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Power system (13) - NOTES
Power Electronics (203122357)
Parul University
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the greater demand by raising the line voltage than to provide another set of conductors. The additional insulation required for the raised voltage can be easily obtained in the suspension arrangement by adding the desired number of discs. (vi) The suspension type insulators are generally used with steel towers. As the conductors run below the earthed cross-arm of the tower, therefore, this arrangement provides partial protection from lightning. Disadvantages of Suspension Insulator
- Suspension insulator string costlier than pin and post type insulator.
- Suspension string requires more height of supporting structure than that for pin or post insulator to maintain same ground clearance of current conductor.
- The amplitude of free swing of conductors is larger in suspension insulator system, hence, more spacing between conductors should be provided. Strain Insulator When suspension string is used to sustain extraordinary tensile load of conductor it is referred as string insulator. When there is a dead end or there is a sharp corner in transmission line, the line has to sustain a great tensile load of conductor or strain. A strain insulator must have considerable mechanical strength as well as the necessary electrical insulating properties. Fig 2- Strain Insulator
Shackle Insulator or Spool Insulator The shackle insulator or spool insulator is usually used in low voltage distribution network. It can be used both in horizontal and vertical position. 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. POTENTIAL DISTRIBUTION OVERA STRING OF SUSPENSION INSULATORS: A string of suspension insulators consists of a number of porcelain discs connected in series through metallic links. Fig. 2(i) shows 3-disc string of suspension insulators. The porcelain portion of each disc is in between two metal links. Therefore, each disc forms a capacitor C as shown in Fig.2(ii). This is known as mutual capacitance or self-capacitance. If there were mutual capacitance alone, then charging current would have been the same through all the discs and consequently voltage across each unit would have been the same i., V/3 as shown in Fig. 2(ii). However, in actual practice, capacitance also exists between metal fitting of each disc and tower or earth. This is known as shunt capacitance C1. Due to shunt capacitance, charging current is not the same through all the discs of the string [See Fig 2(iii)]. Therefore, voltage across each disc will be different. Obviously, the disc nearest to the line conductor will have the maximum voltage. Thus referring to Fig 2(iii), V 3 will be much more than V 2 or V 1. The following points may be noted regarding the potential distribution over a string of suspension insulators: (i) The voltage impressed on a string of suspension insulators does not distribute itself uniformly across the individual discs due to the presence of shunt capacitance. (ii) The disc nearest to the conductor has maximum voltage across it. As we move towards the cross-arm, the voltage across each disc goes on decreasing. (iii) The unit nearest to the conductor is under maximum electrical stress and is likely to be punctured. Therefore, means must be provided to equalize the potential across each unit.
Power system (13) - NOTES
Course: Power Electronics (203122357)
University: Parul University
