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📘 Unsymmetrical Fault Analysis – Sequence Networks

Unsymmetrical faults are more common than 3-phase faults. We analyze them using symmetrical components.

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🔹 Types of Unsymmetrical Faults

• Single Line to Ground (LG)
• Line to Line (LL)
• Double Line to Ground (LLG)

LG is most common (≈ 70%).

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🔹 Why Sequence Networks?

Because fault causes: • Unbalanced currents • Different phase values So we split system into:

• Positive Sequence Network (Z1)
• Negative Sequence Network (Z2)
• Zero Sequence Network (Z0)

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🔹 1️⃣ LG Fault Formula

For solidly grounded system:

I_fault = 3V / (Z1 + Z2 + Z0)

Usually V = 1 pu So: I_fault = 3 / (Z1 + Z2 + Z0) All three sequence networks are connected in series.

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🔹 2️⃣ LL Fault Formula

Zero sequence not involved.

I_fault = √3 V / (Z1 + Z2)

Zero sequence current = 0

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🔹 3️⃣ LLG Fault Formula

More complex. All three networks connected but not simple series. Fault current:

I_fault = 3V / (Z1 + (Z2 Z0)/(Z2 + Z0))

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🔹 Example – LG Fault

Given: Z1 = 0.2 pu Z2 = 0.2 pu Z0 = 0.1 pu Fault current: I_fault = 3 / (0.2 + 0.2 + 0.1) = 3 / 0.5

I_fault = 6 pu

Compare with 3-phase fault: I_3ph = 1 / 0.2 = 5 pu Observe: LG fault current > 3-phase (because Z0 small)

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🎯 Key Observations

• LG fault most common
• Zero sequence path depends on grounding
• If neutral not grounded → Z0 very large
• LL fault has no zero sequence

Unsymmetrical Fault = Sequence Network Combination