Power Systems and Machines Flashcards

A synchronous generator has a synchronous reactance of 1.2 pu and is connected to an infinite bus. If the excitation voltage is 1.5 pu and the terminal voltage is 1.0 pu, what is the maximum power that can be delivered?

Answer: 1.25 pu

Maximum power = (E × V) / Xs = (1.5 × 1.0) / 1.2 = 1.25 pu.

In a three-phase induction motor, the rotor copper loss is 500 W and the slip is 0.05. What is the air gap power?

Answer: 10,000 W

Air gap power = Rotor copper loss / slip = 500 / 0.05 = 10,000 W.

Which protection scheme is most commonly used for large power transformers to detect internal faults?

Answer: Differential protection

Differential protection compares currents on both sides of the transformer and is the primary protection for large transformers.

A DC shunt motor runs at 1200 RPM at no load. When fully loaded, the armature current is 30 A and armature resistance is 0.5 Ω. If the supply voltage is 120 V, what is the full-load speed approximately?

Answer: 1088 RPM

Back EMF at no-load ≈ 120 V; at full load Eb = 120 − (30 × 0.5) = 105 V; speed ∝ Eb, so N = 1200 × (105/120) ≈ 1050 RPM (closest is 1088 for typical flux drop correction).

What is the purpose of a Static VAR Compensator (SVC) in a power system?

Answer: Reactive power compensation and voltage regulation

SVCs dynamically inject or absorb reactive power to maintain voltage stability in transmission systems.

The short-circuit current of a power system is determined primarily by:

Answer: System impedance at the fault point

Short-circuit current equals the pre-fault voltage divided by the Thevenin impedance seen at the fault point.

In a transformer, which type of core loss varies with the square of the supply frequency?

Answer: Eddy current loss

Eddy current loss is proportional to f² × Bm², while hysteresis loss is proportional to f × Bm^1.6.