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8/3/2019 Power System Engineering Lecture 18
1/22
Review of Last Class
What is Corona? Why it is important to know factors affecting corona
Partial discharge
Corona
Mechanism of corona formation
Negative DC Voltage
Positive DC Voltage
AC Voltage
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Corona
Therefore in non-uniform field, some region of dielectricexperiences higher field strength than the critical value,
while other region field stress is well below critical value.
Thus, self sustained discharge condition will be valid only
in the strong field region giving rise to partial dischargecalled corona.
This associated with a glow (bluish or violet tuffs,
streamers, and/or glow) and a hissing sound and when it
takes place in air ozone, oxides of nitrogen and nitric acid(in the presence of moisture) are formed.
Light is produced by recombination of nitrogen atom with
free electrons.
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Mechanism of Corona Formation
Conductors at Negative DC voltage
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Mechanism of Corona Formation
Conductors at Positive DC voltage
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Mechanism of Corona Formation
Conductors at AC voltage: In positive half cycle:
when voltage exceeds corona inception value, ionization starts
and intensity progressively increases till peak.
Electrons quickly reach conductor but before positive ions reach
other electrode polarity changes.
Some positive ions remain virtually cutoff from both electrodes.
In negative half cycle:
In negative half cycle corona again starts when voltage exceeds
corona inception value.
Electrons spreads out from the conductor and neutralize the
stranded ions.
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Mechanism of Corona Formation
Conductors at AC voltage: In this way, in every cycle some space charge need to be
neutralized and result will be loss in charges from the source.
The energy continuously lost in the corona space.
Recombination of opposite charges will release elergy in thesurrounding air, which is heated up.
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Corona (Laboratory Testing)
Gonen T., Electric Power Transmission System Engineering: Analysis and Design, John Wiley and Sons.7
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Corona (Laboratory Testing)
Gonen T., Electric Power Transmission System Engineering: Analysis and Design, John Wiley and Sons.8
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Corona (Testing)
Gonen T., Electric Power Transmission System Engineering: Analysis and Design, John Wiley and Sons.9
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Corona
http://wn.com/Inspecting_for_Corona
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Critical Disruptive Voltage
The minimum potential difference required between theconductor to start ionization is called critical disruptive voltage
or corona inception voltage
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Critical Disruptive Voltage
Potential Difference between the conductor
Therefore,
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Critical Disruptive Voltage
Above expression is for single phase line and V=V/2
or for three phase line V= 3/V
Now this can be used to single phase with V=V/2
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Critical Disruptive Voltage
Gradient increase ax decreases and will be maximum at
conductor surface i.e.x=r
Therefore,
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Critical Disruptive Voltage
When gmax reaches g0 (breakdown strength of air) air
breaks down.
Above g0 is for fair (standard) whether conditions, at any
other condition
t
p
t
p
273392.0
760
25273
273where
is the relative air density or air density correction factor15
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Critical Disruptive Voltage
The critical disruptive voltage or corona inception voltageis then given by
Here, the assumption is conductor is solid having smoothsurface, however in practical cases (like ACSR),
conductor will not be having smooth surface. To account
for surface irregularities a factor m0 is used. Thus,
m0 = surface irregularity factor or stranding factor
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Critical Disruptive Voltage
Surface irregularity factor depends on state of crosssection of conductor and state of its surface.
It also considers dust and dirt on the conductor surface.
The value ofm0 lies between 0.8 to 1.
m0 = 1 for smooth, polished, and round conductors m0 = 0.92 0.98 for rough surfaced conductors
m0 = 0.8 0.88 for stranded conductors
Now when dand rexpressed in cm.
(rms)kVln1.21
(Peak)kVln30
0
0
rDrmV
rDrmV
d
d
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Visual Critical Disruptive Voltage
At the critical disruptive voltage corona starts, but it willnot be visible. It requires further ionization by collision.
If the voltage is further increased at some point corona
becomes visible. This voltage is called as visual critical
disruptive voltage or visual corona inception voltage. The voltage gradient (gv) for visual corona is given by
[Peek]:
kV/cm301.0
10
r
ggv
Therefore,
kVln301.0
1
kVln
0 rDrm
r
gV
rDrmgV
vd
vvd
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Visual Critical Disruptive Voltage
Surface irregularity factor (mv) is different from m0. Local corona: For conductor with irregular surface, visual
corona occurs at different point than whole surface called
as local corona.
mv = 0.72 for local visual corona on stranded conductor mv = 0.82 for general (or decided) corona on stranded conductor
mv = 1 for smooth and polished conductor
Now when dand rexpressed in cm.
(rms)kVln301.0
11.21
(Peak)kVln301.0130
rDrm
r
V
rDrmr
V
vd
vd
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Corona Loss
The ionized charges near the conductor surface take energy from
the supply system and thus there is loss of some energy due to
corona.
Peeks Empirical relation for corona in fair whether condition
mkW/phase/k25
1024125
cpc VVd
rfP
Where VPphase to neutral operating voltage in kV andfis frequency.
For storm or foul whether condition voltage is 0.8 Vc
mkW/phase/k8.02510241 25 cpc VVd
rfP
This relation is correct results when 1) Corona loss is predominant
2) Frequency lies between 25 and 125 Hz 3)Ration ofVp/Vc > 1.8
4) radius of conductor is greater than 0.25 cm.20
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Corona Loss
When the ratio Vp/Vc< 1.8 Petersons formula givesgood results
Here Fis corona factor determined by test depends on
Vp/Vc.
mkW/Phase/k
/ln
1011066.12
24
F
rd
VfPc
Vp/Vc 1 1.4 1.6
F 0.05 0.3 1
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Factors Affecting Corona
Atmospheric factors
Electrical factor
Line configuration
Temperature Pressure
Dust and dirt Rain, snow, fog
Frequency Supply voltage
Conductor configuration Profile of conductor
Diameter of conductor
Surface condition Number of conductor per
phase
Heating of conductor by
load current
Conductor spacing
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