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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Guard ring transmission line
Improves power factor
Reduces earth capacitance of the lowest unit
Reduces transmission losses
Improves regulation
Reduces earth capacitance of the lowest unit
A guard ring (or grading ring) is a metallic ring connected to the cross-arm and surrounding the bottom unit of a suspension insulator string. Its primary purpose is to bypass the leakage current and equalize the potential distribution across the string by compensating for the unequal charging currents that flow through the shunt earth capacitances.
A guard ring (or grading ring) is a metallic ring connected to the cross-arm and surrounding the bottom unit of a suspension insulator string. Its primary purpose is to bypass the leakage current and equalize the potential distribution across the string by compensating for the unequal charging currents that flow through the shunt earth capacitances.
Ishunt=V⋅jωCe — Current through shunt earth capacitance
Vstring=∑i=1nVi — Total voltage across string
In a string of suspension insulators, the voltage distribution is non-uniform due to the existence of shunt capacitance between each metal pin and the earthed tower structure. The unit nearest to the cross-arm bears the highest voltage stress. The guard ring introduces an additional capacitance between the line conductor and the metal fittings, which injects current that offsets the current leaking to the tower, thereby balancing the potential gradient.
The voltage distribution across an insulator string is non-linear without grading.
The unit closest to the line conductor suffers the maximum voltage stress.
Guard rings improve string efficiency by increasing the capacitance of the bottom units.
Without guard rings, the bottom insulator is prone to puncture due to high potential gradient.
Improves string efficiency
Prevents corona discharge on the bottom insulator
Ensures uniform potential distribution
Increases the cost of the insulator assembly
Adds weight to the cross-arm structure
Extra High Voltage (EHV) overhead transmission lines
Suspension-type insulator strings
String Efficiency η=n×Voltage across bottom unitVoltage across whole string×100%
Option A is incorrect because guard rings do not directly modify power factor of the line.
Option C is incorrect because reduction of transmission line losses is achieved by increasing conductor size or voltage level, not by insulator guard rings.
Option D is incorrect because regulation is a function of the line impedance (R+jX), whereas guard rings influence voltage distribution on insulators.
B is correct — The guard ring effectively increases the capacitance of the lowest unit, balancing the potential gradient across the entire string.
Always remember that for insulator strings, string efficiency decreases as the number of units increases. Guard rings are mandatory for lines above 33 kV to prevent dielectric breakdown.