Electrical Power Systems

364 Electrical Power Systems
The value of voltage gradient, G max at which complete disruption of air occurs is called
disruptive strength of air. The break down strength of air at 760 mm pressure and temperature
of 25ºC,
Gº max = 3 ´ 10 6 V/m (maximum) or
3´ 10
2
6
V/m (rms).
The value of Gº max depends upon the density of air. The dielectric strength of air is
proportional to its density over a wide a range, and thus directly proportional to the barometric
pressure and inversely proportional to the absolute temperature. Thus the air density factor at
a barometric pressure of p mm of mercury and temperature of tºC can be given as:
d =
14.9 CORONA POWER LOSS
0. 392 p
...(14.38)
b273 + tg
The power dissipated in the transmission line due to corona discharges is called corona loss.
Estimation of accurate corona loss is very difficult because of its extremely variable nature. It
has been found that corona loss under fair weather conditions is very small as compared to the
losses found under foul weather conditions. According to .W. Peek, the corona loss under
fairweather conditions is given by the formula,
P c = 244
d (f + 25)(V n – V 0) 2 r
D ´ 10–5 KW/km/phase ...(14.39)
Where
Pc = corona power loss
f = supply frequency (Hz)
d = air density factor
Vn = rms phase-voltage (line-to-neutral voltage) in KV
V0 = rms value of disruptive critical voltage per phase in KV.
r = radius of the conductor (meters)
D = Spacing (or equivalent spacing) between conductors (meters)
It is also to be noted that for a single phase line
V n = 1
and for a three phase line
2
V n = 1
3
´ line voltage
´ line voltage
Peek's formula for corona loss is valid when corona losses are predominant and the ratio V n /
V0 is greater than 1.8 when Vn
V0
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