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Active Contributor

Join Date: Nov 2012
Posts: 17

Earth Fault Current of a Generator

01/04/2013 9:24 AM

If the generator of 11kV having a NGR of 63.5Ω,if any one one phase is grounded,then the earth fault current is limited to 100A due to 6350V applied across NGR....

If TWO Phases grounded voltage applied across the NGR is 11kV,then what is the earth fault current...?

If THREE Phases grounded voltage applied across the NGR is 0V due to vector sum of three phases,then what is the earth fault current...?

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#1

Re: Earth Fault Current of a Generator

01/04/2013 10:32 AM

If it isn't in your text book, then this may help: Fault (power engineering) - Wikipedia, the free encyclopedia

Wasn't what you needed? My point is that as with your previous questions, this can be found by performing a simple search.

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#2
In reply to #1

Re: Earth Fault Current of a Generator

01/04/2013 11:09 PM

You don't tell how much earth fault current flows....?

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#3
In reply to #2

Re: Earth Fault Current of a Generator

01/05/2013 3:28 AM

It's your homework, not mine.

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#4

Re: Earth Fault Current of a Generator

01/05/2013 8:54 AM

assuming the sequence impedances of the generator are all equal zero .

X1 = 0 & X2 = 0 & X0 = 3*63.5 = 190.5

for (L-G) fault the fault current will be 100 as you said .

for (L-L-G) fault the fault current will be (6350)/(x1+x2//x0) = infinity

for 3-phase fault the fault current will be (6350)/x1 which equals infinity or the maximum capacity of the generator

so for proper calculations you should know the sequence impedance of that generator

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#5

Re: Earth Fault Current of a Generator

01/05/2013 1:08 PM

For 1 phase to ground, that is simple and Ok for 100A

For 2 phases to ground, there will be inter-phase current that does not go thru the NGR, but directly via ground resistance (which could be very small resistance compared to the NGR of 63.5Ω!) and high, limited by earth resistance at site...

For 3 phases, the fault is similar to 2 phase fault in the sense that it will be via ground and not thru the neutral dround resistance! This will be almost a short circuit condition on the generator terminals, limited only by earth resistance between the phases: Site conditions and type of ground location where it occurs. This will not be ZERO!!! even though you thing that the vectors eliminate at the neutral, that is only for Neutral returning currents but not the phase current flowing within the generator!!

Up to you to figure out the equations! I give only a tip of what is the problem...

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#6

Re: Earth Fault Current of a Generator

01/05/2013 6:24 PM

If more than one phase is grounded at once, expect the conductor overcurrent protection device to operate on at least one of the phases. The imbalance device will pull out the final phase if its overcurrent device didn't operate at the point of grounding one or both of the other two phases. If none of these devices operates then expect the generator to be on fire/scrap metal/exploding.

Please makse sure all other CR4 readers are standing well back before carrying out this particular madness.

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