Question

Autumn 2013 T1 G1 5 kV 50 MVA 5/20 kV 50 MVA 20 kV 50 MVA X 0.1 p.u Figure A13 In the system shown, per-unit series equivalent impedances are shown for each element. Before a balanced 3-phase fault occurs in the location shown, the pre-fault current is at its rated value at 90 % lagging power factor flowing from generator G1 The generator is operating at full voltage. How much current in kA is flowing from generator G1 after the fault occu rs? Useful Relationships

The following are the answers:

Answer 1

Given that, Winter 2012 Three - phase fault occurs P positive equivalent circuit \r\n fault current = I_f implies 1/j 0.4 = -j 2.5 pu Basement current = Base MVA/squareroot 3 times Base voltage implies 100 MVA/squareroot 3 times 50 kV I_base = 1.8193 kA Fault current at generator = I_f (pu) times I_base = -j 2.5 times (1.8193 k) = -j 16.5 kA therefore hence solved.

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