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What are the Types of Insulators Used in Transmission Lines?
The element of the transmission line by which the line conductors are insulated from supports is known as line insulator. The insulators used in the transmission lines are of various types. Some of the important types of insulators are given as follows −
Pin Type Insulator
Suspension Type Insulator
Pin Type Insulators
The pin type insulator is shown in the figure. The pin type insulator is fitted to the cross-arm on the pole. The pin type insulator has a groove on the upper end of the insulator for housing the line conductor. The line conductor passes through this groove and is bounded by the annealed wire made up of the same material as the line conductor.
The pin type insulators are mainly used for the transmission and distribution of electrical power at the voltages up to 33 kV. If the working voltage is greater than 33 kV, then the pin type insulators become too bulky and hence uneconomical.
Also, for the pin type insulators, the value of safety factor (i.e. ratio of puncture strength to flash-over voltage) is about 10.
Suspension Type Insulators
The suspension type insulator is shown in the figure. The suspension type insulators are generally used with the steel towers. These insulators consist of a number of porcelain discs connected in series by metal links in the form of a string. The line conductor is suspended at the bottom end of this string and the other end of the string is fixed to the cross-arm of the steel tower.
It is a usual practice to use suspension type insulators when the operating voltage is high (> 33 kV). In the suspension type insulator, each insulator disc is designed for low voltage (say 11 kV). Therefore, the number of discs in a string would depend upon the working voltage. For example, if the operating voltage is 132 kV, then 12 discs in series will be provided on the string.
The chief advantages of the string insulators are as follows −
The suspension type insulators are economical when the operating voltage is greater than 33 kV.
If any disc of the string is damaged, the whole string does not become useless since the damaged disc can be replaced by the new one.
The suspension arrangement provides greater flexibility to the transmission line. It is because the connection of the string at the cross-arm is such that the insulator string is free to swing in any direction and can take up the position where mechanical stresses are minimum.
If the demand on the transmission line is increased, then the voltage level of the same transmission line can be increased by providing another set of conductors and the addition insulation required by the raised voltage can be easily obtained in the suspension arrangement by adding the desired number of discs.
In case of suspension type insulator, the line conductors run below the earthed cross-arm of the tower, hence, the suspension type insulator provides partial protection from lightening.
The strain insulators are used when there is a dead end of the transmission line or there is a corner or sharp curve and the line is subjected to the greater tension. The strain insulator is shown in the figure.
For low-voltage transmission lines (say < 11 kV), shackle insulators are used as the strain insulators while for the high voltage lines, the strain insulator consists of an assembly of suspension insulators. The insulation discs of the strain insulators are used in the vertical plane. In the places, where the tension in the transmission line is exceedingly high, then two or more strings of discs are used in parallel.
Shackle insulators are mainly used in low-voltage distribution lines. The shackle insulators can be used either in a horizontal position or in a vertical position. The shackle insulators can be directly fixed to the pole at the cross arm and the line conductor in the grove is bounded by a soft binding wire.
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