Question

A surge of 15 kV magnitude travels along a cable towards its junction with an overhead line. The inductance and capacitance of the cable and overhead line are respectively 0.3 mH, 0.4 $\mu$F and 1.5 mH, 0.012 $\mu$F per km. The voltage rise at the junction due to the surge is _____ kV.

• A. 27.84

Answer 1

The correct option is A 27.84
Natural impedance of the cable = $\sqrt{\frac{0.3\times {10}^{-3}}{0.4\times {10}^{-6}}}=27.38\Omega$

Natural impedance of the line = $\sqrt{\frac{1.5\times {10}^{-3}}{0.012\times {10}^{-6}}}=353.55\Omega$

The voltage rise at the junction is the voltage transmitted into the overhead line as the voltage is zero before the surge reaches the junction
$E^{\prime \prime }=\frac{2\times 353.55\times 15}{353.55+27.38}=\frac{2\times 353.55\times 15}{380.93}=27.84kV$