power system engineering lecture 16

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/


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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.jpg


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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

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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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power system engineering lecture 16

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