Insulators Used In Overhead Power Lines
Electrical Overhead Insulator:
- Overhead line insulators are used to isolate the line conductors from each other and the supporting structures electrically.
- Permittivity and dielectric strength of the insulating material are very in such a way that it can withstand high electrical stress.
- Insulators also protect the transmission line from overvoltages that occur due to lightning, switching, or other causes under severe conditions.
Overhead line insulators should have the following properties:
- high mechanical strength in order to withstand the conductor load, wind load etc.
- high electrical resistance in order to minimize the leakage currents
- high relative permittivity of insulating material so that the dielectric strength is high
- high ratio of puncture strength to flashover
There are the following types of insulation used for the overhead line:
- Pin Insulator
- Suspension Insulator
- Strain Insulator
- Stay Insulator
- Shackle Insulator
1. Pin type insulators:
Fig showed below the pin type insulator,
- It is secured to the cross-arm on the pole.
- There is a groove on the upper end of the insulator for housing the conductor.
- The conductor passes through this groove and is bound by the annealed wire of the same material as the conductor.
- Pin-type insulators are used for the transmission and distribution of electric power at voltages up to 33 kV.
- Beyond operating voltage of 33 kV, the pin-type insulators become too bulky and hence uneconomical.
2. Suspension type insulators:
Pin type of insulator is not economical beyond 33 kV. To overcome from it, suspension type insulator is used,
- It consists of a number of porcelain discs connected in series by metal links in the form of a string.
- The conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross-arm of the tower.
- Each unit or disc is designed for low voltage, say 11 kV (RMS Phase).
- The number of discs in series would obviously depend upon the working voltage.
3. Strain insulators:
- When there is a dead-end of the line or there is a corner or sharp curve, the line is subjected to greater tension.
- In order to relieve the line of excessive tension, strain insulators are used.
- For low voltage lines (< 11 kV), shackle insulators are used as strain insulators.
4. Stay Insulator:
- For low voltage lines, the stays are to be insulated from the ground at a height.
- The insulator used in the stay wire is called the stay insulator and is usually of porcelain and is so designed that in case of breakage of the insulator the guy-wire will not fall to the ground.
5. Shackle Insulators
Shackle insulators are used in low voltage distribution lines as strain insulators. A shackle insulator can be used vertically as well as horizontally and it can be directly fixed to a pole with a bolt or to the cross arm. However, the use of such insulators is decreasing after increasing the use of underground cables for distribution purpose.
Some additional types of insulators are: post insulators, glass insulators, polymer insulators, long-rod insulators, stay insulators etc
