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Examoogle • User • info@examoogle.com • EE-2024-8821
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Most occurred fault in overhead line is
LG
LL
LLG
LLL
LG
Quick Summary: The most frequently occurring fault in overhead power transmission and distribution lines is the Line-to-Ground (LG) fault. Statistically, approximately 70-80% of all system faults are LG faults, usually caused by transient events such as lightning strikes or flashovers due to tree branches or pollution.
The most frequently occurring fault in overhead power transmission and distribution lines is the Line-to-Ground (LG) fault. Statistically, approximately 70-80% of all system faults are LG faults, usually caused by transient events such as lightning strikes or flashovers due to tree branches or pollution.
If=Z1+Z2+Z0Vph — Represents the fault current for an LG fault in a three-phase system using symmetrical components.
An LG fault occurs when a single phase conductor makes contact with the ground or a grounded structure (like a tower or pole). This can be caused by insulation breakdown, wind-blown debris, or external objects. Because the earth provides a low-impedance path, a large fault current flows through the ground back to the system neutral, which is detected by protective relays to trigger circuit breaker operation.
LG faults account for 70-80% of total faults.
LL faults account for 10-15% of total faults.
LLG faults account for 5-10% of total faults.
LLL faults are the rarest (approx. 2-5%) but are the most severe.
Most LG faults are transient in nature.
System stability is easier to maintain compared to symmetrical faults.
Requires high-speed clearing to prevent equipment damage.
Can lead to significant voltage dips.
Used in designing relay coordination settings for distance and overcurrent protection.
| Feature | LG (Common) | LLL (Severe) |
|---|---|---|
Fault Type | 70-80% | 2-5% |
LLL (Three-phase to ground) faults are considered symmetrical and are the most severe, involving all three phases.
The sequence network for an LG fault involves connecting the positive, negative, and zero sequence networks in series.
A is correct — The LG (Line-to-Ground) fault is the most common fault in overhead transmission systems due to its sensitivity to external environmental disturbances.
Always remember the order of frequency: LG > LL > LLG > LLL; however, remember that the order of severity (current magnitude) is exactly the inverse: LLL > LLG > LL > LG.