Power Systems and Machines Flashcards

A 50 MVA, 11 kV synchronous generator has a subtransient reactance of 0.15 pu. What is the subtransient fault current (in pu) for a three-phase bolted fault at its terminals?

Answer: 6.67 pu

Subtransient fault current = 1.0 / X''d = 1.0 / 0.15 = 6.67 pu.

Which of the following best describes the 'equal area criterion' in power system stability?

Answer: A graphical method to assess transient stability by comparing accelerating and decelerating energy areas on a P-δ curve

The equal area criterion states that a system is transiently stable if the decelerating area on the P-δ curve equals or exceeds the accelerating area.

What is the slip of a three-phase induction motor running at 1440 RPM with a synchronous speed of 1500 RPM?

Answer: 0.04

Slip s = (Ns − Nr) / Ns = (1500 − 1440) / 1500 = 60/1500 = 0.04.

In per-unit system analysis, which quantity remains the same on both sides of a transformer?

Answer: Per-unit impedance (on the same MVA base)

When converting to a common MVA and kV base, per-unit impedances are the same on both sides, simplifying transformer analysis.

A DC series motor is preferred for traction applications because it provides:

Answer: High starting torque at low speed

In a series motor, torque ∝ Ia², giving very high torque at startup when current is high, ideal for traction.

The Ferranti effect in a long transmission line refers to:

Answer: The receiving-end voltage exceeding the sending-end voltage under no-load or light-load conditions

Under no-load, the capacitive charging current causes a voltage rise along the line, making receiving-end voltage higher than sending-end.

Which winding configuration in a three-phase transformer eliminates the triple-harmonic (zero-sequence) currents from flowing into the external system?

Answer: Delta (Δ) winding

A delta winding provides a closed path for triple-harmonic (zero-sequence) currents to circulate internally, preventing them from entering the network.