power transmission lec-1

8/3/2019 Power Transmission Lec-1

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10/2/2006Lec# 1 Syed Abdul Rahman

Kashif 1

Insulators

Lecture No. 1


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Sub- transmission and Distribution line

Distribution line 13.8 kV

Transformer

240/120V line

Fuse and disconnector

Telephone line

Distribution Cable 13.8 kV


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H ow e l ec t r ic i t y ge t s t o y ouH ow e l ec t r ic i t y ge t s t o y ou

When electricity leaves a powerplant (1), its voltage is increased

at a step-up substation (2).

Next, the energy travels along a

transmission line to the area

where the power is needed (3).

Once there, the voltage is

decreased, or stepped-down,

at another substation (4), and a

distribution power line (5) carries

the electricity until it reaches a

home or business (6).

EEI, Getting Electricity Where Its Needed,May 2000


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Transmission Lines

Double circuit69 kV line

Distribution line12.47kV

Wooden tower

Shieldconductor


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Power System

generation transmission distribution

the network of electric power


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Transmission lines Extra-high-voltage lines

Voltage: 345 kV, 500 kV, 765 kV Interconnection between systems

High-voltage lines Voltage: 115 kV, 230 kV

Interconnection between substations, power plants

Sub-transmission lines Voltage: 46 kV, 69 kV

Interconnection between substations and large industrial customers

Distribution lines Voltage: 2.4 kV to 46 kV, with 15 kV being the most commonly

used

Supplies residential and commercial customers

High-voltage DC lines Voltage: 120 kV to 600 kV

Interconnection between regions (e.g., Oregon-California)


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Insulators

Overhead transmission lines aresupported on the towers. Sincetowers are at ground potential, thelines must be insulated with thetower structure. Insulatorsare used

to insulate tower from the bareconductors.


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Ideal insulators Properties There should not be any pores or air spaces. There should not be any impurities. There should be perfectly homogeneous material. Leakage current through insulators should be

minimum. Insulators should be able to withstand over-

voltage and normal working voltage. It should be mechanically strong to bear the

conductor load.


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Material for Insulator

Porcelein Toughened glass

Moulded material polymers

Porcelein and toughned glass is usedfor high voltages where mouldedmaterials are used for low voltages.


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Porcelain

Should be thoroughly vitrified and glazed. Vitrification is very important because

presence of pores and dirt particles

reduces the di electric strength of theporcelain.

Sealed in impurities reduces the dielectricstrength and mechanical strenght too.


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Porcelain

Glazing is important to provide asurface which can be kept relativelyfre from dirt and moisture.


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Porcelain

It should have the required electricalcharecteristics as well as the strongmechanical strength because it willhave to support the weight of theoverhead line conductorsunder all

conditions of wind and weather.


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Porcelain

Dielectric strength of the porcelainis of order of 12 to 28 kV/mm.

Ultimate strength in compression isabout 690 MN/sq.m

And 48 MN/sq.m in tension


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Porcelain

Porcelain consists of 20% silica, 30%feldspar and 50% clay and having thedielectric strength of 120-280 kV/cm.

Any impurity will reduce the dielectricstrength of the insulator. It is difficult to manufacture perfectly

homogenous porcelain in the thicknessrequired and therefore, it is necessary togo for different pieces and cemented

together.


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Toughened glass

Toughened glass, which is normallyhaving dielectric strength of 1200kV/cm, is another material used for

insulators. The glass is toughened tomake skin more resistant to damage

by chipping and arc.


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Advantages of Glass

Insulators High dielectric strength Longer life High thermal sock resistant thus reduced

damage from the flashover Lower coefficient of thermal expansion Greater mechanical strength under the

compression but in tension it is same asporcelain

Fault can be easily seen from the nakedeyes.


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Disadvantages

Moistures are readily condensed onthe surface.

It is expensive than porcelain.

Its resistance to continuousmechanical load and temperature

changes is poor compared to porcelainand toughened glass.


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Ratings

Insulators are rated by threevoltages:

working voltage (or rated voltage)

puncture voltage

flashover voltage.


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Working voltage

The workingvoltage rating is thevoltage at which an insulator isdesigned to bear the steady state

voltage stress. If the line voltage isVLLthe working voltage will be

VLL/ sqrt (3).


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Flashovervoltage

The flashovervoltage is the voltageat which flashover occurs through airsurrounding the insulator


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Puncture voltage

Thepuncturevoltage is the voltage atwhich the insulator breaks throughbetween conductor and pin. It

destroys the insulator. This rating isdetermined by applying the voltage

while insulator is emerged in oil. This is done because before the

puncture, there will be flashover.


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Ratings Flashover voltage is less than puncture

voltage and higher than working voltage ofinsulators.

A safety factor is defined relating the

flashover and working voltages.Mathematically, it is written as

Flashover voltage Safety factor =

Working voltage


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TYPES OF INSULATORS

There are four types of insulators whichare used in overhead transmission lines:

Sackle type,

Pin type,

Suspension type

Strain type.


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Shackletype

Sackletype insulators are normallyused in 230-440-V lines.

A sackle insulator looks like a dumblyshape and is used in one unit.


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Pin type Insulator

They are used for medium voltage. Small in size.

Simple construction. Low price


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Pin Type Insulator

Groove for condutor


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Pin Type Insulator Pin bolt is used to clamp the insulator to

cross arm on the pole. There should be sufficient thickness of

porcelain between line conductor and the

insulator pin to give a safety factor of 10against puncture. The groove at the top of insulator is used for

conductor placing. The insulator and pin must have a sufficient

mechanical strength to bear the weight ofconductor, wind pressure and ice loading .


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Pin type Insulator The cost of such insulators increases if the

voltage rating increased. Pin type insulators are uneconomical for

higher voltage. They are used upto 33 kV. Safety factor for 11 kV line is 8.2 in dry

condition and 5 in wet condition. They are now availabe upto 50 kV. Repalcement is not easy.


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Pin type Insulator


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String type Insulators Also called suspension type insulator. As the voltage increases cost of pin type

insulator increases so string type

insulators are used. It is hung from cross-arm and carrying

power conductor at the lowest extremity.

It is free to swing and therefore largercross-arm is required, than the pin typeinsulators.


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String type Insulator


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Locking Key

Insulator's Head

Expansion Layer

Imbedded Sand

Skirt

Petticoats

Iron Cap

Ball Socket

Compression

Loading

Cement

Insulating Glassor Porcelain

BallCorrosion Sleevefor DC Insulators

Steel Pin


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Suspension type Insulator

Line Voltage

Number of Insulators per String

69 kV 46

115 kV 79

138 kV 810

230 kV 12

345 kV 18

500 kV 24

765 kV 3035

Advantages of String type


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Advantages of String type

Insulator

Economical for voltage above 33 kV. Each insulator is designed for 11 kV

and hence for any operating voltage, astring can be made.

Failure of any unit can be replaced

without changing the whole string.


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Advantages

Since it is allowed to swing in the air,mechanical stress at a point ofattachment is reduced.

Flexible in extension of voltage ratingby adding more units. Since the conductors lay below the

cross-arm, the line outages due tolightening strokes are reduced.


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String InsulatorsS h i e l d

c o n d u c t o r

I n s u l a t o r

P h a s e

c o n d u c t o r

T o w e r

6 9 k V

L i n e

Line post- composite


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Insulator

(1) is the clevisball, (2) is the socket

for the clevis, (3) is the yoke

plate, and

(4) is thesuspensionclamp. (Source:Sediver)



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Insulator

C o m p o s i t e

I n s u l a t o rC r o s s a r m

C o m p o s i t ei n s u l a t o r

S t e e l t o w e r

T w oc o n d u c t o r

b u n d l e

S h i e l d c o n d u c t o r

Hewlett type


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Hewlett type

Insulator


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Strain insulators Strain insulators are similar to suspension

type insulators in shape but they are usedin vertical plane rather than horizontal

plane as suspension type of insulators areused.

Strain insulators are used at dead ends or

at anchor towers. When the tension in theconductor is very high, two or moreinsulators are used in parallel.


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Electrical Equivalent


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VOLTAGE DISTRIBUTION Normally, the string units are similar in

shape and size, the capacitance of eachunit can be taken as the same.

The capacitance between metal part andstructure (at the earth potential) isformed which is not negligible because thecross-arms are not longer.

If the cross-arm length is long enough, thecapacitance between the metal structureand the earth (tower) can be neglected.


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VOLTAGE DISTRIBUTION Under this condition, the current flows in

each string will be the same and thepotential distribution will also be the same

in each insulator. But this assumption is not true and the

current in lower string is larger than the

topmost string and this uneven distributionof current causes the different voltageacross the string.


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VOLTAGE DISTRIBUTION

Let the capacitance of each unit (alsocalled mutual capacitance)is mCanddefined a ratio mas


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VOLTAGE DISTRIBUTION Let us calculate the voltage across

each string (unit). In is current through capacitance

between nth pin to the earth and in is current through nth string.

Vn is the voltage across nth string and vn is the voltage between nth pin to

the earth.


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Voltage distribution If m=5 voltage across each unit in terms of the voltage

of the top String is V2= 1.2 V1 V3= 1.64V1 V4=2.408V1 This shows that the voltage across the unit

nearest to the conductor is the highest andtherefore the unit near to the conductor is highlystressed and the topmost string is less utilized.


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Electrical Equivalent


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String Efficiency

Normally,efficiency

is defined as aratio of output to input.

However, string efficiencyis a

measure of utilization of material inthe string. If the number of units

used in the string is n, the stringefficiency is defined as


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String Efficiency

From this definition, if the voltage distribution is

same across each unit, the string efficiency will be100%.


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Problem A string insulator has 4 units and

each unit is having capacitance C. Thepin to-earth capacitance is C/10; find

the voltages across each unit of thestring, and the string efficiency.

METHODS TO IMPROVE STRING


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METHODS TO IMPROVE STRING

EFFICIENCY

String efficiency can be improved bythe following four ways: Increasing the value of m

Grading of units Static shielding

Conducting glazes


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Increasing value of m .If the value of mis increased, which can be

achieved by increasing the cross-arm length sothat the capacitance between pin to earth isdecreased, the voltage distribution across the unitcan be improved and thus higher efficiency can beachieved. However, increasing the cross-armlength after certain value will not be economical.Mathematically, we can achieve 100% efficiency

(equal voltage across each unit), .if the value of mis infinity. It is found that the value of mgreaterthan 10 is not economical


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Grading of units Since the current in each unit is different which is not

possible to eliminate, voltages across each unit can beequalized using the different capacity units. As the currentin the lowest string is highest, the low-impedance unit (orhigh-capacitance unit) near to the conductor will improve thestring efficiency. This shows that if discs are arranged in

decreasing order of capacitance from bottom (near to theconductor) to top, the voltage across each unit can beequalized. In other words, if the product of a capacitivereactance to the current flowing through discs is the same,the voltage across each unit will be the same. This is

possible only if the capacitance of the upper unit is less thanthat of the lower unit.


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Capacitance Grading Grading of units is also called capacitance grading. This method of improving the efficiency will result

in the need for large stock of different sizes ofunits, which outweighs the advantages of string

insulators. Good results can be obtained by using insulators of

one size for most of the units and larger units forthe one or two adjacent to the line.

In practice this method is used for very highvoltage lines.


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Grading of unit


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Grading of units


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Static shielding. In this method, current from pin to earth is

equalized by injecting current from the line to thepin, as shown in Figure, so that equal current flowsthrough the unit and therefore the voltage acrosseach unit can be the same. This is achieved by theemployment of a grading or guard ring, whichusually takes the form of a large metal ringsurrounding the bottom unit, and connected to the

line. Due to this arrangement, capacitancesbetween line to pin of the units are formed.


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Guard Ring


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Static Shielding


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Conducting glazes. Since there are uneven voltages in each unit due

to unsymmetrical current, each mutual capacitance(also called self-capacitances) could be shunted bya resistor of such a magnitude that thecapacitance currents were swamped by the effectof the leakage current through the resistors.Then a more uniform voltage division can beobtained. This can be achieved by coating

insulators with a conducting glaze. This method ispractically impossible due to difficulty ofproducing glaze for long time.


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TESTING OF INSULATORS

Three type of tests are performed onthe insulators.

Flashover tests,

Sample tests and routine tests.


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Flashover tests Flashover tests are a design test

made on three insulators, only toprove the correctness of the design. 50% dry impulse flashover test. Impulse withstand test.

Dry flashover and dry one-minute test. Wet flashover and one-minute rain test.


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Sample Tests Sample tests are to prove the quality

of manufacture and are made on 50%of the insulators supplied.

Temperature cycle test. Mechanical test . Electro-mechanical test. Puncture test. Porosity test.


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Routine tests Routine tests are carried out on all

insulators. Electrical routine tests.

Mechanical routine tests.


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Note See the details of the test from your

text book


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Impulse Ratio Insulator must be tested under two

different voltage conditions, namelyimpulse and power frequency, and

that the required impulse withstandlevel is much greater than the powerfrequency value.


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Impulse Ratio


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Problem A suspension type insulator is having 5

units and the value of pin-to-earthcapacitance is C. Find the line-to-pin

capacitances to equalize the voltageacross each unit.


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Solution The line-to-pin capacitances S\, S2, 53

and S4as shown in Figure can be obtainedby using Equation. However, here the sameis calculated with basic principle. Letpotential across each disc is V. The linevoltage will be 5 V. Potential of point Awillbe V. For equal voltages across each unit,the current through each disc having same

capacitance will be same. Therefore, thecurrent through S1 will be same as currentthrough pin-to earth capacitance. Hence


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Solution


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Solution


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References The Transmission and Distribution of

Electrical Energy by H. Cotton. Power Transmission and Distribution

Systems by S.M. Singh Power Transmission and Distribution

by J.B Gupta

power transmission lec-1

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