Methods of Improving String Efficiency

There are following methods used to improve string efficiency of suspension insulators

( 1 ) Using long cross arm

• The unequal voltage distribution across the string is done by charging current passes through shunt capacitance ( Capacitance between insulator metal pin and tower ).
• Charging current I = V / X_C   =  VωC
• If the value of shunt capacitance decreases, the charging current automatic decreases. 
• The shunt capacitance is inversely proportional to distance between two plates.

       C α ( a / d )

       Where d = Distance between two plates

• As the distance increases, the capacitance decreases and vice versa. 
• If we use long cross arm, the shunt capacitance decreases to some extent and voltage distribution becomes uniform.

Disadvantages

• The cost of cross arm increases if its length increases.

( 2 ) Grading of Insulators

• The unequal voltage distribution across the string is due to charging current passes through shunt capacitance ( C1 ) between metal pin and earth. 
• This charging current cannot be eliminated. 
• The charging current reduced as we goes far away from conductor.

       General equation Voltage across capacitance V = I X_C

      Where I = charging current

                V = I / ωC

• As the voltage near the conductor is maximum, its capacitance must be maximum in order to equal the voltage distribution across the string. 
• Therefore the capacitance at the bottom of the conductor must be maximum and its value decreases from bottom to top ( cross arm ). 
• The capacitance between metal pin and the tower is not considered in this case.

Disadvantages of Grading of Insulators

• It is difficult to manufacture different types ( voltage rating ) of insulators.
• There are number of insulators requires in spare in the case of replacement / maintenance.

( 3 ) Guard ring

• The string efficiency can be increased by using guard ring which is connected between conductor and top of the insulator metal pin. 
• As the guard ring itself acts as conductor, there is a capacitance between guard ring and insulator metal pin. 
• The effect of this capacitance partially offset the effect of shunt capacitance ( insulator pin and cross arm ) therefore the net charging current passes through shunt capacitance reduces and string efficiency increases.

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