A surge of 15 kV magnitude travels along a cable towards

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A surge of 15 kV magnitude travels along a cable towards its junction with a overhead line. If inductance and capacitance of the cable and overhead line are respectively 0.3 mH, 0.4 µH and 1.5 mH, 0.012 µH per km, the voltage rise at the junction due to the surge, will be ________ kV

1. 27.87
2. 2.787
3. 278.7
4. 2787

Correct Option: A

The surge travels from the cablet owards the overhead line and hence there will be positive voltage reflection at the junctions.
Natural impedance of the cable

= 0.5 √	0.3 × 10^-3	= 27.38Ω
	0.4 × 10^-6

Natural impedance of the line

= √	1.5 × 10^-3
	0.12 × 10^-6

= √	1.5 × 10^-3	= 35.3Ω
	0.12 × 10^-7

The voltage rise at the junction is the voltage transmitted into the over head line as the voltage is zero before the surge reaches the junction.

E" =	2 × 35.3 × 15
	(35.3 + 2.7)

=	2 × 35.3 × 15	= 27.87kV
	(35.3 + 2.7)