Capacitance is the natural and unavoidable effect of transmission lines. Formation of capacitance(Fig:1a) occurs due to the presence of ceramic material between the two metallic pins, which act as a dielectric between the metal plates. The Figure:1b below is the equivalent circuit for the above condition.
Here, the current through all the capacitors (insulator) is the same “i” and voltage across the capacitor (insulator) is also the same “v”.
However, due to the presence of a tower, there is one more type of capacitance called “shunt capacitance” between the insulator pins and tower(Fig:2a). In this capacitor, air acts as a dielectric. To better understand this, take a close look at the circuit depicted below(Fig:2b).
Due to the presence of shunt capacitance, current flow through each capacitor (insulator) changes, hence the voltage. Consequently, the transmitted voltage appears unequally across the capacitor (insulators). Here, more current is passing through the bottom most capacitor (insulator), so voltage across it would be higher. Similarly, as we go up towards the upper most capacitor (insulator), the voltage will decrease. This means the bottom most capacitor (insulator) is usually under high voltage stress and the uppermost capacitor (insulator) experiences the least voltage stress. Due to this unequal voltage distribution, the chances of insulator breakdown are increased.
To overcome this issue, guard rings are used.
In the below circuit diagram, one metallic ring is attached to the transmission conductor near the bottom most insulator. Unequal voltage distribution is mainly due to the shunt capacitor. This metallic ring forms another capacitance equal and opposite to the shunt capacitance. This capacitance provides approximately an equal amount of current to the circuit as the shunt capacitance drawing from the circuit.
As a result, approximately the same current flows through all the vertical capacitors. Thus, the voltage stress distribution becomes equal throughout the insulators.