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Electrical Engineering QUESTION #2190
Question 1
A transmission line has $Z_1 = (0.4+j1.2)\,\Omega/\text{km}$ over $80\,\text{km}$. A distance relay's Zone-1 is set to protect $80\%$ of the line. The Zone-1 reach impedance magnitude is approximately:
  • $80.9\,\Omega$✔️
  • $64.0\,\Omega$
  • $101.2\,\Omega$
  • $50.0\,\Omega$
Correct Answer Explanation
$|Z_1/\text{km}| = \sqrt{0.4^2+1.2^2} = \sqrt{1.6} \approx 1.2649\,\Omega/\text{km}$. Total $|Z_{line}| = 80\times1.2649 \approx 101.2\,\Omega$. Zone-1 reach $= 0.8\times101.2 \approx 80.9\,\Omega$.