Methods of Increasing String Efficiency

       We have seen that higher the value of the string, efficiency, more uniform is the potential distribution over a string of suspension insulators. The line unit is always under the maximum electrical stress. To avoid possibility of puncture of line unit due to excessive stress, efforts are made to have uniform potential distribution. Hence some methods are used in practice to get higher string efficiency. These methods are,
1. Reducing ratio of shunt capacitance to self capacitance
2. by grading the insulators
3. use of guard ring to provide static shielding.
1. Reducing Ratio of Shunt Capacitance to Self Capacitance

       The voltage across the line unit depends on the value of k which is the ratio of shunt capacitance to self capacitance. The string efficiency us dependent on the voltage across the line unit. Lesser the value of k, higher is the string efficiency and more uniform is the potential distribution. The voltages across the various units of string are almost equal for very low values of k.
       Now k = C1 /C i.e. the ratio of shunt capacitance to self capacitance. So to reduce the value of k, C1 must be reduced. This is possible by increasing the distance between the insulator and the earth i.e. tower. This can be achieved by increasing the length of cross-arms as shown in the Fig.1. More the length d of the cross-arm, less is the value of C1 and less is the value of k.

       But this method has practical limitations such as :
1. Use of long cross-arm increase the cost.
2. Due to long cross-arm overall strength of the tower reduces.
       Hence the practice, the minimum value k which can be achieved by this method is 0.1.
       And due to this limitations, this method is rarely used in practice.
2. Grading the insulators
       By correct grading of the insulators, more uniform voltage distribution across the string can be achieved. In the method of grading, the insulators are so selected that the self capacitances i.e. mutual capacitance of the various units are different and the values of mutual capacitances decrease from the line unit. So top unit has minimum mutual capacitance while the line unit has maximum mutual capacitance. The voltage for the given current across the capacitance is inversely proportional to the capacitance. So more the capacitance, lesser is the voltage across the capacitance.
       Thus keeping line unit capacitance to be maximum, current through it is minimum. This reduces the voltage across the line unit.
       So by probably grading the insulators i.e. by using different sized insulators in a string, uniform voltage distribution can be achieved and string efficiency can be improved. The grading is shown in the Fig. 2.

       The design of such string using different sized insulators is practically complicated and inconvenient. Hence the method is used only for very high voltage system such as 200 Kv and above. Though the design of such string is inconvenient, using standard insulators for most of the units and larger units adjacent to the line conductor, better results can be obtained.
3. Use of Guard Ring
       In this method a large metal ring surrounding the line unit and connected to the metal part of the bottom of the line unit is used. Such a ring called ''guard ring''. The guard ring is shown in the Fig.3. This is also called static shielding of the string.

       Earlier it has been mentioned that capacitance between insulator and the line is neglected. But use of guard ring increases the capacitance between the metal part of the insulator and the line.
       These capacitance are shown as C'2,C'3 in the Fig.3. These capacitances are greater for the lower units. Due to this, voltage across these units is reduced. An equal distribution of voltage is not possible by this method.
       But the guard ring used can be designed in such a way that shunt capacitance current I1, I2, I3 etc. are equal to the currents through newly introduced capacitors i.e. i1, i2, i3 etc. as shown in the Fig.3. Due to this, charging current through the mutual capacitors remains same, giving uniform voltage distribution. But such a design is practically difficult.
       Thus the primary aim of the guard ring is to reduce the electrical stress on the lower units.


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