Power Systems and Machines Flashcards

In a power system, the 'swing equation' describes:

Answer: The relationship between rotor acceleration, mechanical power input, and electrical power output

The swing equation M(d²δ/dt²) = Pm − Pe governs the dynamic behavior of the generator's rotor angle during disturbances.

An overhead transmission line has a distributed capacitance to ground. Under steady-state AC operation, this capacitance causes the line to:

Answer: Generate reactive power (leading)

Shunt capacitance generates leading reactive power (VArs), which can raise voltage levels along the line.

A three-phase, 415 V induction motor draws 20 kW at a power factor of 0.8 lagging. What is the reactive power consumed?

Answer: 15 kVAR

Q = P × tan(θ) = 20 × tan(cos⁻¹0.8) = 20 × 0.75 = 15 kVAR.

Which type of DC generator has the best voltage regulation under varying load?

Answer: Compound generator (cumulative)

A cumulative compound generator combines series and shunt field effects to maintain nearly constant terminal voltage from no-load to full-load.

In symmetrical component analysis, a single line-to-ground (SLG) fault is characterized by:

Answer: Positive, negative, and zero sequence currents all being equal

For an SLG fault, all three sequence currents (I1, I2, I0) are equal and the sequence networks are connected in series.

What does the 'short circuit ratio' (SCR) of a synchronous generator indicate?

Answer: The ratio of field current for rated open-circuit voltage to field current for rated short-circuit current

SCR = If,OC(rated V) / If,SC(rated I), and it is approximately equal to 1/Xd (pu), indicating the machine's sensitivity to excitation changes.

The purpose of a tap changer in a power transformer is to:

Answer: Adjust the turns ratio to regulate secondary voltage under varying load or supply conditions

Tap changers vary the effective turns ratio, allowing operators to maintain the secondary voltage within acceptable limits despite changes in load or primary voltage.