SOLUTION

The non-uniform distribution of voltage across the units in a string of suspension type insulators is due to the existence of stray capacitance between the metallic junctions of the units and the tower body.

Voltage distribution Over a String Type Insulator

A suspension-type insulator consists of a number of porcelain discs connected in series by metal links in the form of a string. The conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross arm of the tower.

When ac voltage is applied across the insulator string, the voltage across each unit is not the same. This is because there is a capacitance between the units as well as the capacitance between each unit and earth or framework.  The insulator units have capacitance between about 6 to 10 times the capacitance of the unit to earth.

Due to the shunt capacitance, the charging current is not the same through all the discs of the string. Therefore, the voltage across each disc will be different. Presence of self-capacitance and shunt capacitance results in the

1. Non-uniform potential distribution of voltage across the individual discs of suspension-type insulators.
2. The disc nearest to the conductor has a maximum voltage across it.
3. The disc nearest to the conductor is under maximum electrical stress and is likely to be punctured.
4. Since the insulator capacitances are ineffective for DC, the voltage across each disc would be the same for DC voltage across the string.
5. Non-uniform voltage distribution will mean a few units are stressed more while other units are lightly stressed. This will either reduce the flashover voltage of the string or necessitate the use of more number of disc units to insulate a certain phase voltage.
6. Non-uniform potential distribution of voltage across the individual discs of suspension-type insulators is due to the existence of stray capacitance between the metallic junctions of the units and the tower body