The potential distribution in a string of suspension insulators is not uniform which results in a reduction in string efficiency. String efficiency can be improved using longer cross arms of transmission line poles, by grading the insulators, or by application of a guard ring.
Given below is an overview of the role of the guard ring in improving string efficiency.
The guard ring is a metal ring that is electrically connected to the conductor and surrounds the bottom insulator. A guard ring establishes capacitance between the conductor wire and metal fittings of the insulator. The figure below is an illustration of guard ring connected to a line.
Where in above figure
- i1, i2 are shunt capacitance currents
- i′1, i′2 are line capacitance currents
The guard ring is connected in a manner that shunt capacitance currents (i1, i2) are equal to metal fitting line capacitance currents i′1, i′2.
Once guard ring is properly installed in place, the same charging current I flows through each unit of string by the virtue of which a uniform potential distribution across the units is achived.