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Methods of Improving String Efficiency

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Methods of improving string efficiency

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

Using long cross arms
Grading of Insulators
Guard Ring

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Using Long cross arms

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

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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

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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.

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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.

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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.

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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.

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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.

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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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Thank You

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