Electric Power Generation, Transmission, and Distribution

whereEcis the corona onset field,ais the radius of the conductor, andmis an experimental factor to
take account of the surface conditions. Typical values ofmare 0.8–0.9 for a dry-aged conductor, 0.5–0.7
for a conductor under foul-weather conditions, anddis the relative air density factor.
The above corona-onset condition emphasizes the great sensitivity of corona activities to the con-
ductor surface condition and, hence, to changes in weather conditions. In effect, although the line
voltage and the nominal conductor surface gradient remain constant, the surface condition factor varies
continuously due to the exposure of line conductors to atmospheric conditions. The changes are
particularly pronounced during foul weather as a result of the numerous discharge sites associated
with water drops, snow, and ice particles deposited on the conductor surface.
Adequate corona performance of HV lines is generally achieved by a proper control of the field
intensity at the surface of the conductor. It can be well illustrated by the simple case of a single-phase,
single-conductor line for which the field intensity at the conductor surface is

Lateral Distance from Center Line (m)

Line #

1 65' 41' 745 kV

735 kV

735 kV

525 kV

525 kV

45'

34'

34'

50'

65'

40'

65'

40'

4  1.165"

4  1.2"

4  1.602"

4  1.382"

4  1.302"

2

3

4

5

Conductor

Cond.

Height

Phase

Spacing U

Rain

(BPA)

Rain

(BPA)

Morning

Fog (AEP)

Calculated Profile

Measured Profile

(Apple Grove)

5

4

3

2

1

(4)

Light Rain

(Hydro-Québec)

Rain

(Hydro-Québec)

AN Level in dBA above 2



10

−^5

N/m

2

0

50

52

54

56

58

60

62

10 20 30

FIGURE 15.11 Comparison of calculated and measured AN performances of HV lines. (From Trinh, N.G.,IEEE
Electr. Insul. Mag., 11, 5, 1995b; Trinh, N.G. and Maruvada, P.S.,IEEE Trans., PAS-96, 312, 1977.)