Insulators Used for Overhead Lines | Electrical Engineering

In this article we will discuss about:- 1. Introduction to Overhead Line Insulators 2. Insulator Materials 3. Factors 4. Failure.

Introduction to Overhead Line Insulators:

The overhead line conductors are bare and not covered with any insulating covering/coating. The line conductors are, therefore, secured to the supporting structures by means of insulating fixtures, called the insulators, in order that there is no current leakage to the earth through the supports.

Insulators are mounted on the cross-arms and the line conductors are attached to the insulators so as to provide the conductors proper insulation and also provide necessary clearances between conductors and metal work. The insulators must provide proper insulation and necessary clearances against the highest voltage in worst atmospheric conditions to which the line is likely to be subjected.

The insulators also prevent short- circuiting between the different phase conductors and provide necessary mechanical support for the line conductors. Thus insulator is undoubtedly one of the most important and vulnerable links in transmission and distribution practice and, therefore, proper selection is of utmost importance for the successful operation of overhead transmission and distribution system.

The important properties that an overhead line insulator must possess are:

1. High mechanical strength so as to bear the load due to the weight of line conductors, wind force and ice loading if any.

2. High relative permittivity so as to provide high dielectric strength.

3. High insulation resistance in order to prevent leakage of currents to earth.

4. High ratio of rupture strength to flash-over voltage.

5. Ability to withstand large temperature variations i.e., it should not crack when subjected to high temperatures during summer and low temperatures during winter. The dielectric strength should remain unaffected under different conditions of temperature and pressure.

The material used should not be porous and should be impervious to the fluids and gases in the atmosphere. Simultaneously it should be free from internal impurities and cracks etc. as these lower the value of dielectric strength to a great extent.

Insulator Materials:

The material most commonly used for overhead line insulators is porcelain but toughened glass, steatite and special composition materials are also used to a limited extent.

Porcelain is produced by firing at a controlled temperature a mixture of kaolin, feldspar and quartz. It is mechanically stronger than glass. It gives less trouble from leakage, and is less susceptible to temperature variations and its surface is not affected by dirt deposits.

On the other hand, it is not as homogeneous as glass, owing to the fact that each component shell of a porcelain insulator is glazed during manufacturing process and its satisfactory performance in service depends to a considerable extent on the preservation of this glaze which is only of the order of 25 microns in thickness.

Also fault cannot be detected easily as it is not transparent. In tension this material is usually weak and does not withstand tensile stresses exceeding 5 kg/mm². The dielectric strength and compressive strength of a mechanically sound porcelain insulator are about 6.5 kV/mm of its thickness and 700 kg/ mm² respectively.

Normally, it is difficult to manufacture homogeneous porcelain in the thickness required for some types of insulators and, therefore, for a particular operating voltage, a two or more piece construction is adopted in which each piece is fired and glazed separately and then they are cemented together.

If this insulating material is manufactured at lower temperature, its mechanical prop­erties improve but the material remains porous and on putting it in service it may deterio­rate. If this material is manufactured at high temperature, its porosity is reduced but the material becomes brittle.

The porosity of this insulating material also reduces its dielectric strength, also any impurity or air bubble left within the material results in a lower dielectric strength. So a compromise is made between the mechanical strength and the porosity of the material and a suitable temperature of the kiln is designed.

Glass is cheaper than porcelain in the simpler shapes and if properly toughened and annealed gives high resistivity and dielectric strength (14 kV per mm of thickness of the material). Owing to high dielectric strength, the glass insulators have simpler design and even one piece design can be used.

Glass is quite homogeneous material and can withstand higher compressive stresses as compared to porcelain. It has also a lower coefficient of thermal expansion which minimises the strains due to temperature changes and owing to its transparent nature flaws in the material can be readily detected by visual examination.

The main disadvantage of glass is that moisture more readily condenses on its surface and facilitates the accumulation of dirt deposits, thus giving a high surface leakage. Also in large sizes the great mass of material combined with the irregular shape, may result in internal strains after cooling. Glass insulator however, can be used up to 25 kV under ordinary atmospheric conditions and well up to 50 kV in dry atmosphere.

Steatite is a naturally occurring magnesium silicate, usually found combined with oxides in varying proportions. It has a much higher tensile and bending stress than porcelain and can advantageously be used at tension towers or when the transmission line takes a sharp turn.

The special artificial material is used in insulators for low voltages and has an important advantage that these can be easily moulded into any shape without any internal stress.

Metallic fittings can also be firmly embedded in the material without any difficulty. The disadvantage of insulators made from special artificial material is that they deteriorate rapidly in bad climatic conditions and on being subjected to flash-over their carbonised surface forms a conducting path.

Factors Involved in Overhead Line Insulator Design:

Overhead line insulators are required to withstand both electrical and mechanical stresses. In addition, the surface leakage path must have sufficiently high resistance so as to avoid any current leakage to earth. The design of the insulators must be such that the stress developed owing to contraction and expansion in any part of the insulator does not lead to any defect.

Electrical breakdown may occur either by flash-over or puncture. In a flash-over, an arc occurs between the line conductor and earth (i.e., supporting pin of the insulator) and the discharge jumps across the air-gaps in its path.

In a puncture, the discharge occurs from conductor to pin through the body of the insulator. In case of electrical breakdown due to flash- over, the insulator continues to act in its proper capacity after the event unless fractured by the heat of the arc, but after a puncture, it is permanently damaged due to excessive heat.

Thus it is very important to provide sufficient thickness of porcelain in the insulator to resist puncture by the combined effect of the line voltage and any probable transient voltage rise on the line. The ratio of puncture strength to flash-over voltage, termed as the factor of safety, must be high so as to provide a good margin for the protection of insulators from complete failure.

It is desirable that porcelain may not come in direct contact with a hard metal screw thread. Normally cement is used between metal and the porcelain. The cement so used must not cause any fracture by expansion or contraction.

Arcing Horn and Grading Ring:

In the event of flash-over the insula­tor is cracked or broken up due to the heat of the arc. Grading ring, in ad­dition to equalisation of voltage distri­bution across the insulator units, when used in conjunction with arcing horn fixed at the top end of the string serves the purpose of arcing shield and pro­tects the insulator string from flash-over whenever over-voltage (under normal or abnormal condition) appears between the tower structure and the line conductor.

They are designed to keep the arc away from the insulator string until it is interrupted by the device protecting the line. The arrange­ment of the arcing horns on a 7-unit string of suspension insulators. The combination of the arcing horn and grading ring provides path through the air medium and dis­charges the energy contained in the abnormal voltage and thus saves the insulator string.

Failure of Overhead Line Insulators:

Failure of overhead line insulators, resulting in an interruption in the continuity of power supply, is a matter of serious concern to the power engineers. It may occur due to cracking of porcelain, porosity, puncture, mechanical stresses, flash-over etc.

1. Cracking of Insulators:

This failure is very common in case of pin type insulators and the cemented-cap type suspension insulators. This occurs due to unequal expansion of steel, porcelain and cement during the varying conditions of cold and heat and dryness and damp­ness. This develops high stresses in the porcelain near the joint and it results in tension failure. This can be avoided to some extent by using elastic cushions between the shells.

2. Porosity of Material:

Porosity in the porcelain which may be due to under-firing or other causes, always leads to failure after a comparatively short period of service. The pores usually absorb moisture from the atmosphere or the cement, thereby decreasing the insula­tion resistivity of the material. This gives rise to leakage current flowing through the porcelain, resulting in a gradual rise in the temperature until the porcelain is punctured. Such a failure can be avoided by glazing the insulator, to some extent.

3. Improper Vitrification:

This is another cause of the puncture of the material and it can be avoided by carrying out suitable routine tests during the course of manufacture.

4. Flash-Over:

The most common cause of the insulator failure is the flash-over that causes unequal expansion of the porcelain thereby shattering the insulator with big cracks and causing interruption of the supply. This can be avoided by providing arcing horns or rings which take up the arc and divert it away from the insulators.

5. Mechanical Stresses:

Although the compressive strength of the porcelain is quite high, yet its tensile strength is not adequate and the insulator is always weak in tension and usually fails in that fashion. Such a failure is very rare because defective pieces are weeded out in the routine factory tests.

6. Short-Circuits:

In the case of pin insulators, the birdage is very common. Birdage means the short-circuiting of the conductor to earth through the large birds or similar objects. Such a situation can be avoided by providing bird guards near the insulator on the cross-arm, by increasing the clearance of the conductor from earthed parts or using suspen­sion insulators, instead of pin type, where the clearance between the earthed structure and the conductor is very large to warrant any birdage.

7. Deposition of Dust:

If the insulator material is not properly glazed, the water will stick over it resulting into deposition of dust etc. over it which is partially conducting and reduces the flash-over distance. The deposits of dust and like matter e.g. salt, cement, dust etc. on the interior surfaces can actually cause much havoc under the condition of fog and mist. This can be easily avoided by cleaning the insulators periodically.