Types of Insulators

Types of Insulators

The use of proper insulator is an important part of the mechanical design of the overload lines. The various types of the insulators are,

1. Pin type insulators

2. Suspension type insulators

3. Strain insulators

4. Shackle insulators

5. Stay insulators 

 

1. Pin Type Insulators

A typical pin type insulator is shown in the Fig. 5.3.1.

For lower voltages upto 11 kV generally one piece pin type insulator is used.

But for higher working voltages like 33 kV, 45 kV, 66 kV and beyond it two piece, three piece, four piece pin type insulators can be used. But its use is restricted upto 33 kV as for higher voltages, the pin insulators are uneconomical. The pin insulators become very bulky for higher voltages.

On the upper end, there is a groove for housing the conductor. The pin insulators are very firmly secured to the cross arm on the transmission pole with the help of steel bolts. To avoid the direct contact of hard metal with porcelain, the lead screws are used. In such case two methods are used to secure insulator to the bolt.

1. The porcelain insulator has cement threads which are lined with a soft material like lead. The pin is screwed into such cement screw.

2. Solid lead screw is casted on the head of the pin and is screwed directly into the porcelain.

Such type of insulators are used for the transmission lines which are running straight.

The conductor passes through the groove on the upper end and is bound by the binding wire of soft copper or soft aluminium i.e. of the same material as that of conductor. This is shown in the Fig. 5.3.2. The binding wire should cover at least 4 to 5 turns before its ends are wrapped with the conductor.

When the insulators are wet their outer surface is Fig' 5'3'2 Attachment of conductor almost conducting. To keep the inner side of the insulator dry, the rain sheds are provided or now a days surfaces are provided with petticoats. But design of these sheds or petticoats is such that their surface must lie along the equipotential surfaces of the electrostatic fields while the body is so shaped to lie along the lines of the electrostatic fields. Due to this, leakage resistance of each shell remains almost same, keeping  capacitance of each shell also same. Due to this, there is uniform voltage distribution along the shells. Such petticoats help to avoid flashover and leakage current also.

 

2. Suspension Type Insulators

As the voltage level increases, pin type insulators become very bulky and their cost also increases rapidly. Hence the most popular insulators used for very high voltage transmission lines are suspension type insulators. These insulators have number of porcelain disc units. These units are connected to one another in series with the help of metal links. This forms a string of porcelain discs. The top most insulator unit is connected to the cross arm of the tower while the lowest insulator is made to hold the conductor along the conductor shoe. Each unit is designed for the low voltage for say 11 kV but a string of such units give us the proper insulation against very high voltage levels.

The two important types of suspension type insulators are,

1. Cemented cap type

2. Hewlett or inter-linking type

The cemented cap type is the most commonly used suspension type insulator. The Fig. 5.3.3 shows the cemented cap type suspension insulator.

It consists of a single disc shaped piece of porcelain. At the bottom, it is grooved so as to increase the flash over distance. A galvanised cast iron cap is cemented at the top. The space is provided in the cap, which can be used to hold the pin of another unit. The cap is cemented to the insulator. And the pin is either cemented or connected by means of steel wire spring ring. The main drawback of this type is that the cubical expansions of three materials iron, porcelain and cement are different from each other and due to this fine cracks in the insulator and early failure is possible.

The Hewlett type insulator is more simple in design. It consists of a porcelain disc. The top portion of the disc consists of two curved tunnels, the planes of which are at right angles to each other. Lead covered steel U links are passed through the tunnels. These links are bolted to the two similar units at the top and bottom. No cementing is required in this type of insulator. The mechanical strength of this type is also very high due to the use of steel links. Another advantage of this type is that even if porcelain  breaks due to the links, the units are held togther and there is no interruption in the working. But the main disadvantage of this type is that the porcelain in between is under high electrostatic stress and hence there is possibility of puncture earlier than the cemented cap type. Hence cemented cap type is more preferred than Hewlett type. The Hewlett type insulator is shown in the Fig. 5.3.4 .

The overall string of suspension type insulators is shown in the Fig. 5.3.5.

The top unit is fastened to the cross arm of the tower. The lowest unit holds the line conductor. This bottom most unit, which is nearest to the line conductor is also called line unit.

a. Advantages of Suspension Type Insulators

The various advantages of the suspension type insulators are,

1. For higher voltages, these are cheaper than the pin insulator.

2. Each unit is designed for low voltage level such as 11 kV but by connecting such units in series to form a string, insulator for any higher voltage level can be designed. 

3. In case of failure of any of the units, the replacement work can be done very easily and entire string need not be replaced.

4. If the line voltage is required to be increased at some later stage to satisfy increased load demand then just by adding additional units to the string, same insulator can be used. Adding such units is very easy.

5. This type of insulator provides greater flexibility to the line. The string is suspended and is free to swing in any direction. So it takes the position so that mechanical stresses on the line are minimum.

6. When used with the steel towers, the line conductors are less affected by lightning. This is because the conductor is lower than the earthed cross arm and the arrangement acts as a lightning arrester.

b. Comparison of Pin and Suspension Type

The comparison is given in the following table,

 

3. Strain Insulators

These insulators are used when there is dead end of the line or corner or line is at a sharp curve or the line is crossing the river etc. These insulators reduce the excessive tension on the line under such abnormal conditions. For low voltage lines below 11 kV shackle insulators are used but for higher voltages strain insulators are used. Assembly of the suspension insulators is used as a strain insulator. The discs of the strain insulators are in a vertical plane instead of the horizontal plane as in the suspension insulators. In case of conditions like crossing of the river, there is excessive tension on the line. In such a case, two or more strings of the insulators are used in parallel.

The Fig. 5.3.6 (a) shows the use of strain insulator. The Fig. 5.3.6 (b) shows the plan of the line.

 

4. Shackle Insulator

These are also called spool insulators. These are primarily used for low voltage distribution lines. These insulators can be used in horizontal position or in vertical position. These are used at the dead end of the aerial wire of service connection to a house or a factory where there is excessive mechanical stress on the line.

The Fig. 5.3.7 shows the shackle type insulator.

The insulator is round. It has through hole in the centre for the bolting purpose. On each side of the insulator, there is galvanised iron plate of 25 mm wide. The other end of plates are placed around the cross arm of the channel or pole. The conductor is in the groove and it is secured with the help of soft binding wires. Similar to the strain insulators, these insulators are effective when there is dead end of the distribution line or the distribution line changes its angle. 

 

5. Stay Insulators

The stay insulators are also called egg insulators. In case of low voltage lines, it is necessary that the stays are to be insulated at a height of not less than 3 meters from ground. The stay insulators are used on stay wire to create insulation between pole and stay clamp. It is usually made of porcelain. It has two holes for the stay wires and the design is such that in case the insulator breaks then the stay wire will not fall on the ground.

The Fig. 5.3.8 shows the stay insulator and the method of inserting the stay wires into the stay insulator.

Review Questions

1. Write a note on different types of overhead insulators.

2. Give the list of various insulators and compare pin and suspension type insulators.

3. Draw and explain the construction of Hewlett type suspension type insulator.

4. With the help of neat figure, explain pin type insulator.

5. Draw with neat sketches and explanation of pin and suspension type insulators. Compare their merits and demerits.