unless the voltage operating is 230Kv or above.
High-voltage transmission lines can produce gaseous effluents during rain,mist and other wet conditions. A corona on an electrical conductor can produce small amounts of Ozone(o3). And again,just like radio interference the amount of ozone created is small and for 230 kV and lower voltage lines.
The main effect of Corona lies on insulation failure i.e insulation breakdown.This
occurs at breakdown voltage. Corona starts at critical disruptive voltage given by
e=m*g*d*r*ln(S/r)
.
During Corona formation, there comes a sound from the transmission line. A hissing sound is generated by small electrical discharges, often caused by snow or mist or in times of rain.
During corona,there will be pulses of voltage and current on the transmission line, there can also be radio frequency noise. This interferes with radio and television reception. This interference is not significant unless the voltage operating is 230Kv or above.
High-voltage transmission lines can produce gaseous effluents during rain,mist and other wet conditions. A corona on an electrical conductor can produce small amounts of Ozone(o3). And again,just like radio interference the amount of ozone created is small and for 230 kV and lower voltage lines.
The main effect of Corona lies on insulation failure i.e insulation breakdown.This
occurs at breakdown voltage. Corona starts at critical disruptive voltage given by
e=m*g*d*r*ln(S/r)
e is the "visual critical corona voltage" required to initiate a visible corona discharge between the wires.
m is an irregularity factor to account for the condition of the wires.
Polished, smooth | m=1 |
roughened, dirty or weathered wires | m=0.98 to 0.93 |
underground cables | m=0.87 to 0.83 |
r is the radius of the wires
S is the distance between the wires
d is the air density factor. It is calculated by the equation given by
d=3.92b/(273+t).
b-pressure in cm of Hg.
t-temperature in degrees celsius
The minimum value of voltage at which corona is visible is 'visual critical voltage' which is given by same formula as above with a small change i.e; replacing 'g' by
g=g0*d*(1+0.301/sq.rt(d.r))
g0 is the disruptive critical potential gradient,app. 30Kv/cm for air.
d=3.92b/(273+t).
b-pressure in cm of Hg.
t-temperature in degrees celsius
The minimum value of voltage at which corona is visible is 'visual critical voltage' which is given by same formula as above with a small change i.e; replacing 'g' by
g=g0*d*(1+0.301/sq.rt(d.r))
g0 is the disruptive critical potential gradient,app. 30Kv/cm for air.
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