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GATE 2022 EE Q.13: solution and answer

GATE 2022 · EEQ.13 · 1 mark · MCQ

The most commonly used relay, for the protection of an alternator against loss of excitation, is

  1. (A)

    offset Mho relay.

  2. (B)

    over current relay.

  3. (C)

    differential relay.

  4. (D)

    Buchholz relay.

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Answer and solution

ANSWER

The offset Mho relay is the most commonly used relay for loss-of-excitation protection of an alternator, so the answer is (A).

Official answer: ASource: Official answer key, GATE 2022 EE (IIT Kharagpur)

  1. When the field of an alternator is lost, the machine keeps running above synchronous speed and draws magnetising (reactive) power from the grid, behaving as an induction generator. Stator current stays near normal, while the terminal voltage may sag.

  2. As seen at the alternator terminals, the apparent impedance moves from the first quadrant of the - plane, where it lies in normal operation, into the fourth quadrant, where is negative. It settles close to the axis, with of the order of half the direct-axis reactance.

  3. Detecting this requires an impedance-sensing (distance-type) relay. An offset Mho relay has a circle with characteristic , the circle being shifted below the axis so that it covers the fourth-quadrant region where the loss-of-excitation locus settles. It operates when the apparent impedance enters this circle.

  4. Overcurrent relay: the stator current stays near normal during field loss, so a current-sensing relay cannot discriminate this condition. Option (B) is rejected.

  5. Differential relay: it responds to the difference between currents entering and leaving the stator winding, which signals an internal winding fault. Field loss is not a winding fault, so it sees no difference current. Option (C) is rejected.

  6. Buchholz relay: it is a gas-actuated relay for oil-immersed transformers and has no role in alternator protection. Option (D) is rejected.

  7. Hence the standard loss-of-excitation relay is the offset Mho relay: option (A).

KEY IDEA

Loss of field makes the alternator's apparent impedance move into the fourth quadrant of the - plane. An offset Mho (impedance) relay with its circle shifted below the axis detects this.

WATCH OUT FOR

Choosing the overcurrent or differential relay because they are the usual alternator protections. Loss of field leaves the stator current near normal and is not a winding fault, so neither responds. Only an impedance-sensing relay sees the change.

From the GATE 2022 EE paper. Every question of this paper · Download the full paper · All 5 Protection questions, with notes

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