The overhead line insulators provide insulation between the overhead line conductors and the earthed cross-arm to which they are connected 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 insulators in overhead Lines
Depending on the applications and the operating voltage levels ,the overhead line insulators are divided into three types.They are,
(i) Pin type insulators
(ii) Suspension type insulators and
(iii) Strain type insulators
(i) Pin type insulators- Electrical Insulators
Pin type insulators are used for supporting line conductors.For low voltages up to and including 11 KV one piece insulators can be used and two piece insulators are commonly used for 25 KV and for 44 KV up to and including 66 KV three and four piece insulators can be employed.The pin type insulator figure shown in fig.
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.On the top of the insulator the conductor bound into a groove,which is cemented on to a galvanized steel pin.A soft metal thimble is used to avoid the direct contact between the porcelain and the metal pin.The potential difference between the sheds can be increased by increasing distance between the sheds.
Advantages Of Pin type insulators :
1.Pin type insulators are simple in construction and cheap in cost.
2.It provides economic and most efficient method of supporting conductors.
3.In many cases on pin type insulator is used instead of two suspension type insulators.
Disadvantages Of Pin type insulators :
For voltages below 50 KV,they give satisfactory performance ,beyond that,they become bulky and uneconomical.
(ii) Suspension type insulators- Electrical Insulators
At high voltages pin type insulators becomes more bulky,cumbersome and uneconomical.In this case,suspension type insulators are used to insulate the high voltage transmission lines.Normally,these are used up to and including 400 KV .The suspension type insulators shown in fig.
Suspension type insulators consists of porcelain discs arranged in series as shown fig.,by metal links to form a string.The conductor is suspected at its lower end and it's other end hangs from cross-arm of the supporting structure tower.Thw whole arrangement of a suspension type insulators are called as string(String Efficiency).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:
1.If any one of the insulator in the string fails, only that insulator can be replaced instead of whole string.
2.These are much cheaper in cost and used for operating voltages above 50 kV.
3. It gives more flexible operation and by this arrangement the mechanical stresses are reduced.
4. In suspension type insulator, additional insulation can be obtained by adding one or more discs to that string.
5.When these are used in conjunction with the steel towers, provides protection against lightening.
(iii) Strain type insulator- Electrical Insulators
Strain type insulators are used when the line is subjected to greater tension such as dead ends,river crossings,sharp curves and when there is a change in the direction of line.The main function of this insulator is to reduce excessive tension on the line. It is basically an assembly of suspension insulator used in horizontal position as shown in figure.
A strain insulator 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 11 KV,Shackle type insulator can be used.But for high voltages,strain type insulators should be used.strain type insulators also called as tension insulators.
In addition to that there are other two types of electrical insulator available mainly for low voltage application, e.i. Stay Insulator and Shackle Insulator.
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