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That it true that it will operate using the bypass logic, although in my case, the bypass logic has been disabled in the settings. Why would there need to be current in the wye of the distribution transformer if the a phase on its delta has been effectively connected to earth by the fault in a...

Sketchhttp://files.engineering.com/getfile.aspx?folder=65ed0f10-864f-4678-b0cb-eb43f65061c9&file=1422746585795-1152159921.jpg

Would you agree then looking at the logic diagram for this relay that the REF element would not operate without zero sequence current in the phase CTs? It is used in the transmission system. If delta/Wye transformers are used for distribution would you agree that current would flow back into the...

For clarification, all testing to this point has been by secondary injection. The current reversal was done to prove stability before a trip. I now understand that I have mistakenly reversed the neutral current and not the phase current. I have had another look at the manual at figure 4.11 which...

If the faulted power TX in question was supplying a delta connected distribution TX, wouldnt the healthy phases of the faulted TX then supply the fault via the delta on the distribution TX? Supplying zero sequence current back into the fault? I think I have seen the light!

Thank you for your response, although I don't agree with it, reason being that the relay that I am testing (SEL787) requires zero sequence current to flow in the phase current CTs for polarisation. How could this be if it relies on an external source to supply this?

You say that the only time Ia would flow is if there were an external source? (in a faulted winding). That doesnt make sense to me as I would expect that the winding would still produce a voltage of some reduced quantity between the fault to earth and the A phase bushing? Or should I be thinking...

So the faulted winding would produce the fault current?

Davidbeach thank you for your perseverance. It's not that I don't think that the current would not change dependant on the location of the fault. Is more that I'm trying to work out how the currents flow in a faulted Tx. Please see my attached sketch. I thought that the fault current would have...

So why does the current change directions? What is feeding the fault if both the phase and neutral currents are pointing in toward an in zone fault?

So for an out of zone fault the current would flow from the phase terminals of the TX out to the fault, though the fault to earth, then back into the star point via the neutral on the TX. For an in zone fault both currents point in?

Yes. Thank you for your responses. I understand that the current does change direction. My question is, why

The pre fault condition in the test scenario would be a small amount of zero sequence current (above the required pick up value) flowing through the phase CTs to fulfill the requirements of the element under test. This current is required to determine the direction in the neutral CT. The neutral...

I am presently testing an REF scheme on a 25MVA TX and was interested to know if anyone could tell me why the current measured by the neutral CT reverses direction when a winding fault occurs. Is it to do with the fault changing the earthed reference point on the winding causing it to act like a...

Thanks waross. I think that it might be time to give up on this one as I am begining to see just how complex this could be. This isn't an assignment, it's something that was raised in class in the protection training that I am currently doing. The problem could be summed up with a response from...

Thanks waross for that detailed explanation. I now understand what would happen in an overloaded situation. If we were to then look at a 3 phase wye winding, using your impedance of 100 ohms per winding in the secondary, and adding a load to A phase so that the total A phase impedance were 25...

Thanks guys for responding. Marmite, I have seen this 2/3 and 1/3 ratio when I first came across this problem and found the reasoning behind it hard to accept. This is actually the reason I started this thread, to find a solution that made sense to me. Waross, thanks for your short story. I am...

Thanks for the tip ijl

Thanks ijl, any ideas where I could look for more info?

Thanks guys for the comments. ijl, we seem to be on the same track with your last comment, only if we assume that the primary side is connected to a 3 phase supply voltage then current would flow in a phase primary. Because of the ungrounded wye connection current would also flow, in a reverse...