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Per-Unit Impedance Conversion

The per-unit impedance of a power system component is 0.25. If the base kV and base MVA are halved, then the new value of the per-unit impedance will be
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Answer & Explanation
Correct AnswerC. 0.5

Quick Explanation

Per-unit impedance transforms as Zpu,new=Zpu,old(Snew/Sold)(Vold/Vnew)². If both base MVA and base kV are halved, the factor is (1/2)×(2)²=2, so the new per-unit impedance is 0.25×2=0.5. Hence option C.

Formula / Key Relation

Zpu,new=0.25×0.5×(2)²=0.5
Per-Unit Impedance Conversion explanatory diagram

Detailed Explanation

Correct Answer

C — 0.5

Concept and Reasoning

Per-unit impedance transforms as Zpu,new=Zpu,old(Snew/Sold)(Vold/Vnew)². If both base MVA and base kV are halved, the factor is (1/2)×(2)²=2, so the new per-unit impedance is 0.25×2=0.5. Hence option C.

Calculation / Key Relation

Zpu,new=0.25×0.5×(2)²=0.5

Exam Takeaway

Base-kV change is squared; base-MVA change is linear.

Common Mistake

Do not halve per-unit impedance just because both bases are halved.

Quick Trick

Base-kV change is squared; base-MVA change is linear.

Common Mistake

Do not halve per-unit impedance just because both bases are halved.

Exam Tip

Base-kV change is squared; base-MVA change is linear.

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