# Fault calculations - power

Discussion in 'Homework Help' started by Juniordesigner, Aug 29, 2008.

1. ### Juniordesigner Thread Starter New Member

Aug 22, 2008
2
0
Does anyone have an idea of what to do for this question. Any help or guidance would be really appreciated.

A generator is subjected to a sustained fault with phases b and c short circuited together and to neutral.Show that the zero sequence impedance may be determined from the following relation: Z0=Ea/In where Ea is the voltage from the unfaulted conductor to neutral and In is the neutral current, i.e., the sum of the currents Ib and Ic.

Last edited: Aug 29, 2008
2. ### subtech AAC Fanatic!

Nov 21, 2006
123
5
Speaking VERY generally...
Consider the following information.

1. Shorting gen terminals' A and B to the neutral will yield current 3Io in the neutral.
(Not just Io, see below)

2. The zero sequence (neutral) current results from unbalance in the phases and in this case is limited only by the reactances (impedances) of the machine windings.
(remember that sequence quantities are equal in all three phases, never in just one phase alone)

3. If Io exists equally in all phases, in this example it exists in the unfaulted phase as well.( phase A)

4. The unfaulted phase voltage remains normal. Only the two faulted phase voltages will collapse.(If the fault were truly at the gen terminals, the B-N and C-N voltages would be 0.)

5. Ia0= 1/3 X 3Io

6. Zo = ....

I think this should get you there. If not, post again.

Best wishes