Question

A lossless transmission line with 0.3 pu reactance per phase uniformly distributed along the length of line, connecting a generator bus to a load bus, is protected upto 85% of its length by a distance relay placed at the generator bus. The generator terminal voltage is 1 pu. If no generator is at the load bus, the threshold pu current for operation of distance relay for a solid three phase to ground fault on the transmission line will be________ (upto 2 decimal places)

• A. 3.92

Answer 1

The correct option is A 3.92

$Perunitfaultcurrent,I_f=\frac{1}{Z_{th}}=\frac{1}{0.3}=\frac{10}{3}$

$\therefore for100\%oftransmissionline=\frac{10}{3}pu$

$Relayisoperatedfor85\%lengthoftransmissionline,$

$Z_f=0.85Z_l=0.85\times 0.3=0.255pu$

$For{85}^{\circ }{/}_{\circ }oftransmissionline,$

$I_f=\frac{1}{0.255}=3.92pu$