📘 Unsymmetrical Faults – Quick Revision (GATE Focus)
This page gives short, exam-focused summary of LG, LL and LLG faults. Memorize these formulas and network connections.
🔹 1️⃣ Types of Unsymmetrical Faults
- LG (Single Line to Ground) – Most common (~70%)
- LL (Line to Line)
- LLG (Double Line to Ground)
🔹 2️⃣ Sequence Networks Used
| Fault Type | Sequence Networks | Connection Type |
| 3-Phase | Z1 only | Single Network |
| LG | Z1, Z2, Z0 | Series |
| LL | Z1, Z2 | Series |
| LLG | Z1, Z2, Z0 | Mixed (Z2 || Z0) |
🔹 3️⃣ Important Formulas
✔ LG Fault
I_fault = 3V / (Z1 + Z2 + Z0)
If V = 1 pu: I_fault = 3 / (Z1 + Z2 + Z0) ---✔ LL Fault
I_fault = √3 V / (Z1 + Z2)
Zero sequence current = 0 ---✔ LLG Fault
I_fault = 3V / (Z1 + (Z2 Z0)/(Z2 + Z0))
---🔹 4️⃣ Key Concept Points
- LG fault current depends strongly on Z0 (grounding).
- If system not grounded → Z0 very high → small LG current.
- LL fault does NOT involve zero sequence.
- 3-phase fault uses only positive sequence.
🎯 GATE Exam Tips
- Remember which networks are connected.
- LG → All three in series.
- LL → No zero sequence.
- Check grounding before solving.
Unsymmetrical Fault = Sequence Network Combination Logic
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