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Bias setting of transformer differential relay cannot avoid maloperation due to

a) External fault

b) CT saturation

c) CT mismatching

d) Over-voltage on source side of the transformer

1. A only
2. D only
3. B and C only
4. B only

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Correct Answer - Option 2 : D only

Bias setting of transformer differential relay:

The relay operate when magnitude of the difference in the currents at two ends of a protected element exceeds a certain percentage Nr/No of half the magnitude of their sum. Hence the name is percentage differential relay.

Where No = number of turns of the operating coil and Nr = number of the restraining coil.

In order that the differential relay does not operate during conditions other than genuine internal faults, the restraining characteristic and which are necessary are:

a) Stability for external faults

b) Stability on magnetizing inrush, and

c) Stability during over excitation in rush.

Stability for external faults: A differential relay may maloperate for external faults due to the following:

  • Current transformer saturation
  • Current transformer mismatch


Stability on magnetic inrush: When a transformer primary is switched on to the supply source with the secondary is kept open, a transient magnetizing inrush current flows only on the primary side which appears as an internal fault to the differentially connected relays.

Stability during over-excitation inrush: With the advent of CRGO steel in transformer manufacture, modern power transformers are designed to operate at about 90 percent of the saturation flux levels at rated voltages. Therefore, during abnormal system conditions short duration over voltage conditions can occur, resulting in saturation of transformer core.

In order to ensure that the relays do not maloperate during above conditions some relays are provided with higher harmonic restraint features. But the maloperation can’t be avoid due to over-voltage on source side of the transformer.

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