We have already discussed on the Potential Distribution across the various discs in a Suspension type Insulator. When Suspension type insulator is connected to hold a power conductor carrying electrical power at high voltage, a charging current will flow though the series connected Self Capacitors. As the charging current through each of the Capacitor (Porcelain disc) is same, therefore the potential distribution across each Porcelain disc will be same i.e. each Porcelain disc will have equal voltage stress.
But actually, in addition to self capacitance of porcelain disc there also exists Capacitance in between the metallic link of suspension insulator and grounded tower body. This capacitance is known as Shunt Capacitance. Now due to this Shunt Capacitance, the charging current through each porcelain disc will no longer be same rather it will decrease as we move from the disc nearer to the power conductor to the disc farthest from the power conductor. Thus the voltage distribution across the discs of Suspension Insulator is not uniform due to Shunt Capacitance.
Here comes the Grading Ring or Guard Ring. Grading Ring or Guard Ring equalizes the potential distribution across each disc in Suspension Insulator. Grading Ring nullifies the effect of shunt capacitance of string insulator (suspension type insulator is also called string insulator).
How Grading Ring Works?
Carefully observe the figure below.
As the non uniformity in potential distribution across the various discs are because of current flowing in shunt capacitance, therefore if it is possible to supply the amount of current in shunt capacitor, then throughout the string same current will flow. Grading Ring does this job. Because of Grading Ring, capacitance between the Grading Ring and metallic link of disc forms. The Grading Ring is constructed in such a way that shunt capacitance current i2 and i3 are equal to the metal fitting line capacitance current. Because of this the charging current flowing through the whole string of discs will be same due to which a uniform potential distribution is achieved.