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Examoogle • User • info@examoogle.com • EE-2024-8821
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Lowest occurred fault in overhead line is
LG
LL
LLG
LLL
LLL
Quick Summary: Three-phase symmetrical (LLL) faults are the least frequent occurrences in overhead power transmission lines. Statistically, approximately 70-80% of all power system faults are single line-to-ground (LG) faults, while symmetrical three-phase faults account for less than 5% of total faults.
Three-phase symmetrical (LLL) faults are the least frequent occurrences in overhead power transmission lines. Statistically, approximately 70-80% of all power system faults are single line-to-ground (LG) faults, while symmetrical three-phase faults account for less than 5% of total faults.
Pfault∝Zfault−1 — Relationship between fault current and system impedance
Isym=Z1Vph — Symmetrical fault current calculation
Overhead lines are subjected to various external disturbances like wind, falling trees, and lightning. LG faults are most common because they involve only one phase-to-ground insulation breakdown. Conversely, LLL faults require simultaneous contact or insulation failure of all three phases, which is statistically improbable due to the physical separation between conductors and the complexity of such a coincidental event.
LG faults are the most common (approx. 70-80%).
LL faults account for approx. 10-15%.
LLG faults account for approx. 5-10%.
LLL faults are the least common (< 5%) but the most severe.
Symmetrical faults are easiest to calculate using the per-unit system.
Easier design for protection relays since they rely on balanced conditions.
LLL faults cause maximum mechanical stress on transmission towers.
Extreme thermal stress on circuit breakers during current interruption.
System stability studies.
Circuit breaker rating selection.
Symmetrical faults are also known as 'balanced faults' because the system remains balanced after the fault occurs.
Option A (LG) is the most frequent fault type, not the lowest.
D is correct — The three-phase (LLL) fault is the rarest type of fault occurring in overhead transmission lines.
Remember the hierarchy: LG > LL > LLG > LLL for frequency, but LLL > LLG > LL > LG for severity of current magnitude.