The Electrical insulators in overhead lines provide insulation between the overhead line conductors and the earth cross-arm to which they are connect and also provide support to the conductors. An ideal insulator should not allow leakage of current from the line conductor to the earthed cross-arm besides providing adequate clearance between the line conductors and metal work.
Types of Electrical insulators in overhead Lines
Depending on the applications and the operating voltage levels, the overhead line insulators are divide into three types.
A) Pin type insulators
B) Suspension type insulators and
C) Strain type insulators
D) Shackle or spool type Insulator
A) Pin type insulators:
These insulators is for supporting line conductors for low voltages up to and including 11KV one piece insulators can be use and two piece insulators are commonly use for 25KV and for 44KV up to and including 66KV three and four piece insulators can be employed.
Meanwhile, Pin type insulator consists of single or multiple shells arranged in parallel and mounted on a cross-arm of the pole or tower. The designing of sheds or petticoats should be such that even when the outer surface is wet due to rain, the inner dry surface provides sufficient leakage resistance. Therefore,On the top of the insulator the conductor bound into a groove, which is cement on to a galvanize steel pin. Therefore, A soft metal thimble is use to avoid the direct contact between the porcelain and the metal pin. Meanwhile, The potential difference between the sheds can be increase by increasing distance between the sheds.
Advantages of Pin type insulators:
- Pin type insulators are simple in construction and cheap in cost.
- It provides economic and most efficient method of supporting conductors.
- In many cases on pin type insulator is use instead of two suspension type insulators.
Disadvantages of Pin type insulators:
For voltages below 50KV, they give satisfactory performance, beyond that, they become bulky and uneconomical.
B) Suspension type insulators:
At high voltages pin type insulators becomes more bulky, cumbersome and uneconomical. In this case, suspension type insulators are use to insulate the high voltage transmission lines. Therefore, Normally these are use up to and including 400KV. The suspension type insulators is in below figure.
Suspension type insulators consist of porcelain discs arranged in series as shown figure, by metal links to form a string. The conductor is suspect at its lower end and it’s other end hangs from cross-arm of the supporting structure tower. Meanwhile,The whole arrangement of a suspension type insulators are call as string. The number of insulators in a string depends upon the working voltage, weather condition, size of insulation etc., and if necessary, the discs to that string can be added easily.
Advantages of Suspension type insulators:
- If any one of the insulator in the string fails, only that insulator can be replaced instead of whole string.
- These are much cheaper in cost and used for operating voltages above 50kV.
- It gives more flexible operation and by this arrangement the mechanical stresses are reduce.
- In suspension type insulator, additional insulation can be obtain by adding one or more discs to that string.
- When these are use in conjunction with the steel towers, provides protection against lightening.
C) Strain type insulators:
Strain type insulators is use when the line is subject to greater tension such as dead ends, river crossings, sharp curves and when there is a change in the direction of line. Meanwhile,The main function of this insulator is to reduce excessive tension on the line. It is basically an assembly of suspension insulator use in horizontal position as shown in figure.
Strain insulators are designed with considerable good strength and with necessary dielectric properties. In case, when the tension is exceedingly high, two or more strings of insulator can be used in parallel. For voltages up to 11KV, Shackle type insulator can be used. But for high voltages, strain type insulators should be used. Strain type insulators also called as tension insulators.
D)Shackle or spool type Insulator
The shackle insulator (also known as a spool insulator) is usually used in low voltage distribution network. It can be used in both the horizontal or vertical positions. The use of such insulator has decreased recently after increasing the using of underground cable for distribution purpose.
The tapered hole of the spool insulator distributes the load more evenly and minimizes the possibility of breakage when heavily loaded. The conductor in the groove of shackle insulator is fixed with the help of soft binding wire.