BEE Power Systems Engineering 5 — Questions and Answers
Question 1: In an economic dispatch problem, what condition must be satisfied for optimal allocation of real power generation among multiple units?
- All units operate at the same output in megawatts
- The incremental cost (λ) of each online unit must be equal (Correct answer)
- Units with lowest capacity factor are loaded first
- Reactive power output of each unit must match its real power output
Correct answer: The incremental cost (λ) of each online unit must be equal
Economic dispatch requires that the incremental fuel cost (∂F/∂P) be equal for all online generators, ensuring no cost savings are possible by shifting load between units.
Question 2: What is the purpose of a tap-changer on a power transformer in a transmission substation?
- Change the transformer's kVA rating under load
- Regulate the secondary voltage by adjusting the turns ratio while energized (Correct answer)
- Protect the transformer from short-circuit currents
- Reduce magnetizing inrush current at energization
Correct answer: Regulate the secondary voltage by adjusting the turns ratio while energized
An on-load tap-changer (OLTC) adjusts the transformer turns ratio in small steps while the transformer is energized, maintaining secondary voltage within acceptable limits.
Question 3: In a radial distribution system, what is the primary disadvantage compared to a looped or meshed network?
- Higher investment cost per unit of capacity delivered
- A single fault isolates all downstream customers, resulting in longer outages (Correct answer)
- Greater reactive power losses due to multiple parallel paths
- More complex protection coordination requirements
Correct answer: A single fault isolates all downstream customers, resulting in longer outages
Radial feeders have a single supply path, so any fault or equipment failure between the source and load causes a complete outage for all downstream customers.
Question 4: Which characteristic of a synchronous generator determines its ability to supply reactive power to the system and support voltage?
- Rotor inertia constant (H)
- Excitation (field current) level (Correct answer)
- Governor droop setting
- Armature resistance
Correct answer: Excitation (field current) level
Increasing field excitation causes the generator to become over-excited, supplying reactive power (capacitive) to the system and boosting bus voltage.
Question 5: What is the 'skin effect' in conductors carrying AC current, and how does it affect transmission line design?
- Dielectric heating of the conductor's insulation, requiring thicker jackets at higher voltages
- Concentration of current toward the conductor surface, increasing effective resistance at higher frequencies (Correct answer)
- Magnetic saturation of the conductor core that reduces inductance
- Corrosion of the outer conductor strands that reduces tensile strength
Correct answer: Concentration of current toward the conductor surface, increasing effective resistance at higher frequencies
At AC (60 Hz), current density is highest at the conductor surface and decreases inward, raising effective resistance above the DC value and motivating stranded ACSR conductor designs.
Question 6: In power systems, what is the difference between a 'bolted fault' and an 'arcing fault'?
- A bolted fault occurs at high voltage, while an arcing fault only occurs at low voltage
- A bolted fault has zero impedance at the fault point producing maximum current; an arcing fault has arc impedance that limits current (Correct answer)
- A bolted fault affects all three phases simultaneously; an arcing fault is always single-phase
- A bolted fault is a sustained fault; an arcing fault is always transient and self-clearing
Correct answer: A bolted fault has zero impedance at the fault point producing maximum current; an arcing fault has arc impedance that limits current
A bolted (solid) fault assumes zero fault impedance, giving the highest possible short-circuit current; arcing faults have arc resistance that reduces fault current below the bolted level.
Question 7: A 60 Hz power system experiences a sudden loss of a large generating unit. Which control mechanism acts first to arrest the frequency decline?
- Automatic generation control (AGC) adjusting unit set-points
- Governor response providing primary frequency control within seconds (Correct answer)
- Economic dispatch re-optimizing the generation schedule
- Load shedding relays disconnecting interruptible customers
Correct answer: Governor response providing primary frequency control within seconds
Governor (primary) response acts within 5–30 seconds using droop control to automatically increase output on remaining units and arrest frequency decline before AGC intervenes.
BEE Power Systems Engineering 5
In an economic dispatch problem, what condition must be satisfied for optimal allocation of real power generation among multiple units?