power system engineering lecture 19

8/3/2019 Power System Engineering Lecture 19

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Review of Last Class

Mechanism of corona formation Negative DC Voltage

Positive DC Voltage

AC Voltage

Critical disruptive voltage or corona inception voltage

http://www.ei12.elecrama.com/ei12infinite/index.aspx


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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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Fair (or standard) whether

conditions and solid

conductor with smooth

surface

gmax = g0

g0 = 30 kV/Cm (Peak)g

0= 21.1 kV/Cm (rms)

Other (or foul) whether conditions

than standard and solid conductor

with smooth surface

gmax = g0

r

DrgVc ln0

t

p

273392.0

r

D

rgVc lnmax

r

DrgVc ln0

Other whether conditions than

standard and conductor with

irregular surface (ACSR)

gmax = m0 g0

The value m0 is

0.8 to 1

r

DrgmVc ln00


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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 rDr

r

gmV

rDrgmV

vd

vvd


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r

D

rgVc lnmax


or corona inception voltage

gmax = m0 g0

The value m0 is 0.8 to

1

r

DrgmVc ln00

Visual critical disruptive

voltage or visual corona

inception voltage kV/cmin

301.010

rggv

r

DrgmV vvd ln


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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

102412

5 cpc VVd

rfP

Where VPphase to neutral operating voltage in kV andfis frequency.

For storm, wet 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.


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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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Atmospheric Factors

Temperature:

Pressure:

Dust, dirt:

Rain, snow, fog:

t

p

t

p

273392.0

760

25273

273

mkW/phase/k2510241 25 cpc VVd

rfP

r

DrgmVc ln00


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Electrical Factor

Frequency Voltage

t

p

t

p

273392.0

760

25273

273

mkW/phase/k2510241 25 cpc VVd

rfP

r

DrgmVc ln00


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Line configuration

mkW/phase/k

2510241

25

cpcVV

d

rfP

r

DrgmVc ln00

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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Merits and Demerits of Corona

Merits Act as safety valve during lightning and switching surges.

The waves gets dissipated as corona loss.

Other applications:

Van de Graaff generator, Electrostatic precipitator, Electro printing,Ionization counting, Electrostatic deposition

Demerits

Corona loss reduces efficiency.

Ionized air around the conductor works as conducting mediumincreases effective diameter of conductor.

This increases capacitance and decrease surge impedance

loading.

Interference with communication lines.

power system engineering lecture 19

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