BEE Power Systems and Machines 4 — Questions and Answers
Question 1: In a power system, HVDC transmission is preferred over HVAC for very long distances primarily because:
- There are no reactive power or skin effect losses, reducing overall transmission losses (Correct answer)
- DC transformers are more efficient than AC transformers
- DC lines require more conductors
- DC systems have lower capital costs for short distances
Correct answer: There are no reactive power or skin effect losses, reducing overall transmission losses
HVDC eliminates reactive power losses, capacitive charging, and skin effect, making it more efficient for distances beyond the breakeven point (~600–800 km).
Question 2: A single-phase transformer has a turns ratio of 10:1 (primary:secondary). If the primary voltage is 2200 V and the primary current is 2 A (ideal transformer), what are the secondary voltage and current?
- 220 V and 20 A (Correct answer)
- 22000 V and 0.2 A
- 220 V and 2 A
- 2200 V and 20 A
Correct answer: 220 V and 20 A
V2 = V1/n = 2200/10 = 220 V; I2 = I1 × n = 2 × 10 = 20 A for an ideal transformer.
Question 3: The synchronous speed of a 4-pole, 60 Hz induction motor is:
- 1800 RPM (Correct answer)
- 3600 RPM
- 900 RPM
- 1200 RPM
Correct answer: 1800 RPM
Ns = 120f / P = 120 × 60 / 4 = 1800 RPM.
Question 4: In power system load flow analysis, which bus has both real and reactive power injections specified as known?
- PQ bus (load bus) (Correct answer)
- PV bus (generator bus)
- Slack bus (reference bus)
- Floating bus
Correct answer: PQ bus (load bus)
At a PQ bus, both P and Q are specified; voltage magnitude and angle are the unknowns to be solved.
Question 5: What is the main advantage of using bundled conductors in extra-high-voltage (EHV) transmission lines?
- Reduced corona discharge and lower effective reactance (Correct answer)
- Increased line resistance
- Simplified tower design
- Lower conductor weight per span
Correct answer: Reduced corona discharge and lower effective reactance
Bundled conductors increase the effective conductor radius, reducing the electric field gradient and suppressing corona, while also lowering inductance.
Question 6: Which characteristic distinguishes a reluctance motor from a conventional synchronous motor?
- It has no field winding or permanent magnets; it runs synchronously due to rotor saliency (Correct answer)
- It uses slip rings for excitation
- It cannot operate at synchronous speed
- It requires an external DC source for starting
Correct answer: It has no field winding or permanent magnets; it runs synchronously due to rotor saliency
A reluctance motor has a salient-pole rotor with no excitation; torque is produced by the tendency of the rotor to align with the lowest reluctance path.
Question 7: The voltage regulation of a transformer is defined as:
- (No-load secondary voltage − Full-load secondary voltage) / Full-load secondary voltage × 100% (Correct answer)
- (Full-load voltage − No-load voltage) / No-load voltage × 100%
- Primary voltage / Secondary voltage × 100%
- Rated kVA / Core loss × 100%
Correct answer: (No-load secondary voltage − Full-load secondary voltage) / Full-load secondary voltage × 100%
Voltage regulation = (VNL − VFL) / VFL × 100%, indicating how much the secondary voltage changes from no-load to full-load.
BEE Power Systems and Machines 4
In a power system, HVDC transmission is preferred over HVAC for very long distances primarily because: