power system engineering lecture 14

8/3/2019 Power System Engineering Lecture 14

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Review of Last Class

Parabolic approximation Ice and wind loading

Stringing Chart


2/28

Support at Same Heights

Tat Ends

orMax

If the towers at same height and span is 2l, i.e. half span is l

Conductor

Length

Sag

Conductor

Tension


3/28

The resultant weight of the conductor is:

Effect of Ice Covering and Wind

Conductor alone

Conductor + Ice

kg/m22

wcT www

kg/m22 wicT wwww

ww

wc or(wc+ wi) wT


4/28

Stringing Chart

The curves of sag and tension with temperature variation are called

the Stringing Charts.

Stringing chart is helpful in providing sag and tension at any

temperature, if the sag and tension is know at any particular

temperature.

They are useful in erecting line conductors at specified temperature

and loading conditions.

At high temperature, sag is more and tension is less whereas at low

temperature sag is less but tension is more.


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Stringing Chart

w Load per unit length

f Stress (tension per mm2)

S Conductor length (half span)

d Sag

Temperature

A Area of cross section of conductor

Coefficient of linear expansion

E Youngs modulus

Subscripts 1 and 2 denote temperatures at maximum load conditions and

under stringing conditions (installation or erection of transmission line).


6/28

Stringing Chart

For short spans

and

At temperature

1

At temperature 2

T=A f22

32

6 fA

lwlS

2

1

2

32

11

6 fA

lwlS

2

2

2

32

22

6 fA

lwlS

1

2

11

2 fA

lwd

2

2

22

2 fA

lwd

and

and


7/28

Stringing Chart

Due to increase in temperature from

1 to

2,increase inlength is:

There is another effect increase in temperature from 1

to

2,decrease tension or stress fromf1 tof2 :

Therefore new length is:

lS 1212

lE

ffS

E

ff

2121

0ll

E

lfl 0

lE

fflSS

211212


8/28

Stringing Chart

Simplifying

This is cubic equation inf2 which can be solved usingmathematical algorithm. Then the sag is:

lE

ffl

fA

lwl

fA

lwl

21122

1

2

32

1

2

2

2

32

2

66

E

fA

Elwfff

A

Elw 122

1

2

22

112

2

22

22

2

66

2

2

22

2 fA

lwd


9/28

Stringing Chart

Sag

Tensio

n

Temperature


10/28


11/28

Ruling or Equivalent Span

There are several situations when span length is not same. Therefore, tension in each span will be different.

This is not possible in case of suspension type insulator,

because it will swing to equalize the tension.

Therefore, the uniform tension in each span is calculated bydefining the equivalent span (or ruling span).

Le is the equivalent or ruling span

Li is the each individual span in line

n

n

eLLLL

LLLLL

32

33

2

3

2

3

1

1


12/28

Ruling or Equivalent Span

Also approximate ruling span is:

Lavg is the average span in line

Lmax

is the maximum span in line

The ruling span is then used to calculate the horizontal component of

tension, which is to be applied to all the spans between the anchor

points. Then the sag at each span is computed using

Span should not be more than twice the ruling span or less than half

the ruling span.

avgavge LLLL max

3

2

H

lwd ii

2

2


13/28

Conductor Vibrations

In addition to Horizontal swing due to wind pressure,there are two types of vertical vibrations:

Aeolian or resonant vibrations

It is caused by vortex phenomena in light winds.

Low magnitude (up to 5 cm) and high frequency (5-40 Hz)

Less harmful because of small magnitude

These vibrations are common in conductor and more or less always

present.

TheArmour rods or dampers are used to damp these vibrations.

Armour rods Stock-bridge dampers
http://4.bp.blogspot.com/-VtvYTbxPanU/TWYyxc5qWTI/AAAAAAAAEec/LQ3jxxxeOgU/s1600/damper_2.jpg


14/28

Conductor Vibrations

Galloping (or dancing conductors)

Generally happened due to asymmetrical layer of ice formation.

When this asymmetrical ice coated conductor exposed to light winds

(particularly when the slope of ground is higher).

High magnitude (up to 6 m) and low frequency (0.25-2 Hz).

These vibration may cause flashover between the conductors.

To avoid this flashover horizontal configuration is preferred.

Also if conductor is perfectly circular the effect can be minimized.

The stranded conductors can be wrapped up with PVC to make

conductor perfectly circular.


15/28

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.


16/28

Insulator Ratings

Three voltages ratings Working voltage

Puncture voltage

Flashover voltage

Flashover voltage is less than puncture.

VoltageWorking

VoltageFlashover

FactorSafety


17/28


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

Glass:

Toughened glass is another choice having higher

dielectric strength (120 kV/mm), mechanical

strength and life. Flaws can be detected easily by visual inspection.

Main disadvantage is moisture rapidly condenses

on the surface giving high surface leakage

current.


18/28


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 ledto pollution and caused 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


20/28

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/


22/28

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 are used inparallel.


23/28

Shackle, Post and Polymeric insulators

http://www.electrotechnik.net/

Shackle insulators or

spool insulators

Post type insulators Polymeric insulators
http://4.bp.blogspot.com/-xoQVmPkrCrk/TV4eTUZg52I/AAAAAAAAEeA/VKlkK4RWPvU/s1600/shackle+Insulator.jpg


24/28

Potential Distribution over the String

IfVis voltage across the conductor and

ground. We have:

C

mCm

groundtoeCapacitanc

insulatorpereCapacitanc

Also

m

VV1

112


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Potential Distribution over the String Similarly,

213

131

mmVV


26/28

Potential Distribution over the String Similarly,

3214

1561

mmmVV


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String Efficiency

Let m = 5

m

VV1

112

21313

1 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

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

power system engineering lecture 14

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