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Protective Relay Applications Flashcards

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  1. When testing a directional overcurrent relay (67), which reference quantity is typically used for directional discrimination?

    Answer: Polarizing voltage or current

    Directional relays use a polarizing quantity (voltage or current) as a reference to determine whether the fault is in the forward or reverse direction.

  2. A generator protection relay setting for loss-of-field (40) is based on the measurement of:

    Answer: Reactive power absorption (leading current)

    Loss of field causes the generator to absorb reactive power (operate in an under-excited leading power factor condition), which the impedance-based relay detects.

  3. What is 'CT saturation' and how does it affect differential relay performance?

    Answer: CT core saturates under high current, producing distorted secondary current that can cause false differential trips

    CT saturation during external faults produces distorted secondary currents that appear as differential current, potentially causing false relay operation.

  4. In transformer differential protection, what is the purpose of the harmonic restraint feature?

    Answer: To block tripping during magnetizing inrush current which contains significant 2nd harmonic

    Inrush current during transformer energization contains high 2nd harmonic content, and harmonic restraint uses this to distinguish inrush from internal fault current.

  5. Which ANSI device number is assigned to a breaker failure relay?

    Answer: 50BF

    ANSI device number 50BF designates the breaker failure protection function, which initiates backup tripping if the primary breaker fails to open.

  6. A breaker failure relay initiates after the primary relay trips. What starts the breaker failure timer?

    Answer: The trip signal issued by the primary relay

    The breaker failure timer typically starts when the primary relay issues a trip signal, timing out if current still flows after the expected breaker opening time.

  7. What does the pickup current setting of an overcurrent relay represent?

    Answer: The minimum secondary current at which the relay element begins to operate

    The pickup current is the minimum secondary current value at which the relay element energizes and begins timing (or trips instantaneously).