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Power Systems Engineering Flashcards

7 cards from real BEE practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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  1. In a power system, the per-unit impedance of a transformer is 0.05 pu on a 100 MVA base. If the base is changed to 200 MVA, what is the new per-unit impedance?

    Answer: 0.10 pu

    Per-unit impedance scales linearly with the MVA base: Z_new = Z_old × (MVA_new / MVA_old) = 0.05 × (200/100) = 0.10 pu.

  2. Which relay type is most commonly used for primary protection of transmission lines because its operating time is inversely proportional to fault current magnitude?

    Answer: Inverse time overcurrent relay

    Inverse time overcurrent relays operate faster for larger fault currents, making them ideal for graded transmission line protection.

  3. A balanced three-phase system has line voltage of 415 V. What is the phase voltage?

    Answer: 239.6 V

    Phase voltage = Line voltage / √3 = 415 / 1.732 ≈ 239.6 V in a balanced Y-connected system.

  4. What is the primary function of a shunt capacitor bank installed on a distribution feeder?

    Answer: Improve power factor and reduce reactive power demand

    Shunt capacitor banks supply reactive (VAR) power locally, improving power factor and reducing reactive current flow from the source.

  5. In the symmetrical component method, the positive-sequence, negative-sequence, and zero-sequence impedances of a generator are Z1, Z2, and Z0. For a single line-to-ground fault, the fault current is proportional to:

    Answer: 3 / (Z1 + Z2 + Z0)

    For a single line-to-ground fault, Ia = 3Ea / (Z1 + Z2 + Z0), so fault current is 3/(Z1+Z2+Z0) times the prefault voltage.

  6. What does the term 'load flow' or 'power flow' analysis determine in a power system?

    Answer: Steady-state voltage magnitudes and power flows throughout the network

    Load flow analysis solves for the steady-state voltage magnitude, angle, and real/reactive power flows at every bus under specified loading conditions.

  7. Which of the following describes the Gauss-Seidel method as applied to power flow solutions?

    Answer: An iterative method updating bus voltages one at a time using the most recent values

    The Gauss-Seidel method updates each bus voltage sequentially using already-updated values from the current iteration, iterating until convergence.