Insulators

Introduction: The The insu insula lato torr for for over overhe head ad line lines s prov provid ides es insu insula lati tion on to the the powe powerr condu conducto ctorr from from the groun ground. d. The ins insula ulator tors s are are connec connected ted to the cros cross s arm arm of the suppor supportin ting g struct structur ure e and the power power conduc conductor tor passes passes throu through gh the clamp 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, two, thr three or more more piec pieces es cons constr truc ucti tion on is adop adopte ted d in whic which h each each piec piece e glaz glazed ed separately and then they are cemented together. Porcelain is mechanically strong and less effected by temperature and has minimum leakage problem. Toughened oughened glass is also sometimes sometimes used for ins insulat ulators ors because because it has higher dielectric dielectric strength strength ( 35kV for one-tenth one-tenth inch thickness thickness ) which makes makes it possible possible to make use of single piece construction, whatever be the operating voltage. Glass being transparent, transparent, it is very easy to detect any flaw like trapping of air etc. it has lower co-effi co-efficie cient nt of ther thermal mal expan expansio sion n and, and, as a resu result lt the strain strains s due due to temper temperatu ature re 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 metal and the porcelain. porcelain. It is see that cement to use does not cause fracture by expansion or contraction.

Types of insulators: i nsulators: 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 sufficient leakage resistance. It is desirable that the horizontal distance between the tip of the lowermost shell should be less as

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comp compar ared ed with with the the vert vertic ical al dist distan ance ce betw betwee een n the the same same tip tip and and the the cros cross s arm, 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. 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 resul resultan tantt force force due due to the combin combined ed effec effectt of the weight weight of the the conduc conductor tor,, wind wind pressure pressure and the ice loading if any per span length. The pin type of insulators is uneconomical 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. The These ses s insu insula lato tors rs cons consis ists ts of one one or mor more insu insula lato torr unit units s flex flexib ibly ly 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 10 n X 5.75n in size. The diameter is taken as 10 n 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. Suspe Suspens nsion ion ins insula ulator tors s being being free free to swing swing,, the cleara clearance nces s requi require red d 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.





The operating of the existing transmission can be increased by adding suit suitab able le numb number er of disc discs s in the the stri string ng inst instea ead d of repl replac acin ing g all all the 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 strin string g with with graded graded unit unit method method differ differenc ence e capaci capacitie ties s are are requi require red. d. This 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 very econ econom omic ical al,, very very easy easy and and very very simp simple le meth method od used used in very very high high volt voltag age e 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.



SUSPENSION INSULATOR STRING



NAME: SENEVIRATHNE M. R. K. C. INDEX NO: 050424X DATE OF PERFOMANCE : 28 / 06 / 07 DATE OF SUBMISSION : 11 / 07 / 07

Insulators

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