power system engineering lecture 15

8/3/2019 Power System Engineering Lecture 15

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Review of Last Class

Mechanical Design Important factors affecting mechanical design

Sag tension calculations

Ice-wind loading

Stringing chart Sag template

Equivalent of ruling span

Vibrations in conductors

Line supports

Electrical Design

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Line Support

Wooden Poles

Cheapest, used for small spans (30-40m)

Tendency to rot at ground and life can not be predicted.

Reinforced concrete poles

Long life and low maintenance

High cost of transport because of weight

Tubular steel poles

Longer spans than wooden, longer life, light weight, high strength

Need galvanization

Lattice steel towers

Economical for long spans, tall supports and HV transmission

Galvanized and painted

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Typical Towers

Main stem

Cross

arms

Insulator

Guy wire

(Optional)

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Type of Towers

Type A tower (Tangent tower with suspension string)

Used on straight runs and up to 20 line diversions

Type B tower (small angle tower with tension string)

Used for line diversions from 20 to 150

Type C tower (Medium angle tower with tension string)


Type D tower (Large angle tower with tension string)


Type E tower (Dead End tower with tension string)

Used for line termination and starting

Special tower

Suspension tower: (Span about 1000m) for river or mountain crossing

Transposition tower: Transposition of line

Jaoa B. G. Silva, Innovative solutions for ovehead line supports, Cigre Meeting, SC B2, Iceland, July 2011.4


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Innovative Designs

Jaoa B. G. Silva, Innovative solutions for overhead line supports, Cigre Meeting, SC B2, Iceland, July 2011.5


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Electrical Design

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Insulators for Overhead Line

Insulators are used to insulate towers from the liveconductors

The insulators are attached to the tower and support the

line conductors.

Important characteristics:

It should be completely homogeneous materials without voids

and impurities.

Leakage current through it should be minimum.

Breakdown strength of the material should be high and it

should withstand over-voltages and normal working voltages.

It should be mechanically strong to bear the conductor load and

should have longer life.

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Insulator Ratings

Three voltages ratings Working voltage

Puncture voltage

Flashover voltage

Flashover voltage is less than puncture voltage.

VoltageWorking

VoltageFlashoverFactorSafety

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Porcelain:

Porcelain is widely used as it is cheap.

It is thoroughly vitrified to remove voids and

glazed before use to keep surface free of dust and

moisture.

Breakdown strength is around 6-12 kV/mm

(air: 3kV/mm)

Glass:

Toughened glass is another choice having higher

dielectric strength (120 kV/mm), mechanicalstrength and life.

Flaws can be detected easily by visual inspection.

Main disadvantage is moisture rapidly condenses

on the surface giving high surface leakagecurrent.9


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Polymeric Insulation:

Silicone rubber and EPDM (Ethylene propylene diene

monomer) are used for insulation purpose.

Low cost, light weight, higher life, improved

dielectric performance under contamination or

pollution.

They are used in combination with fiber glass rod.

These are under field trials and may take time to be

used extensively.

Tracking and erosion of the shed material, which canlead to pollution and can cause flashover.

Chalking and crazing of the insulators surface, which

resulted in increased contaminant collection, arcing,

and flashover.

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Types of Insulators

Pin type insulators

Suspension type insulators

Strain type insulators

Shackle insulator

Post type insulators

Composite polymeric insulators

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Pin Type Insulator

Supported on steel bolt or pin which is

firmly supported on cross-arm.

Conductor is tied to insulator on groove

by annealed binding wire.

Generally used for 11 kV and 33 kV

lines.

They can be made in one piece up to 33

kV and two pieces for higher voltages.

Pin type insulators are uneconomical for

higher voltages. The leakage or creep age distance is

from line to pin radially along the

surface.

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Suspension Type Insulators Consists of one or more insulating units hung from

cross arm and conductor is connected at lowest unit.

String is free to swing (lower mechanical stresses);

thus long cross arms are required.

Economical voltages above 33 kV. Each typical unit

is designed for 11 kV.

Failed unit can be changed

without changing whole

string.

V shaped insulator strings

can also be used to avoid the

swings.

400 -> 19 units -> 3.84 m

http://www.electrotechnik.net/13


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Strain Type Insulator

The insulators are similar to

suspension type insulator but used

in horizontal position.

Generally used at the towers with

dead end, angle towers, and road

and river crossings.

They can take tension off the

conductors. When tension is very

high two or more strings are usedin parallel.

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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 andground. 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 mmVV

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

StringAcrossVoltageEffciencyString

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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

CCCCCC

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 different

capacities, 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, ma 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 to

achieve 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 15

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