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POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP.

Chapter four

Over head insulatorThe overhead line conductors should be supported on the poles or towersin such a way that currents from conductors do not flow to earth throughsupports i.e., line conductors must be properly insulated from supports.This is achieved by securing line conductors to supports with the help ofinsulators.The insulators provide necessary insulation between line conductors andsupports and thus prevent any leakage current from conductors to earth.

*The function of the insulators are :.

1- Insulate the conductors from each other and from the towers underhighest voltage and under bad air estimate circumstance2-Carry the conductors under the bad estimate mechanical stresses

*The materials that used for made the insulators of overhead line are1- porcelain2-Tougheneed glass.Although the two above materials are brittle and inelastic but it are thebest materials that used to insulate the conductors.

Types of insulator:

1-Pin type insulatorsPin type insulators are used for transmission and distribution of electricpower at voltages upto 33 kV. Beyond operating voltage of 33 kV, thepin type insulators become too bulky and hence uneconomical.


.

2-Suspension- type insulator.The cost of pin type insulator increases rapidly as the working voltage isincreased. Therefore, this type of insulator is not economical beyond 33kV. For high voltages (>33 kV), it is a usual practice to use suspensiontype insulators consist of a number of porcelain discs connected in seriesby metal links in the form of a string. The conductor is suspended at thebottom end of this string while the other end of the string is secured to thecross-arm of the tower. Each unit or disc is designed for low voltage, say11 kV. The number of discs in series would obviously depend upon theworking voltage. For instance, if the working volt-age is 66 kV, then sixdiscs in series will be provided on the string.


3. Strain insulators. When there is a dead end of the line or there is corner or sharp curve, theline is subjected to greater tension. In order to relieve the line ofexcessive tension, strain insulators are used. For low voltage lines (< 11kV), shackle insulators are used as strain insulators. However, for highvoltage transmission lines, strain insulator consists of an assembly ofsuspension insulators as shown in Fig. 8.8. The discs of strain insulatorsare used in the vertical plane. When the tension in lines is exceedinglyhigh, as at long river spans, two or more strings are used in parallel.

4. Shackle insulators. In early days, the shackle insulators were used as strain insulators. Butnow a days, they are frequently used for low voltage distribution lines.Such insulators can be used either in a horizontal position or in a verticalposition. They can be directly fixed to the pole with a bolt or to the crossarm. The conductor in the groove is fixed with a soft binding wire.


*The eclectic behavior of each Insulator is as like a capacitanceAppeared in the string

1- C ( self capacitance )

Between the cap of insulator and the pin in the bottom of insulator and itsvalue of about 30 pf

2- Ce ( air capacitance )Where Ce is another capacitance between the tower and the connectionpoint between the pin of each insulator and the cap of the next insulatorand it is have a value of about ( K where k (0.1The insulator make the distribution of voltage through the insulator of thestring is different (where the insulator that nearest to the conductor has ahigher voltage then the next and up)

*There is another capacitance ( air capacitance ) between the conductorand the connection point between the pin of each insulator and the cap ofthe next insulator and is very little and it can be neglected.


(1)

At point B

(2)


At point C

(3)

At point D

(4)

If we have n insulator

(5)

(6)


Where V is the voltage of line (string voltage ƪ is the efficiency of thestring.

Also there is another method (general method ) to solve this problem,where

(7)

(8)

Where X is the number of insulator that you need to get the voltage acrossit.

*Methods of increasing the string efficiencyThe string efficiency increase when there is a uniformly distributionvoltage across the insulator, that means when if V (phase votageof the conductor) Is equal to n ( n : number of insulator and is the voltage of the lastinsulator )We can get ƪ = 100 % ( maximum efficiency )

The voltage across the capacitors ( insulator) are equally ( uniformly). But

this case is a theoretical case because of the air capacitance (ce) that make

the voltage across the insulator are not uniformly. Also the string

efficiency decrease if the number of insulator (n) increase because of the

not equally .

Distribution voltage across the insulators. Many method are used to

increase the string efficiency such as :.


1-Reducing the value of k :.

When

We can reduce the value of k by reducing the value of by increasing

the length of cross- arm for the tower but this method leads to reduce

rigidity for the cross-arm and increase the cost

2-Grading of units :.This method depend on the change

Of the self capacitance where

The capacitance that near to

The conductor is greater than

The others due to the location

Of insulator in the string and

The capacitance (self capacitance)

That near to the cross-arm is

Less than the others

If we have four insulator (n =4) and the self capacitance that nearest to

the cross- arm is C then we make a grading for self capacitance in order to

get an equal voltage across the insulators (maximum efficiency)

(


At point A

(9)

At point B

(10)

At point C

Ƶ= y+3kcƵ= c(1+3k)+3kc

(11)

And so onThis method is difficult in practical application because of different valueof the capacitance .


3) Guard Ring :.

It is possible to make the voltage distribution across the units of string ofinsulator uniformly by a guard ring which is a ring of conductor put orplaced across or around the nearest insulator to the conductor .The main reason of different the Voltage across the insulator of Thestring is the air capacitance (Ce) And so that the guarding reduce Ce byadding a number of Air capacitance between the Conductor or guard ringand Metal point of insulator and these air capacitances are in opposite bythe effect with Ce and make the voltage across the insulators moreuniformly .


Ex1) Over string of five insulator and the ratio of Ce/C=0.1. If the stringis connected to line of voltage 33kv.

1- What is the voltage distribution over the unit of string and whatwill be the string efficiency.

2- When the string is supplied by a guard Ring and this lead to add atwo air capacitance of value 0.2c,0.1c respectively to the nearest tothe conductor. Find the new voltage distribution and the newefficiency.

So1)

(1)

(2)

(3)

(4)

0.1c

cc0.1c

0.1c

c0.1c

c

A

B

C

D

Conductor


}

*Also we can solve it by


2) After the guard Ring

At point C0.1c c

A

cB

0.1c

cC

c

0.1c

D0.1c

0.2 c

0.1c

c


At point D

1.299

NowAfter guard Ring Before guard Ring


Ex2) Each of the 4- insulator have after guard ring

3C=5CeCalculator cd, cf, cg in

So1) at point A

At point B

A

C0.6C

C

B

0.6C

C

C

0.6C

C

cd

cg

cf

Conductor


At point C

Ex3) over string of 3 insulators and the ratio of if the string isconnected to line voltage of 33kv. Also if the break down voltageof each insulator is 7kv.

1- Find the voltage distribution over the unit of the string and what isthe string efficiency.

2- When the string is supplied by a guard ring and this load to add at aircapacitance of 0.15 c,0.15c respectively to the nearest to theconductor . find the new voltage distribution and the new efficiency

So1)

= 19050 voltage

*1

*2

0.15c

0.15c

C

C

C

V

conductor


We see that the break down voltage is 7000 volt

Then the case will be

(Break down voltage)

ThenThe string failed to insulator the conductor from the tower

2) at point A

At point B

0.15CC

CV

conductor

C

A

0.15C

C

B

0.15C C

conductor

0.15C

0.15C


}

This string successes in insulation the conductor


Ex4) Over string of 3- Transmission line has four insulator andCe=0.12C. if the voltage between the metol cap of the last insulator andthe earth is 33kv.

1- Find the voltage distribution across the insulators and what is thestring efficiency

2- If the string is supplied by a guard ring and this lead to add a twoAir capacitance of o.2C, 0.1 C respectively to the nearest to theconductor. Find the new voltage distribution and the new stringefficiency

So1)

K= 0.12

0.12c

0.12c

0.12c

C

C

C

V

C


(

KV

At point B

(1)


At point C

Also as F before

0.12C

0.12C

0.12C

C

C

C

C

0. 2CC

B

A

0. 1C

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COLLEGE OF ENGINEERING /ELECTRICAL DEP. · POWER SYSTEM ANALYSIS MISSAN UNIVERSITY\ COLLEGE OF ENGINEERING /ELECTRICAL DEP. ... *The materials that used for made the insulators of