Types of insulators are used in an overhead transmission and distribution line to provide insulation to power conductors from the ground and tower or supporting structure. like Pin type of insulators, Suspension, and Strain type insulators.
There are 5 types of insulators used in transmission lines as overhead insulation
- Pin type Insulator
- Suspension type Insulator
- Strain type Insulator
- Shackle Insulators
- Stay Insulators
Table of Contents
Generally, insulators are connected to cross an arm of a supporting structure, and the power conductor passes through the clamp of the insulator.
Pin, Suspension, and Strain types of insulators are used in medium to high voltage lines. While Stay and Shackle Insulators are mainly used in low voltage applications.
Insulators have an important role in the successful operation of the transmission lines. insulators used in the power system must have high mechanical strength and high dielectric strength to withstand conductor weight and operating voltage. Insulators must have high resistance to avoid leakage of current to earth.
Insulators are made up of either porcelain or glass, the material used for porcelain are silica 20% feldspar 30%, and clay 50%. The porcelain that is used for making insulators must be free from defects and impurities, it should be checked because the presence of pores or air bubbles will lower down dielectric strength of insulators.
The dielectric strength of porcelain should be 15KV to 17KV for every one-tenth inch of thickness. porcelain is used for manufacturing insulators because it is mechanically strong less affected by temperature and has a minimum leakage problem.
Sometimes the glass is also used for insulation purposes because it has higher dielectric strength which makes it possible to use single-piece construction.
Glass is transparent because it is very easy to find the defects and pores in the glass, which leads to a reduction in the dielectric strength of insulators.
The major drawback of the Glass type of insulator is moisture gets condensed very easily on its surface which limits its use up-to 33 KV operating voltage.
Pin type insulator
Pin type insulator consists of single or multiple shells that are mounted on the spindle to be fixed to the cross arm of the tower.
Multiple shells are provided to obtain a sufficient length of leakage path so that flash overvoltage between conductor and pin type insulator is increased.
The design of shells is made such that if the uppermost shell gets wet due to rain, the lower shells are dry and provide sufficient leakage resistance.
In pin types of insulators, the power conductor passes through the groove at the top of the insulator.
Pin type insulator is used for transmission and distribution of electric power at voltage up to 33kv beyond 33kv operating voltage, pin type insulator become too bulky and uneconomical.
The cost of insulators beyond the 33kv increase is given by
Cost is directly proportional to VX.
Advantages of Pin Insulator
- Pin type insulator has high mechanical strength.
- It has a good creepage distance.
- Pin type insulator used on the high voltage distribution line.
- Construction is simple and requires less maintenance.
- Pin type insulators can be used vertically as well as horizontally.
Disadvantages of Pin Insulator
- Pin type insulator used up to 33kv operating voltage
- Beyond 33kv costs increase and become uneconomical.
- Replacement of insulators is expensive
- Pin type insulators are only used in the distribution line.
Suspension type insulator
Because of the limitations of pin type insulators for higher voltage, suspension type Insulators are used.
Suspension type insulator consists of one or more insulator units connected together to form a string and hung to the cross arm of the supporting structure or tower and carry a power conductor at its lowest extremity. This type of composite unit is also called a string type insulator.
The conductor is suspended at the bottom end of the string while the other end of the string is attached to the cross arm of the supporting structure.
In suspension types of insulators, each disc of porcelain is designed to operate at 11KV, as operating voltage increases the number of discs connected in series increases. e.g 132 KV transmission the number of insulator disc required is 12.
Advantages of suspension type insulator
- Suspension type insulators are economical as compared to pin type insulators for high-voltage overhead transmission line
- Each insulator disc is designed to operate for 11KV. hence for any operating voltage, a string of insulators can be used. for example, for 66kv 6 discs are required.
- In case of failure of one of the unit in a string only that unit/disc need to be replaced rather than the whole string
Strain type insulators
When there is a dead-end of the transmission line or there is a corner or sharp curve on the line, the transmission line is subjected to greater tension. In order to relieve the line of excessive tension, strain type insulators are used.
For low voltage lines (< 11 kV), shackle types of insulators are used as strain insulators. However, for high voltage transmission lines, strain insulator consists of an assembly of suspension insulators as shown in Figure.
The discs of strain insulators are used in the horizontal plane. When the tension in lines is exceedingly high, as, at long river spans, two or more strings are used in parallel.
The shackle or spool types of insulator are easily distinguished by their shape and are usually used on LT lines both low voltage conductors and the house services wire are connected to the shackle insulators.
Stay types of insulators are looks like egg hence also called as egg insulators and used in guy cables were it necessary to insulate the lower part of guy cable from the pole for the safety of people on the ground.
stay insulator installed at 3m above ground level. Stay insulators give protection in the event of accidentally broken live wire that can accidentally energizing a stay wire and remain in contact with the line which doesn’t trip. In such cases.
The bottom portion of the stay would have no voltage due to insulation, Stay insulator will normally be installed in the middle of the stay wire.