power system engineering lecture 18

8/3/2019 Power System Engineering Lecture 18

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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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Conductors at Positive DC voltage

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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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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)



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Corona (Testing)



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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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Potential Difference between the conductor

Therefore,

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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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Gradient increase ax decreases and will be maximum at

conductor surface i.e.x=r

Therefore,

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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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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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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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power system engineering lecture 18

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