It should be a very easy question but I cannot get my head around it.

If one of the 400 kV transmission line has a fault so at the instant (before the breakers are opened), the voltage drop to zero and high short circuit current.

What will happen to the voltage to the overall system? I think the voltage will drop but I am not sure why. My own explaination is that the impedance increases when one line is removed, so the power output is dropping so the voltage of the system will drop as well. It is such an odd answer though.

Appreciate your help.

Richard,

You need to think of the network as a multi-stage potential divider, with all the lines and transformers as impedances and the distant generator(s) as 1pu voltage sources, with your fault connected to "ground".



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Dr. Andrew Roscoe

http://personal.strath.ac.uk/andrew.j.roscoe

With a solidly earthed neutral and a single line to earth fault on phase A
The positive, negative and zero sequence components of the current I1, I2 and I0 are equal, I1 = I2 = I0
Fault current = Ia = I1 + I2 + I0 = 3 E / [Z1 + Z2 + Z0]
Va = 0
Vb and Vc are unchanged.

Rergards
John

Sorry about late reply. I have beem busy and forgot about the question I posted but I still really want to know about this question.

John,

I thought about using symmetrical component but it is not really about the fault on one phase but the line itself is faulted for whatever reason. The question was about the system voltage.

Dr. Roscoe

There are three transmission line, one of the line is faulted and shorted. I can imagine the line voltage at the incidence is close to zero but what I can think of is that the impedance of the parallel transmission lines will increase so the power output will drop and impact on system voltage.

I don't quite understand what you mean though but I think I read something somewhere in the book. I will go read it again to see if I can find a hint. So far, I don't understand what you are stating.

I agree with Mr John. That is the inherent advantage of solidly earthed system. In reality , there may be some change (increase) in other healthy phases depending on overall ground impedance and circuit impedance . Higher the impedance , greater will be the rise in healthy phases.
Regards,
Umesh