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.
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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.
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
• 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
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