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SarkariResultOutBMS · 2024

Previous Year Question

Consider following statements: Assertion: When a line-to-line (L-L) fault takes place at the terminals of an open-circuited generator, the phase voltages are sometimes indeterminate though line voltages are always determinable. Reason: During L-L fault, a zero-sequence voltage is always indeterminate. Of these statements:
  1. Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
  2. Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
  3. Assertion is true, but Reason is false.
  4. Assertion is false, but Reason is true.

Correct answer: A

Explanation

For an L-L fault there is no zero-sequence current, so the zero-sequence network is not fixed by current flow. At open-circuited generator terminals this can leave the common zero-sequence voltage reference indeterminate even though line-to-line voltages remain uniquely determined. Hence both assertion and reason are true and the reason explains the assertion: option A.

Formula

L-L fault: I0=0; line voltages are independent of common zero-sequence voltage shift

Step-by-step solution

Zero-Sequence Reference and Phase Voltage explanatory diagram

Correct Answer

A — Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

Concept and Reasoning

For an L-L fault there is no zero-sequence current, so the zero-sequence network is not fixed by current flow. At open-circuited generator terminals this can leave the common zero-sequence voltage reference indeterminate even though line-to-line voltages remain uniquely determined. Hence both assertion and reason are true and the reason explains the assertion: option A.

Calculation / Key Relation

L-L fault: I0=0

line voltages are independent of common zero-sequence voltage shift

Exam Takeaway

A common phase-voltage shift cancels in line-to-line voltages.

Common Mistake

Do not infer that indeterminate phase reference makes line voltage indeterminate.

Common mistake

Do not infer that indeterminate phase reference makes line voltage indeterminate.

Exam tip

A common phase-voltage shift cancels in line-to-line voltages.

Quick method

A common phase-voltage shift cancels in line-to-line voltages.