Secondary fault - Primary current?

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Lars

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If I have a Phase/Ground fault secondary on a D/Y transformer, how will that primarily affect the transformer?

In my opinion, it will influence only one Phase secondary, but I am in doubt about the primary side.

A concrete example is a Phase/ground fault on a 480V installation, meaning that the voltage is 277V and for example 100A ? This will affect only one winding on the secondary Y transformer, but how about the 3 primary 4160V D windings. How will the current split up?

Thanks
 
Lars said:
A . . . Phase/ground fault on a 480V installation . . . . will affect only one winding on the secondary Y transformer. . . .
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Not true. It will affect all three secondary windings, and therefore all three primary windings.

If this is not intuitive to you, you are not alone. But consider this: Create a fault from Phase A to Neutral. Prior to the fault, current flows from A to B, and from B to C, and from C to A, and from A and B and C to neutral. If the load is balanced, then A, B, and C will have equal currents, and the neutral will have no current. But the amount of current flowing from A to B, for example, will depend on the impedance along the path. That path includes the load on A, the load on B, and the impedances of the two windings. After the fault, currents will flow to the fault point. Currents from all three phases see a lower impedance path then they had before. That is because they can go from the fault point back to the source, without having to travel through the windings of the faulted phase A.

The amount of influence on each winding will depend on the impedances of each and the connected load (i.e., large motor loads will add to the amount of available fault current). Analysis of this type of fault requires a tool, called "Symmetrical Components," in the theory and use of which I am a bit rusty. There are computer programs that will perform the analysis as well.
 
This is why the code requires GFI protection on the secondary of the separately derived source feeder, as the upstream GFI device on the primary may not pick up the secondary L-G fault.
I have to search to find the code section.
 
Charles E. Beck

All right, thanks for your answer.
I see there is no simple explanation to this question, but what about the feeder to the transformer. Will the current in each 3 phases have the same complex split up, if the fault is the same as the fist mentioned?

Thanks again.
 
If the fault is unbalanced (line-to-ground, line-to-line, double-line-to-ground, almost anything other than a bolted 3-phase fault), then the currents in the secondary windings, the primary windings, and the primary feeders will not be balanced.
 
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