Per-Unit System and Single Line Diagrams — MCQs – EE

1. The per-unit system in power engineering is used to:
(A) Express system quantities in actual units
(B) Normalize system quantities for easy comparison
(C) Measure frequency variations
(D) Calculate temperature rise

2. The main advantage of the per-unit system is:
(A) Complex calculations
(B) Simpler comparison and reduced error
(C) Reduced insulation cost
(D) Increased power factor

3. In per-unit system, base quantities are usually selected as:
(A) Voltage and current
(B) Voltage and power
(C) Power and frequency
(D) Resistance and current

4. The per-unit impedance remains the same:
(A) For all voltage levels on the same transformer
(B) Only for primary winding
(C) Only for secondary winding
(D) For different machines with different ratings

5. The per-unit value of a quantity is obtained by dividing:
(A) Actual value by base value
(B) Base value by actual value
(C) Rated value by frequency
(D) Power by voltage

6. In per-unit calculations, if base MVA and base kV are known, then base current and impedance can be:
(A) Calculated directly
(B) Measured experimentally
(C) Ignored
(D) Independent of MVA

7. The per-unit system is particularly useful in:
(A) DC circuits
(B) Power system fault and load flow studies
(C) Lighting circuits
(D) Communication systems

8. The per-unit value of unity power factor load impedance is:
(A) Purely resistive
(B) Purely inductive
(C) Purely capacitive
(D) Zero

9. If all system quantities are expressed in per-unit, then:
(A) The system equations become complex
(B) Transformer turns ratio becomes unnecessary
(C) Unit mismatch occurs
(D) Base values change with load

10. The per-unit system simplifies fault analysis because:
(A) Reactances of all components can be referred to a common base
(B) All impedances are eliminated
(C) Frequency effects are neglected
(D) It increases the number of equations

11. Base kV for a transformer is usually selected as:
(A) The rated voltage of its primary winding
(B) The rated voltage of its secondary winding
(C) The voltage level of the system connected
(D) Any arbitrary value

12. The per-unit reactance of most electrical equipment lies between:
(A) 0.05 to 0.5
(B) 1 to 10
(C) 10 to 100
(D) 0.001 to 0.01

13. The per-unit impedance of a transformer is independent of:
(A) Voltage rating
(B) Frequency
(C) Power factor
(D) Both (A) and (B)

14. The base MVA for the entire system in per-unit analysis is chosen:
(A) Same for all components
(B) Different for each component
(C) Arbitrarily for each bus
(D) Varies with load

15. The single-line diagram of a power system shows:
(A) All three phases
(B) One phase with major components represented by symbols
(C) Only generators
(D) Complete control wiring

16. In a single-line diagram, a bus represents:
(A) A conductor only
(B) A point where several components are connected
(C) Only a generator terminal
(D) Only a transmission line

17. The per-unit system is most advantageous when:
(A) System contains multiple voltage levels and transformers
(B) The system is purely resistive
(C) System has a single voltage level
(D) Only DC analysis is performed

18. A single-line diagram is also called a:
(A) One-wire diagram
(B) Parallel circuit diagram
(C) Control circuit
(D) Load curve

19. In a single-line diagram, transformers are represented by:
(A) Two separate coils
(B) A pair of parallel lines or a symbol with voltage levels
(C) A resistor
(D) A busbar

20. The single-line diagram provides information about:
(A) Physical locations
(B) Electrical connections and voltage levels
(C) Mechanical structure
(D) Load ratings only

21. The base impedance in per-unit system is given by:
(A) Directly proportional to base voltage squared
(B) Inversely proportional to base power
(C) Both (A) and (B)
(D) Independent of base voltage

22. When base MVA changes, per-unit reactance:
(A) Changes inversely with MVA
(B) Changes directly with MVA
(C) Remains constant
(D) Becomes zero

23. The main use of single-line diagrams is in:
(A) Fault analysis and power flow studies
(B) Lighting circuit design
(C) Small control systems
(D) Domestic wiring

24. The elements shown in a single-line diagram include:
(A) Generators, transformers, transmission lines, and loads
(B) Only resistors and capacitors
(C) Communication lines
(D) Measuring instruments

25. The per-unit voltage of the slack bus in load flow study is usually taken as:
(A) 0
(B) 1.0∠0°
(C) 0.8∠0°
(D) 1.1∠0°

26. In per-unit system, the base current is directly proportional to:
(A) Base MVA and inversely proportional to base voltage
(B) Base voltage and inversely proportional to base MVA
(C) Base impedance
(D) Frequency

27. The per-unit value of power is:
(A) Actual power divided by base power
(B) Base power divided by actual power
(C) Product of voltage and current bases
(D) Always unity

28. The main purpose of per-unit normalization is to:
(A) Allow uniform comparison between different rated equipment
(B) Eliminate transformers from the system
(C) Reduce total harmonic distortion
(D) Simplify control circuit

29. In a single-line diagram, circuit breakers are represented by:
(A) Dotted line
(B) Small rectangle or symbol between buses
(C) Transformer symbol
(D) Load symbol

30. The single-line diagram of a power system is mainly used for:
(A) Planning and analysis of power flow and faults
(B) Designing mechanical systems
(C) Economic load dispatch
(D) Energy auditing