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8/3/2019 Power System Engineering Lecture 16
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Review of Last Class
Electrical Design Insulator Characteristics: Homogeneous, low leakage, high
dielectric and mechanical strength and long life.
Ratings: Working voltage, Flashover voltage and Puncture
Voltage
Materials: Porcelain, Glass, Polymeric insulation materials
Type of insulators: Pin, Suspension, Strain
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Pin Type Insulator
http://en.wikipedia.org/wiki/Insulator_(electrical)2
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Suspension Type Insulators
http://www.electrotechnik.net/ http://en.wikipedia.org/wiki/Insulator_(electrical)3
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Strain Type Insulator
http://lifeatllu.blogspot.com/2009_03_01_archive.html4
http://lifeatllu.blogspot.com/8/3/2019 Power System Engineering Lecture 16
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Shackle, Post and Polymeric insulators
http://www.electrotechnik.net/
Shackle insulators or
spool insulators
Post type insulators Polymeric insulators
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http://4.bp.blogspot.com/-xoQVmPkrCrk/TV4eTUZg52I/AAAAAAAAEeA/VKlkK4RWPvU/s1600/shackle+Insulator.jpg8/3/2019 Power System Engineering Lecture 16
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Potential Distribution over String
Capacitance of disc:Capacitance between metal
work of the insulator units;
some times called as mutual
capacitance. Capacitance to ground:
capacitance between metal
work of insulator to tower.
http://www.epemag.net/electricity-generation-pylons.html
C
mCm
groundtoeCapacitanc
insulatorpereCapacitanc
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Potential Distribution over the String
IfVis voltage across the conductor and
ground. We have:
Also
m
VV1
112
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Potential Distribution over the String Similarly,
213
131
mmVV
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Potential Distribution over the String Similarly,
3214
1561
mmmVV
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String Efficiency
Let m = 5
m
VV1
112
213
131 mm
VV
3214
1561
mmmVV
12 2.1 VV
12 64.1 VV
12 41.2 VV
%8.6310041.24
)41.264.12.11(
100
1
1
V
V
linetoadjacentunitacrossVoltagen
StringAcrossVoltageEf fciencyString
10
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Methods of Equalizing the Potential
Methods to improve string efficiency
Selection ofm
Grading of units Static shielding or guard rings
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Selection of m
If the value ofm is increased, which canbe achieved by increasing the cross-arm
length.
Increased cross-arm length decreases the
capacitance between earth and metallicconnections.
However increasing cross-arm length is
not economical after certain distance.
Theoretically, one can achieve equalvoltage distribution when m is infinity.
It is found that value ofm greater than
10 is not economical.
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Grading of Units
Voltage across capacitor isinversely proportional to the
capacitance for given current.
By correct grading of
capacitances complete equalityvoltage can be achieved.
We have,
12
12
112
CCC
VCVCVC
III C
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Grading of Units
CCC
CCC
VCCVCVC
CCCBut
VCVCVCIII C
21
3
2
2
13
13
13
12
23
223
Generalized case:
CnCCn 13211
onsoand,11,8,6 432 CCCCCC
Therefore, ifC1=5C, then
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Grading of Units
Thus if capacitance of one unit is
fixed other capacitances can be
easily determined.
This requires units of differentcapacities, which is uneconomical
and impractical.
Therefore this method is usually
not employed except for very high
voltage lines.
In that case, string is graded in
groups, may be two/three.
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Static Shielding
334
223
112
Cz
Cy
Cx
IIII
IIII
IIII
1
)1(
1
n
CC
VCVnCx
II
x
Cx
Voltages can be equal if
Also,
22
2)2(
2
n
CC
VCVnCy
II
y
Cy
Similarly, and
3
3
n
CCZ
pn
pCCp
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Static Shielding
In practice, it is very difficult toachieve the condition of equal
voltages.
However the partial advantage
can be gained by this method(guard ring) and used normally.
Further, when the horn gap is
also used, it also protect the
insulator from the flashover.
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Numerical Method (FEM, BEM, FDM, etc.)
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