GATE Electrical Engineering - Complete Master Library

 

📘 Power Systems – Question Bank (Page 5)

🔹 Section A: Conceptual Questions (Unsymmetrical Faults)

Q1. What is unsymmetrical fault?

A fault in which phase currents are not equal in magnitude and not 120° apart.

Q2. Which unsymmetrical fault is most common?

Single Line to Ground (SLG) fault.

Q3. Which sequence networks are connected for SLG fault?

Positive, Negative, and Zero sequence networks in series.

Q4. Which sequence networks are used for LL fault?

Positive and Negative sequence networks only.

Q5. Which fault involves all three sequence networks but not in series?

Line-to-Line-to-Ground (LLG) fault.

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🔹 Section B: Important Formulas

SLG Fault Current: I_f = 3V / (Z₁ + Z₂ + Z₀)

LL Fault Current: I_f = √3 V / (Z₁ + Z₂)

LLG Fault Current: More complex → depends on sequence network interconnection

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🔹 Section C: Numerical Problems

Q6. A system has Z1 = Z2 = j0.2 pu and Z0 = j0.1 pu. Find SLG fault current in pu.

If prefault voltage = 1 pu I_f = 3 / (0.2 + 0.2 + 0.1) = 3 / 0.5 = 6 pu

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Q7. Z1 = Z2 = j0.25 pu. Find LL fault current.

I_f = √3 / (0.25 + 0.25) = 1.732 / 0.5 = 3.464 pu

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Q8. Z1 = 0.3 pu, Z2 = 0.3 pu, Z0 = 0.6 pu. Find SLG fault current.

I_f = 3 / (0.3 + 0.3 + 0.6) = 3 / 1.2 = 2.5 pu

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Q9. If Z1 decreases, what happens to fault current?

Fault current increases because I = V / Z.

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Q10. A 220 kV system has Z1 = Z2 = 0.4 pu and Z0 = 0.8 pu. Find SLG fault current in pu.

I_f = 3 / (0.4 + 0.4 + 0.8) = 3 / 1.6 = 1.875 pu

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🔹 Section D: MCQs

Q11. Zero sequence current flows in:
A) LL fault
B) LLL fault
C) SLG fault
D) None

Answer: C) SLG fault

Q12. In LL fault, zero sequence network is:
A) Used
B) Not used
C) Series connected
D) Parallel connected

Answer: B) Not used

Q13. Most frequent fault is:
A) 3-phase
B) LL
C) SLG
D) LLG

Answer: C) SLG

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Page 5 Completed – Unsymmetrical Fault Analysis