Introduction:

The insulator for overhead lines provides insulation to the power conductor from the ground. The insulators are connected to the cross arm of the supporting structure and the power conductor passes through the clamp of the insulator. These insulators are mainly made of either glazed porcelain or toughened glass. The materials used for porcelain are silica 20%, feldspar 30%, and clay 50%. The porcelain should be ivory white, sound and free from defects. It should be vitrified because the presence of pores or air in the porcelain will lower down its dielectric strength. Any sealed air impurity will also lower the dielectric strength of porcelain. It is, therefore, desirable that porcelain to be used for insulator should be air free and impervious to the entrance of liquid and gases. The dielectric strength of porcelain should be 15kV to 17kV for every one tenth inch thickness. Normally it is difficult to manufacture homogeneous porcelain and therefore for a particular operating voltage two, three or more pieces construction is adopted in which each piece glazed separately and then they are cemented together. Porcelain is mechanically strong and less effected by temperature and has minimum leakage problem.

Toughened glass is also sometimes used for insulators because it has higher dielectric strength ( 35kV for one-tenth inch thickness ) which makes it possible to make use of single piece construction, whatever be the operating voltage. Glass being transparent, it is very easy to detect any flaw like trapping of air etc. it has lower co-efficient of thermal expansion and, as a result the strains due to temperature changes are minimized. The major drawback of glass is that moisture condenses very easily on its surface and hence its use is limited to about 33kV.

The design of the insulator is such that the stress due to contraction and expansion in any part of the insulator does not lead to any defect. It is desirable not to allow porcelain to come in direct contact with a hard metal screw thread. Normally cement is used between metal and the porcelain. It is see that cement to use does not cause fracture by expansion or contraction.

Types of insulators:

There are three types of insulators used for overhead lines

Pin type

Suspension type

Strain type

Pin type insulator consist of a single or multiple shells ( petticoats or rain sheds ) adapted to be mounted on a spindle to be fixed to the cross arm of the supporting structure. Multiples shells are provided in obtain sufficient length of leakage path so that the flash over voltage between the power conductor and the pin of the insulator is increased. The design of the shell is such that when the upper most shell is wet due to the rain the lower shell are dry and provide sufficient leakage resistance. It is desirable that the horizontal distance between the tip of the lowermost shell should be less as compared with the vertical distance between the same tip and the cross arm, otherwise in case of an arc-over, the discharge will take place between the power conductor and the pin of the insulator, thereby, the cross arm will have to be replaced rather than the insulator. It is to be noted that the power conductor passes through the groove at the top of the insulator and is tied to the insulator by the annealed wire of the same material as the conductor. The pin

type insulator is normally used up to 33kV. In any case it is not desirable to use them beyond 50kV as the cost of such insulators then increase much faster than the voltage. The cost beyond 50kV is given by

Cos Vx ( x > 2)

The insulator and its pins should be sufficiently mechanically strong to withstand the resultant force due to the combined effect of the weight of the conductor, wind pressure and the ice loading if any per span length.

The pin type of insulators is uneconomical beyond 33kV operating voltage. Also the replacement of these insulators is expensive. For these reasons for insulation overhead lines against higher voltages, suspension insulators are used.

Theses insulators consists of one or more insulator units flexibly connected together and adopted to be hung for the cross arm of the sporting structure and to carry a power conductor at its lowest extremity. Such composite units are known as string insulators. Each insulator is a large disc shape piece of porcelain grooved on the undersurface to increase the surface leakage path between the metal cap at the top and the metal pin at the bottom of the insulator. The cap at the top is increased so that it can take the pin of another unit and in this way a string of any required number of insulators can be built. The cap and the pin are squired by means of cement. The standard unit is 10n X 5.75n in size. The diameter is taken as 10n

as it gives optimum spark over to puncture voltage ratio. Increasing the diameter further increases the flash over or spark over voltage but it lower the above ratio which is undesirable.

Suspension insulators being free to swing, the clearances required between the power conductor and the suspension structure are more as compared to pin type insulators. This means the length of the cross arm for suspension insulators is more as compared with the pin type.

The suspension insulators, in addition to being economical as compared to pin type for voltage more than 33kV, have the following further advantages,

Each insulator is designed for11kV and hence for any operating voltage a string of insulators can be used

In case of failure of one of the units in string, only that particular unit needs replacement rather than the whole string.

Since the power conductor and string swing together in case of wind pressure, the mechanical stress at the point of attachment are reduced as compared with the pin type of insulator where because of the rigid nature of the attachment fatigue and ultimate brittleness of the wire result.

The operating of the existing transmission can be increased by adding suitable number of discs in the string instead of replacing all the insulators as is necessary in case of pin type insulators.

The strain insulators are exactly identical in shape with the suspension insulators. Theses strings are placed in the horizontal plane rather than vertical. As is done in case of suspension insulators. Theses are used to take the tension of the conductors at line terminals, at angle towers, at road crossings and at junctions of overhead lines with cables. These insulators are therefore known as tension or strain insulators. For low voltage of the order of 11kV, shackle insulators are used. But for higher voltages a string

of insulator is used. Whenever the tension in the conductor is very high as at long river crossing, some times two, even three, string of insulators in parallel have been used.

Advantages and disadvantages of grading of units methods:

In string with graded unit method difference capacities are required. This requires large stocks of difference size units, which is uneconomical and impractical. Therefore this method is normally not used except for very high voltage lines. But it is very economical, very easy and very simple method used in very high voltage transmission systems.

String with identical unit and graded ring method is very simple method. It can be easily taken the same voltage distribution throughout the string accurately than string with graded units method. But it is difficult to find the capacitors which have various values. Therefore little bit errors can be occurred. And also it should be used another conductor, that is an uneconomical reason to remove this method in some applications.

Reasons for difference between theoretical and practical values :

The capacitors which we used may not be worked correctly and sometimes they may not be represented the actual values. The components which we used for this practical can be worked with errors. Human errors also can be happen. In our model we missed the lowermost capacitance which is in between the conductor and the ground. So it can be made some errors.

References:

Electrical Power Systems ( Third Edition ) - by C. L. WADHWA

SUSPENSION INSULATOR STRING

NAME: SENEVIRATHNE M. R. K. C.

INDEX NO: 050424X

DATE OF PERFOMANCE : 28 / 06 / 07

DATE OF SUBMISSION : 11 / 07 / 07