Free Power Systems MCQs with Answers

• A. Open circuit
• B. Short-circuit
• C. Long
• D. None of the above

Explanation: For an open-circuit termination, the reflected voltage is in phase with the incident voltage, giving a voltage reflection coefficient of…

• A. Lightning arresters
• B. Relays
• C. Step-down transformer
• D. Switchgear

Explanation: A lightning arrester is connected at the line entry point to divert a travelling surge to earth before it reaches sensitive equipment.

• A. Impedance
• B. Reactance
• C. Either of the above
• D. None of the above

Explanation: Impedance relays are commonly used for phase-fault protection, particularly on short and medium transmission lines.

• A. above 30 kVA.
• B. equal to and above 5 MVA
• C. equal to and above 25 MVA
• D. none of the above

Explanation: Transformer differential protection is normally applied to larger transformers because its cost and complexity are justified at higher…

• A. hydro-electric generators
• B. steam turbine generators
• C. gas turbine generators
• D. all of the above
• E. none of the above

Explanation: Overvoltage protection is applied to generators of all these prime-mover types because excessive voltage can damage insulation and…

• A. Zero
• B. Unity
• C. Infinity
• D. None of the above

Explanation: The reflection coefficient is Γ = (ZL − Z0)/(ZL + Z0). When the load impedance equals the line surge impedance, the numerator is zero, so…

• A. zero current
• B. maximum current
• C. minimum voltage
• D. maximum voltage

Explanation: An AC circuit-breaker arc naturally reaches its weakest point when the current passes through zero, allowing the dielectric strength…

• A. single phase motors only
• B. two phase motors only
• C. two single phase motors running in parallel
• D. three phase motors

Explanation: Single phasing means one phase of a three-phase supply is lost, which can overheat and damage a three-phase motor.

• A. low resistance
• B. high resistance
• C. low impedance
• D. high impedance

Explanation: A series reactor limits fault current mainly through its inductive reactance, so it should have high impedance but very low resistance to…

• A. Inductance
• B. Capacitance
• C. Resistance
• D. Reactance

Explanation: For a short transmission line, shunt capacitance is small because the line length is short, so it is neglected in the model.

• A. generators
• B. busbars
• C. transformers
• D. motors
• E. all of the above

Explanation: Under-voltage relays can initiate protection or control actions in generators, busbars, transformers, and motors, such as tripping or load…

• A. Generators
• B. Motors
• C. Transmission line
• D. Transformers

Explanation: Negative-sequence current produces a reverse-rotating magnetic field that causes severe heating in generator rotors, so generators…

• A. single bus
• B. double bus, double breaker
• C. main and transfer bus
• D. all of the above

Explanation: In a main-and-transfer-bus scheme, the bus coupler connects the main and transfer buses so a circuit can be shifted during breaker…

• A. 11 kV
• B. 33 kV
• C. 60kV
• D. 110kV

Explanation: Outdoor substations become more economical and safer than indoor installations at higher voltages because large clearances and equipment…

• A. single phase to ground faults
• B. phase to phase faults
• C. two phase to ground faults
• D. three phase faults
• E. any of these

Explanation: A short-circuit can occur from any listed fault, including phase-to-ground, phase-to-phase, two-phase-to-ground, or three-phase faults.

• A. load flow
• B. load flow and short-circuit
• C. load flow and stability
• D. load flow, short-circuit and stability
• E. none of the above

Explanation: An A.C. network analyser models power-system operating conditions, including load-flow calculations, short-circuit studies and stability…

• A. reactors
• B. resistors
• C. capacitors
• D. none of the above

Explanation: Reactors add inductive reactance in series with the circuit, limiting the magnitude of short-circuit current.

• A. current
• B. voltage
• C. impedance
• D. all of the above

Explanation: An induction-cup relay develops torque from the interaction of alternating magnetic fluxes, and its operation may be arranged to respond…

• A. zero
• B. infinity
• C. unity
• D. none of the above

Explanation: At an open circuit, the load impedance is infinite, so the voltage reflection coefficient is (ZL − Z0)/(ZL + Z0) = +1.

• A. lubricate the contacts
• B. insulate the contacts from switch body
• C. extinguish the arc
• D. all of the above
• E. none of the above

Explanation: When high-voltage switch contacts separate, an arc forms between them; the oil cools and deionises the arc path, helping extinguish it.