January 26, 2017

Describe the types of Insulators used in Transmission and Distribution .


An electrical insulator is a material whose internal electric charges do not flow freely, and therefore make it nearly impossible to conduct an electric current under the influence of an electric field. This contrasts with other materials, semiconductors and conductors, which conduct electric current more easily. The property that distinguishes an insulator is its resistivity; insulators have higher resistivity than semiconductors or conductors.

Insulators are used in electrical equipment to support and separate electrical conductors without allowing current through themselves. An insulating material used in bulk to wrap electrical cables or other equipment is called insulation. The term insulator is also used more specifically to refer to insulating supports used to attach electric power distribution or transmission lines to utility poles and transmission towers. They support the weight of the suspended wires without allowing the current to flow through the tower to ground.

Types of insulators:

There are so many insulators which are recently used but these are most preferable insulators used in transmission lines . These are the common classes of insulator:

A.   Pin Type Insulator

As its name indicates, the pin type insulator is mounted on a pin which is fixed on the pole or tower. The insulator has a groove where the conductor lies, which secures the conductor to the insulator. The conductor placed on the top of the pin insulator is at live potential, whereas the bottom of the pin is at contact with the supporting structure which is usually at earth potential. This type of insulator is used for low and medium voltage distribution lines up to 33 kV. In distribution systems up to 11 kV, a one part type insulator is generally used, where whole pin insulator is one piece of properly shaped porcelain or glass.  It is important to know that pin insulators cannot be fixed one above the other for higher voltage applications.
Figure 1- Pin Type Insulator on distribution pole.

B. Post insulator

A type of insulator in the 1930s that is more compact than traditional pin-type insulators and which has rapidly replaced many pin-type insulators on lines up to 69kV and in some configurations, can be made for operation at up to 115kV.
Figure 2- Porcelain line post insulators.
This insulator can be used for both indoor and outdoor applications. The conductor contact is on both top and bottom. There are two main types: pedestal post insulator and solid core cylindrical post insulator.

C. Suspension Insulator

The suspension type insulator is utilized for high voltage transmission lines (greater than 33 kV), where the line is located bellow the point of support. Multiple insulators are connected in series, each of which is called a disc. The number of disc units used depends on the voltage. Its main advantages are simplicity in design, high mechanical strength, the ability to change individual discs in case of puncture, and the transmission line does not fall if porcelain breaks accidentally.
Figure 3- Pin and cap insulator in suspension configuration.

D.  Strain (Tension) Insulator

A strain insulator is an electrical insulator that is designed to work in mechanical tension (strain), to withstand the pull of a suspended electrical wire or cable. Therefore, they should be able to withstand very high amounts of mechanical tension. They are usually used at line terminal and at dead end. When the tension load in lines is exceedingly high, such as at long river spans, two or more strings are used in parallel.
Figure 4 - Strain Insulator.
In order to support this lateral load, strain insulators are used. For low voltage lines (less than 11 kV), shackle insulators are used as strain insulators. However, for high voltage transmission lines, strings of cap-and-pin (disc) insulators are used, attached to the crossarm in a horizontal direction.

E. Shackle insulator

In early days, the shackle insulators, also known as spool insulators,  were used as strain insulators. But now a day, they are frequently used for low voltage distribution lines. Such insulators can be used either in a horizontal position or in a vertical position. They can be directly fixed to the pole with a bolt or to the cross arm.
Figure 5- Shackle Insulator.

Shackle Insulators are used in low voltage distribution systems. Usually, they are used at the end of distribution lines or at sharp turns where there is excessive tensile load on the lines.

F.  Bushings

A bushing is an insulating structure, including a through conductor or providing a central passage for such conductor, with provision for mounting a barrier, conducting or otherwise, for the purpose of insulating the conductor from the barrier and of an electrical current from one side of the barrier to the other.
Figure 6- Bushings.

Bushings have many types, including: liquid-filled bushing, liquid-insulated bushing, gas-filled bushing, gas-insulated bushing, oil-impregnated paper bushing, and others.

G. Long Rod  Insulator

Long rod insulator are similar to solid core cylindrical insulator except that the top and bottom fittings are of pin and cap type. Long rod insulators provide an alternative to cap and pin insulators but with longer unit length. Similar to the Pin and cap insulator, it can be used in both suspension and tension (strain) configurations.  Long rod insulators are high performance insulators. According to Elsewedy Electric, long rod insulators are “absolutely puncture-proof” and have “excellent anti-pollution performance”.
Figure 8- Long rod type insulator.

H. Cap and Pin Insulators

Cap and pin insulators of porcelain or glass construction are used for applications above 33 kV, either in suspension or tension (strain) configuration. Any insulator length can be achieved by arranging a number of single disk units in a string.

I. Line post insulator

J. Station post insulator

K. Cut-out

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