3. What Is Corona?
Corona is a luminous discharge
due to ionization of the air
surrounding an electrode,
caused by a voltage gradient
exceeding a certain critical
value.
5. What’s The Fuss?
Corona from conductors and hardware
may cause audible noise and radio noise
6. What’s The Fuss?
Corona from conductors and hardware
may cause audible noise and radio noise
Audible noise from conductors may
violate noise standards
7. What’s The Fuss?
Corona from conductors and hardware
may cause audible noise and radio noise
Audible noise from conductors may
violate noise standards
Radio noise from conductors may
interfere with communications or
navigation
8. What’s The Fuss?
Corona from conductors and hardware may
cause audible noise and radio noise
Audible noise from conductors may violate
noise standards
Radio noise from conductors may interfere
with communications or navigation
Corona loss may be significant when
compared with resistive loss of conductors
9. What’s The Fuss?
Corona from conductors and hardware may cause
audible noise and radio noise
Audible noise from conductors may violate noise
standards
Radio noise from conductors may interfere with
communications or navigation
Corona loss may be significant when compared with
resistive loss of conductors
Corona can cause possible damage to polymeric
insulators
11. Physical Parameters of Corona
Corona is caused by the ionization of the
media (air) surrounding the electrode
(conductor)
12. Physical Parameters of Corona
Corona is caused by the ionization of the
media (air) surrounding the electrode
(conductor)
Corona onset is a function of voltage
13. Physical Parameters of Corona
Corona is caused by the ionization of the
media (air) surrounding the electrode
(conductor)
Corona onset is a function of voltage
Corona onset is a function of relative air
density
14. Physical Parameters of Corona
Corona is caused by the ionization of the
media (air) surrounding the electrode
(conductor)
Corona onset is a function of voltage
Corona onset is a function of relative air
density
Corona onset is a function of relative
humidity
15. Corona and the Electric Field
Corona is NOT solely a function of the
Electric Field
16. Corona and the Electric Field
Corona is NOT solely a function of the
Electric Field
Corona is a function of the electric field on
the surface of the electrode (conductor)
17. Corona and the Electric Field
Corona is NOT solely a function of the
Electric Field
Corona is a function of the electric field on
the surface of the electrode (conductor)
Corona is also a function of the radius of
curvature of the electrode (conductor)
18. Corona and the Electric Field
Corona is NOT solely a function of the
Electric Field
Corona is a function of the electric field on
the surface of the electrode (conductor)
Corona is also a function of the radius of
curvature of the electrode (conductor)
Corona is also a function of the rate of decay
of the electric field away from the electrode
(conductor)
19. Corona and the Electric Field
For the preceding reasons, selecting the
conductor with the smallest electric field at
its surface is not correct.
One utility found out (the hard way) that
simply choosing large diameter conductors
did not work well because the electric field
decayed slowly away from the surface
21. Corona and the Relative Air Density
Corona has an inverse relationship with air
density
22. Corona and the Relative Air Density
Corona has an inverse relationship with air
density
Standard line designs that perform well at
sea level, may have significant corona
issues if used on lines that are installed over
mountainous areas
24. Corona and the Humidity
Corona has an inverse relationship with
humidity at power frequencies
25. Corona and the Humidity
Corona has an inverse relationship with
humidity at power frequencies
Fair weather corona is more prevalent in
low humidity environments
27. Corona Is Dependent Surface
Condition Of The Conductors
Corona is enhanced by irregularities on the
conductor surface
28. Corona Is Dependent Surface
Condition Of The Conductors
Corona is enhanced by irregularities on the
conductor surface
Irregularities include: dust, insects, burrs
and scratches and water drops present on
new conductors
29. Corona Is Dependent Surface
Condition Of The Conductors
Corona is enhanced by irregularities on the
conductor surface
Irregularities include: dust, insects, burrs
and scratches and water drops present on
new conductors
Corona will generally be greater on new
conductors and will decrease to a steady-
state value over a period of approximately
one year in-service
30. Corona Is Dependent Surface
Condition Of The Conductors
Corona is enhanced by irregularities on the
conductor surface
Irregularities include: dust, insects, burrs and
scratches and water drops present on new
conductors
Corona will generally be greater on new
conductors and will decrease to a steady-state
value over a period of approximately one year in-
service
Corona is significantly increased in foul weather.
32. Corona Is Dependent On Local
Electrode Geometry
Non-Critical Surfaces
Non-Critical Surface
Critical Surfaces
Number of Elements =210
Critical Section
34. Corona Is Dependent On Local
Electrode Geometry
r
S
Ground Plane V = 0
Sphere V = V0
35. Corona Is Dependent On Local
Electrode Geometry
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
0
500
1000
1500
2000
2500
3000
Actual Electric Field vs. Distance
Distance (cm)
Electric
Field
(V/cm)
1 cm
2 cm
3 cm
4 cm
5 cm
6 cm
7 cm
8 cm
9 cm
36. Why is it a special problem for
voltage upgrades?
If the voltage of a transmission line is increased
without changing the line design, the electric field
at the surface of the line conductors (and hardware)
will increase. This increase will cause additional
corona
The same can be said for compact transmission
lines.
37. Practical Consequences
1. Larger conductors better – to a point
2. Use conductor bundles to reduce corona
3. Corona phenomena much worse in
foul weather, high altitude
4. Compact lines more susceptible
5. New conductors can lead to poor
corona performance for a while
39. Cautions Regarding Radio Noise
Radio noise from corona should not be
confused with radio noise generated by
spark discharge
40. Cautions Regarding Radio Noise
Radio noise from corona should not be
confused with radio noise generated by
spark discharge
High frequency complaints are almost
always due to sparks
41. Cautions Regarding Radio Noise
Radio noise from corona should not be
confused with radio noise generated by
spark discharge
High frequency complaints are almost
always due to sparks
Causes of spark discharges can be located
and repaired
42. Cautions Regarding Radio Noise
Radio noise from corona should not be
confused with radio noise generated by
spark discharge
High frequency complaints are almost
always due to sparks
Causes of spark discharges can be located
and repaired
Corona is a DESIGN ISSUE
43. Cautions Regarding Radio Noise
Radio noise caused by corona cannot be
reduced by changing the line configuration
near where the noise problem is occuring.
The source of the problem can be several
miles away
44. Audible Noise
Can get “broadband noise” and “hum”
Load can affect audible noise
For measurement see:
ANSI/IEEE Standard 656-1992 – “IEEE Standard
For Measurement of Audible Noise from Overhead
Transmission Lines
45. Audible Noise
May be present as “120 Hz Hum” or
broadband noise
Can be affected by load
48. Audible Noise
May be present as “120 Hz Hum” or
broadband noise
Can be affected by load
49. Audible Noise in Context
50
55
60
65
Audible
Noise dBA
30 meters
from
centerline
Numerous
complaints
Moderate
complaints
No
complaints
Source: BPA study
Common design limit – 53 dBA
50. EPA Regulations
Effect Level Area
Outdoor
activity
Indoor
activity
Ldn < 55 dBA
Residential areas
and farms where
.. quiet is a basis
for use
Leq(24) < 55 dBA
Areas where
people spend
limited time
Ldn < 45 dBA
Leq(24) < 45 dBA
Residential
Other
Ldn (has a -10 dB adder nightime) Leq(24) (24 hour average)
51. x
y
Tradeoff: Magnetic Field vs. Audible Noise
Effect of Changing Conductor Phasing
A
B
C
A or C
B
C or A
White – ABC – ABC Super Bundle
Red – ABC – CBA Low Reactance
500 kV Double Circuit Line
52. 0
5
10
15
20
25
0 20 40 60 80 100 120
Distance from Center Line
Magnetic
Field
(mG)
ABC – ABC Super Bundle
ABC – CBA Low Reactance
Magnetic Field is Reduced
53. 52
54
56
58
60
62
64
0 20 40 60 80 100 120
Distance from Center Line (m)
Audible
Noise
(dBA)
ABC – CBA (Low Reactance)
ABC – ABC (Super Bundle)
Audible Noise is Increased
54. Corona Loss
What is it?
Whenever corona occurs on a transmission line, there is a
corresponding loss of energy called “corona loss.” This loss
is one of the reasons why the transmission of electric energy
between two points is not perfect.
When can it be a problem?
Generally, corona loss is significantly less than resistive loss.
However, at higher voltages, high altitudes and during foul
weather, corona loss can actually exceed resistive loss.
56. A Word On Corona Losses
Whenever corona occurs on a transmission
line, there is a corresponding loss of energy
called “corona loss.” This loss is one of the
reasons why the transmission of electric
energy between two points is not perfect
57. A Word On Corona Losses
Whenever corona occurs on a transmission line,
there is a corresponding loss of energy called
“corona loss.” This loss is one of the reasons why
the transmission of electric energy between two
points is not perfect
Generally, corona loss is significantly less than
resistive loss. However, at higher voltages, high
altitudes and during foul weather, corona loss can
actually exceed resistive loss
58. Example Losses (sea level)
Line
Voltage (kV)
Bundle
n x 2a
Load
MVA
I2R Loss
kW/km
Corona Loss (kW/km)
Average Maximum
362
550
800
2 x 3.16
3 x 3.3
4 x 3.3
400
900
2000
41
52
93
2
4
8
26
78
208
59. Corona On Hardware
Transmission line hardware is usually tested in the
laboratory prior to being selected to determine whether is
corona free. This test is often done on a single phase
system in order to conserve laboratory space.
It has been found that the traditional method of energizing
the system to 110% of the rated line to
ground voltage is not sufficient, especially for lines
of more compact design. Apparently, the hardware is
exposed to higher gradients in the field than those to which
the hardware was tested.
61. Corona On Hardware
Transmission line hardware is usually tested
in the laboratory prior to being selected to
determine whether is corona free. This test
is often done on a single phase system in
order to conserve laboratory space
62. Corona On Hardware
Transmission line hardware is usually tested in the
laboratory prior to being selected to determine
whether is corona free. This test is often done on
a single phase system in order to conserve
laboratory space
It has been found that the traditional method of
energizing the system to 110% of the rated line to
ground voltage is not sufficient, especially for
lines of more compact design. Apparently, the
hardware is exposed to higher gradients in the
field than those to which the hardware was tested
