Overhead Transmission Line Insulators Leave a Comment / Transmission System

The overhead line conductors are supported on the poles or towers. In

order to prevent the flow of current to earth through supports, the

line conductors must be properly insulated from supports. This is

achieved by securing line conductors to supports with the help

of overhead line insulators.

These insulators provide necessary insulation between the line

conductors and supports and hence prevent any leakage current from

conductors to earth. Thus the over head line insulators play an

important part in the successful operation of power system.

In general, overhead line insulators should have the following

desirable properties:

• High mechanical strength in order to withstand conductor load

and wind load.

• High insulation resistance in order to prevent leakage current.

• High relative permittivity of the insulator material used so as to

have high dielectric strength.

• The insulator material should be nonporous; free from impurities

and fractures otherwise permittivity of the insulator material will

be lowered.

• High ratio of rupture strength to flashover voltage.

• The insulator material should not be affected by the change in

temperature. The materials used for insulators used in overhead transmission lines are porcelain, glass, stealite and special composition materials. The most commonly used material is porcelain whereas the other materials viz. glass, stealite etc. are only used to a limited extent. Porcelain is produced by firing at a controlled temperature a mixture of kaolin, feldspar and quartz. This material is preferred over glass since it is mechanically strong; its surface is not affected by dirt deposits and is less susceptible to temperature changes.

The dielectric strength of a porcelain insulator is 60 kV per cm of its thickness and it’s compressive and tensile strengths are 70000 kg/cm2 and 500 kg/cm2 respectively.

Types of Overhead Line Insulators

The most commonly used overhead transmission line insulators are:

• Pin type insulators.

• Suspension type insulators.

• Strain insulators.

• Shackle insulators.

• Egg or stay insulators.

Pin Type Insulator

The pin type insulator is screwed onto a galvanized steel bolt which in turn is installed on the cross-arm of the pole. The electrical conductor is placed in the groove at the top of the insulator and is tied down with annealed (soft) wire of the same material as the conductor as shown in the figure. For lower voltages generally, one-piece type of insulator is used. These insulators may have one, two or three rain sheds or petticoats. These rain sheds are so designed that when these insulators are wet (its outer surface is almost conducting due to rain, water), even then a sufficient dry space is provided by the inner sheds.

For higher voltages, the thickness of the material required for insulation purposes is more and because of practical difficulties, a quite thick single piece insulator cannot be manufactured. Hence, for higher voltages, two or three piece insulators are jointed. In this case a number of shells (pieces) are fixed together by portland cement. These insulators are designed up to 50 kV because beyond this voltage they become uneconomical. The modern practice is not to use these insulators beyond 33 kV. Up to 33 kV, pin-type insulators are preferred over suspension type insulators because firstly they are cheaper in cost. Secondly, they require shorter poles to give the same conductor clearance above the ground since they raise the conductor above the cross-arm while the suspension type insulators suspend it below the cross-arm.

Suspension Type Overhead Line

Insulators

As line voltage increases, the pin-type insulator to be used becomes costly, bulky and complicated in construction. Further, the replacement of the damaged insulator will cost more. Therefore, this type of insulator is not economical beyond 33 kV. For higher voltages (more than 33 kV), it is usual practice to use suspension type insulators. They consist of a number of porcelain discs connected in series by metal links in the form of a string as shown in the figure. The string is screwed at the top to the cross-arm of the tower while the conductor is suspended at the bottom.

Advantages of Suspension Type Overhead Line

Insulators

• Each unit or disc of suspension insulator is designed for 11 kV so

by connecting a number of such discs in series, a string of

insulators can be designed for any required voltage.

• These insulators are cheaper than pin type insulators for voltages

more than 33 kV.

• In the case of failure of any disc the whole string does not become

useless. Rather the damaged disc is replaced easily and at a lesser

cost.

• The string of suspension insulators is more flexible therefore it is

free to swing in any direction. Hence, it takes up a position where

it experiences only a pure tensile stress.

• By the use of suspension type insulators, the line conductors are

less affected by lightning, since they are placed below the cross

arm which is earthed and acts as a lightning arrestor.

• If the load to be transmitted by the line increases, the increased

demand can be met by raising the line voltage than to provide

another set of conductors. This can be achieved by adding one or

more discs to the existing strings.

Disadvantages of Suspension Type Overhead Line

Insulators

• For the same conductor clearance from the ground, higher towers

are required since the conductors are placed at the lowermost

discs.

• Larger spacing between conductors is required due to the large

amplitude of the swing of the conductors. However, these disadvantages are not so serious; therefore, suspension type insulators are invariably employed in the overhead lines working at the voltages more than 33 kV.

Strain Insulators

At the dead ends, on sharp turns, at the river crossings or at the corners, the line is subjected to greater strains. In order to withstand the excessive strain, strain insulators are used in overhead transmission lines. For low voltage lines (below 11 kV) shackle insulators are used but for high voltage transmission lines strain insulators consisting of an assembly of suspension insulators are used. When the pull on the string of suspension insulators is high such in the case of long spans across the river, two or more strings are used in parallel.

Shakle Insulators

These insulators are mostly used at low voltage distribution lines. The conductor is passed through the place left between the clamp and the insulator and is fixed along the groove with the help of soft bending wires of the same material as the conductor.

Egg or Stay Insulators

• Guy or stay wires are used with the poles placed at the dead ends or at sharp turns of the low voltage lines. To insulate the lower part of the gay wire from the pole for the safety of people, a stay insulator is placed in

between the wire. This insulator is placed in the guy wire at a height of three meters from the ground. It has two holes at the right angle to each other through which two ends of the guy wires are looped in such a way that in case the insulator breaks the guy wire will not fall to the ground.

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Potential Distribution over a String of Suspension Insulators

For overhead lines operating at high voltages (33 kV and above) use of number of discs

connected in series, through metal links, is made. The whole unit formed by connecting

a number of discs in series is known as string of insulators. The line conductor is

secured to the bottom disc of the string and the top disc is connected to the cross-arm of

the pole or tower, as illustrated in Fig. 9.15.

The number of discs connected in series in an insulator string depends upon the line

operating voltage (higher the line operating voltage, the larger is the number of discs

required for the insulator string, as given below in tabular form).

The number of discs indicated in above Table 9.1 is actually the usual number used.

However, in the case of transmission lines operating at 66 kV or more, one disc less

than the number indicated in Table 9.1 is used on about eight suspension structures

near the substation. This is accomplished so that in the event of a lightning surge

appearing on the line, the insulator string will flash-over and prevent the surge from

travelling to the substation thus safe-guarding the equipment there.

It is found that the voltage impressed on a string of suspension insulators (the voltage

applied between the line conductor and earth) does not distribute itself uniformly across

the individual discs.

The line unit (unit nearest the line conductor) has the maximum value across it, the

figure progressively decreasing as the unit nearest the cross-arm is approached. The

inequality of voltage distribution between individual units is all the more pronounced with

a larger number of insulator units. This fact may be explained with the help of equivalent

circuit of an insulator string (Fig. 9.16).

Each string insulator unit behaves like a capacitor having a dielectric medium between

the two metallic parts (viz. pin and cap). The capacitance due to two metal fittings on

either side of an insulator is known as mutual capacitance. Further there is also a

capacitance between metal fitting of each unit and the earthed pole or tower. The

capacitance so formed is known as shunt capacitance.

If a string of similar suspension insulators could be situated so far from neighbouring

metal work that the capacitance between this metal work and the metal fitting of the

insulators (i.e. shunt capacitance) would be negligibly small in comparison with the

capacitance of each unit (i.e. mutual capacitance), then the charging current would have

been the same through all of the discs, the discs being connected in series, and

consequently the voltage across individual units would have been the same i.e. applied

voltage V divided by the number of units in the string.

However, in practice this condition is not fulfilled because of nearness of the tower, the

cross-arm, and the line. These shunt capacitances, sometimes called the stray

capacitances; have an important effect on the voltage distribution between the units.

Due to shunt capacitance, charging current is not the same through all the discs of the

string (Fig. 9.16.). So voltage across individual units, being directly proportional to the

current flowing through them, will be different. This unequal potential distribution is

undesirable and is usually expressed in terms of string efficiency.

The ratio of voltage across the whole string and the product of the number of units and

voltage across the unit nearest to the line conductor is known as the string efficiency i.e.

where n is the number of units in the string.

String efficiency may also be defined as:

The voltage distribution across different units of an insulator string and string efficiency

can be mathematically determined with the help of an equivalent circuit of the insulator

string (Fig. 9.16) as below. Fig 9.16 shows the equivalent circuit of a string of

suspension insulators containing 4 units.

Let the mutual capacitance between the links be C and shunt capacitance between links

and earth be C1, voltage across the first unit (nearest the cross-arm) be V1, voltage

across the second unit be V2, voltage across the third unit be V3, voltage across the

fourth unit (nearest the line conductor) be V4 and voltage between conductor and earth

be V volts.

Let C1/C = K or C1 = KC

Applying Kirchhoff’s first law to node A, we get,

ADVERTISEMENTS:

I2 = I1 + i1

or ω C V2 = ω C V1 + ω C1 V1

or ω C V2 = ω C V1 + ω K C V1

or V2 = V1 (1 + K) … (9.2)

Applying Kirchhoffs first law to node B we get,

I3 = I2 + i2

or ω C V3 = ω C V2 + ω C1 (V1 + V2)

∵ Voltage across the second shunt capacitance C1 from the top = V1 + V2

or ω C V3 = ω C V2 + ω K C (V1 + V2)

or V3 = V2 + K (V1 + V2) = K + V2 (1 + K)

or V3 = KV1 + V1 (1 + K) (1 + K)

∵ From Eq. (9.2) V2 = V1 (1 + K)

or V3 = V1 (1 + 3K + K2) …(9.3)

Applying Kirchhoff’s first law to node C, we get,

I4 = I3 + i3

or ω C V4 = ω C V3 + ω C1 (V1 + V2 + V3)

∵ Voltage across the third shunt capacitance C1 from the top = V1 + V2 + V3

or ω C V4 = ω CV1 (1 + 3K + K2) + ω K C [V1 + V1 (1 + K) + V1 (1 + 3K + K2)]

or V4 = V1 (1 + 6K + 5K2 + K3) …(9.4)

Finally voltage between line conductor and earth,

V = V1 + V2 +V3 + V4

= V1 + V1 (1 + K) + V1 (1 + 3 K + K2) + V1 (1 + 6 K + 5 K2 + K3)

= V1 (4 + 10 K + 6 K2 + K3) … (9.5)

The greatest voltage will be obviously V4 which is given as:

Since n = 4, and flash-over voltage of one unit

= Greatest voltage across any unit i.e. V4

Similarly derivation can be had for a string of insulators consisting of any number of

units.

When the number of insulators in the string is large it becomes laborious to work out the

voltage distribution across each unit, for such cases standard formula may be used.

In general case if there are n units in the string, V is the maximum voltage across the

string, V1,V2, V3…….. Vn denote the voltages across the insulator units starting from top,

C is the capacitance between the links and KC be the shunt capacitance between the

links and earth, the voltage distribution across the mth unit (counted from top) is given as

and potential adjacent to the line conductor

Graphical plot of how voltages are distributed across the units of an insulator string is

shown in Fig. 9.17.

The following points may be noted:

1. The unit nearest to the line conductor is under maximum electrical stress and is likely

to be punctured while the one nearest to the cross-arm is under minimum electrical

stress.

2. The voltage distribution across various units depends upon the value of k and number

of discs contained in the string. The greater the value of k, the more non-uniform is the

voltage distribution across the discs and lesser is string efficiency. The inequality in

voltage distribution increases with the number of discs in the string. Thus a shorter string

has higher efficiency than the longer one.

When the insulators are wet the value of mutual capacitance C increases while

C1 remains constant (except for the unit nearest the cross-arm) so the value of K

decreases, more uniform potential distribution is obtained and the string efficiency in-

creases.

The value of K (the ratio of shunt capacitance C1 to mutual capacitance C) varies and

depends upon the length of the insulator string. The larger the number of insulator discs

in a string, the longer will be the string. The longer the string, the greater must be the

horizontal spacing between the insulator disc and the support (pole or tower) to make an

allowance for conductor swing. The greater the horizontal spacing between the insulator

string and the support, the lesser is the shunt capacitance C1 and vice-versa. Thus the

value of K is low for longer strings and high for shorter strings. In practice K varies from

0.1 to 0.1667.

Methods of Improving String Efficiency The maximum voltage appears across the insulator nearest to the line conductor and decreases progressively as the crossarm is approached. If the insulation of the highest stressed insulator (i.e. nearest to conductor) breaks down or flash over takes place, the breakdown of other units will take place in succession. This necessitates to equalise the potential across the various units of the string i.e. to improve the string efficiency. The various methods for this purpose are :

1. By using longer cross-arms. The value of string efficiency depends upon the value of K i.e., ratio of shunt capacitance to mutual capacitance. The lesser the value of K, the greater is the string efficiency and more uniform is the voltage distribution. The value of K can be decreased by reducing the shunt capacitance. In order to reduce shunt capacitance, the distance of conductor from tower must be increased i.e., longer cross-arms should be used. However, limitations of cost and strength of tower do not allow the use of very long cross-arms. In practice, K = 0·1 is the limit that can be achieved by this method.

2. By grading the insulators. In this method, insulators of different dimensions are so chosen that each has a different capacitance. The insulators are capacitance graded i.e. they are assembled in the string in such a way that the top unit has the minimum capacitance, increasing progressively as the bottom unit (i.e., nearest to conductor) is reached. Since voltage is inversely proportional to capacitance, this method tends to equalise the potential distribution across the units in the string. This method has the disadvantage that a large number of different-sized insulators are required. However, good results can be obtained by using standard insulators for most of the string and larger units for that near to the line conductor.

3. By using a guard ring. The potential across each unit in a string can be equalised by using a guard ring which is a metal ring electrically connected to the conductor and surrounding the bottom insulator. The guard ring introduces capacitance between metal fittings and the line conductor. The guard ring is contoured in such a way that shunt capacitance currents i1, i2 etc. are equal to metal fitting line capacitance currents i′1, i′2 etc. The result is that same charging current I flows

through each unit of string. Consequently, there will be uniform potential distribution across the units.

Testing of overhead line insulators

Proper operation of a transmission or distribution line is highly dependent upon the

proper working of insulators. A good insulator should have a good mechanical strength

to withstand the mechanical load and stresses. It should have a high dielectric strength

to withstand operating and flashover voltages. Also, an insulator must be free from pores

or voids, which may damage it. Therefore, to ensure desired performance of insulators,

each insulator has to undergo various tests.

Testing of insulators

Following are the different types of tests that are carried out on overhead line

insulators.

1. Flashover tests

2. Performance tests

3. Routine tests Flashover tests of insulators

Three types of flashover tests are conducted before the insulator is said to have passed

the flashover test.

1. Power frequency dry flashover test

2. Power frequency wet flashover test

3. Impulse frequency flashover test Power frequency dry flashover test

The insulator to be tested is mounted in the same manner in which it is to be used.

Then, a variable voltage source of power frequency is connected between the

electrodes of the insulator. The voltage is gradually increased up to the specified

voltage. This specified voltage is less than the minimum flashover voltage. The voltage

at which surrounding air of the insulator breaks down and become conductive is known

as flashover voltage. The insulator must be capable of withstanding the specified

voltage for one minute without flashover.

Power frequency wet flashover test (Rain test)

In this test also, the insulator to be tested is mounted in the same manner in which it is to

be used. Similar to the above test, a variable voltage source of power frequency is

connected between the electrodes. Additionally, in this test, the insulator is sprayed with

water at an angle of 45° in such a manner that its precipitation should not be more than

5.08 mm/min. The voltage is then gradually increased up to the specified voltage. The

voltage is maintained at the specified value for 30 seconds or one minute and the

insulator is observed for puncture or breakdown. If the voltage is maintained for one

minute, this test is also called as one-minute rain test.

Impulse frequency flashover test

This test is to ensure that the insulator is capable of sustaining high voltage surges

caused by lightning. The insulator under test is mounted in the same manner as in

above tests. An impulse voltage generator which generates a very high voltage at a

frequency of several hundred kilohertz is connected to the insulator. This voltage is

applied to the insulator and spark-over voltage is noted. The ratio of impulse spark-over

voltage to spark-over voltage at power frequency is called as the impulse ratio. This

ratio should be approximately 1.4 for pin type insulators and 1.3 for suspension type

insulators.

[Also read: String efficiency of suspension insulators]

Performance tests of insulators

1. Temperature cycle test

2. Puncture voltage test

3. Mechanical strength test

4. Electro-mechanical test

5. Porosity test

Temperature cycle test

In this test, the insulator under test is first heated in water at 70° for one hour. Then the

insulator is immediately cooled at 7° for another hour. This cycle is repeated three times.

Then the insulator is dried and its glazing is thoroughly observed for any damages or

deterioration.

Puncture voltage test

The purpose of this test is to determine the puncture voltage. The insulator to be tested

is suspended in insulating oil. A voltage is applied and increased gradually until the

puncture takes place. The voltage at which insulator starts to puncture is called

as puncture voltage. This voltage is usually 30% higher than that of the dry flash-over

voltage for a suspension type insulators.

Mechanical strength test

In this test, the insulator under test is applied by 250% of the maximum working load for

one minute. This test is conducted to determine the ultimate mechanical strength of the

insulator.

Electro-mechanical test

This test is conducted only for suspension type insulators. In this test, a tensile stress of

250% of maximum working tensile stress is applied to the insulator. After this, the

insulator is tested for 75% of dry spark-over voltage.

Porosity test

In this test, a freshly manufactured insulator sample is broken into pieces. These pieces

are then immersed into a 0.5% to 1% alcohol solution fuchsine dye under pressure of

150 kg/cm2 for several hours (say 24 hours). After that, the pieces are removed from the

solution and examined for the penetration of the dye into it. This test indicates the

degree of porosity.

Routine tests of insulators

1. High voltage test

2. Proof load test

3. Corrosion test High voltage test

This test is usually carried out for pin insulators. In this test, the insulator is inverted and

placed into the water up to the neck. The spindle hole is also filled with water and a high

voltage is applied for 5 minutes. The insulator should remain undamaged after this test.

Proof load test

In this test, each insulator is applied with 20% in excess of working mechanical load (say

tensile load) for one minute. The insulator should remain undamaged after this test.

Corrosion test

In this test, the insulator with its metal fitting is suspended into a copper sulfate solution

for one minute. Then the insulator is removed from the solution and wiped and cleaned.

This procedure is repeated for four times. Then the insulator is examined for any metal

deposits on it. There should be zero metal deposits on the insulator.