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Examoogle • User • info@examoogle.com • EE-2024-8821
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In overhead line which type of faults occurs nearly 85%?
LLL
LG
LLG
LL
LG
Quick Summary: In overhead transmission and distribution systems, Line-to-Ground (LG) faults are the most common, accounting for approximately 85% of all system faults. This high frequency is primarily due to the exposure of lines to external environmental factors like lightning, falling trees, and birds.
In overhead transmission and distribution systems, Line-to-Ground (LG) faults are the most common, accounting for approximately 85% of all system faults. This high frequency is primarily due to the exposure of lines to external environmental factors like lightning, falling trees, and birds.
If(LG)=Z1+Z2+Z03E — where If(LG) is the LG fault current, E is phase voltage, and Z1,Z2,Z0 are positive, negative, and zero sequence impedances.
An LG fault occurs when a single phase conductor makes physical contact with the ground or a grounded object (such as a steel tower or cross-arm). This creates a low-impedance return path for the fault current, resulting in a sudden surge in current and potential system unbalance, which the protective relaying system must isolate.
LG faults are the most frequent type of unsymmetrical fault.
Most faults are transient in nature, often caused by lightning or flashovers.
Symmetrical faults (LLL) are the least frequent but most severe in magnitude.
Zero sequence impedance plays a crucial role in calculating LG fault current magnitudes.
Most protective relaying schemes are optimized to detect LG faults quickly.
Easier to clear compared to three-phase faults due to lower magnitude.
Frequent interruptions impact power quality.
May cause significant transient overvoltages on healthy phases.
Transmission line protection design.
Switchgear sizing and insulation coordination.
Symmetrical faults (LLL) account for only about 5% of total faults.
LL and LLG faults account for the remaining 10% of faults.
The high incidence of LG faults justifies the widespread use of auto-reclosing circuit breakers in distribution networks.
B is correct — Line-to-Ground (LG) faults represent the vast majority of system faults in overhead lines due to atmospheric and environmental interactions.
Always remember that in power systems, frequency of occurrence is inversely proportional to fault severity; LG faults occur most often but have lower magnitude than LLL faults.