📘 Power Systems – Question Bank (Page 11)
🔹 Section A: Conceptual Questions (Protection)
Q1. What is the purpose of protection system?To detect faults and isolate faulty section to maintain system stability and safety.
Q2. What is selectivity?Only faulty section must be disconnected without disturbing healthy parts.
Q3. What is Plug Setting Multiplier (PSM)?PSM = Fault current / Relay pickup current.
Q4. What is CT saturation?When CT core becomes magnetically saturated leading to distorted secondary current.
🔹 Section B: MCQs
Q5. Percentage differential protection is used for:A) Transmission lines
B) Transformers
C) Load flow
D) Economic dispatch
Answer: B) Transformers
Q6. Distance relay measures:A) Current
B) Voltage
C) Impedance
D) Frequency
Answer: C) Impedance
Q7. Mho relay is:A) Non-directional
B) Directional
C) Time relay
D) Thermal relay
Answer: B) Directional
🔹 Section C: Numerical Problems
Q8. A relay has pickup current 2 A. Fault current is 10 A. Find PSM.PSM = 10 / 2 = 5
Q9. CT ratio = 1000/5. Fault current = 5000 A. Find secondary current.
Secondary current = (5000 / 1000) × 5 = 25 A
Q10. A 220 kV line has impedance 40 Ω. Relay is set to 80% reach. Find relay setting impedance.
Relay reach = 0.8 × 40 = 32 Ω
Q11. Fault MVA at bus = 3000 MVA. Voltage = 132 kV. Find short circuit current.
I = MVA / (√3 × V) = 3000 / (1.732 × 132) = 13.13 kA (approx)
🔹 Section D: Quick Revision Points
- PSM > 1 → Relay operates
- Distance relay protects transmission lines
- Differential protection → Fast & selective
- Backup protection has time delay
Page 11 Completed – Protection Advanced
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