Fault Current Distribution in Delta & Wye Transf. 1

[rayas91]

Hi everyone, sorry for my english..I m not well experienced guy.

I just want to know that is there any much clear document about "current flows and calculations during faults in transformer"
I checked many documents in this forum and in others.But I coulndt find it.

By the way, Can you please answer "show or explain" my question ?

1. When Single phase to ground fault occurs in Wye, Why relay also see current in Phase C as well in Delta Side (I try to follow the circuit but i failed) ?

https://files.engineering.com/getfi...450e-b62c-14e346ad205b&file=SLG_Delta_Wye.JPG

Thanks.

 

[veritas]

It looks like your drawing is wrong. If you have an A-phase to ground fault on the secondary side of a delta-star trfr means you can only have current flowing in the primary A-phase winding due to the ampere-turns balance requirement. That means on the primary side the current flow is A-B phase and not A-C phase.

If there is no current in the secondary B and C phase then there can be no current in the B and C phase of the primary windings either.

 

[stevenal]

It helps to have a correct reference. The transformer depicted is not ANSI. For the transformer depicted, current flows in on A and out B for an a phase fault.

 

[rayas91]

Yeah, But I m still confused. Because I saw many drawing like that in web and documents.

Thanks

 

[rayas91]

veritas , thank you .
stevenal , I just saw your message.

Can you please check this drawing?
on attachment.

 

[stevenal]

The drawings appear to be correct, although I didn't check all the angles. I disagree with the statement that zero sequence current is not correctly transformed. All components are transformed just fine, but the zero sequence current stays inside the delta and does not appear on the lines. Filtering of zero sequence is needed for differential relaying.

If you are looking on line at course notes, you probably don't see the corrections made in class. I suspect some are deliberately wrong just to make sure folks are paying attention.

 

[veritas]

I agree with stevenal. Drawings seem to be fine and zps currents do get transformed but remain in the delta. Also, there is no "false" diff currents. Just real diff currents that will need to be reckoned with when doing differential relaying.

 

[waross]

Respectfully Steven; Perhaps I don't understand what you mean by;
" but the zero sequence current stays inside the delta and does not appear on the lines."
A line to neutral fault or load on the secondary shows up as line to line current on the primary.
It does not "stay in the delta".
The current goes into the "A" winding on line A and out on line C.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rayas91]

veritas , stevenal and waross ;

On first diagram , Current occurs on line A and C ..Yes... But how about phase currents ? If there is a L-E fault on A , Will fault current flow on phase A or phase C? That s the point which I dont understand.

And there is another diagram on attachment. I beliave that is correct diagram. But still I m not sure..
İf this new diagram is correct, values on which I attached as second diagram are not correct...

 

[waross]

You don't seem to have the secondary fault phases lined up with the proper primary phases.
Your notation:
"2I" implies two times the current.
The fault currents are at different phase angles with the result that all three primary currents are equal.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rayas91]

waross,

Yes I got your point.

By the way,

I find out the problem which I faced.
I just followed the diagrams's explanations in some pics and articles.I saw that they did not point out the Delta connection type.Some of them are DAB, others are DAC connection.

In protection relays I saw the correct one.


Thanks for your time.

 

[stevenal]

I'll disagree with Waross. Yes, they are at different angles, 180 degrees apart. With a nice simple angle like that, you can simply assign a direction as shown to use simple arithmetic. C phase will have twice the current of the other two phases, just as B does in the second link above (which does use complex arithmetic).

 

[rayas91]

stevenal,

Thank you..today I will try to do as what you said.

 

[waross]

Respectfully Steven; Is this a case where the angle of the common line changes as the current develops from a load current to a fault current?
If we are looking at the same diagram then with a normal load my comments are correct.
I was commenting on the lower diagram with the phase to phase fault. I should have made that clear.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rayas91]

I was talking about the magnitudes not angles...Because I did not know that is why current occurs on phase C in HV during occuring fault A in LV.

If there is fault on phase A in LV, The fault should flow on phase current A , line current A and line current C. It s what I learned.
Still not much sure.

 

[stevenal]

Waross,

The diagram shows no load, and I didn't assume any. Kirchoff tells us the sum of the currents at the fault equals zero. With only two phases feeding the fault in the lower diagram, those two currents must be equal and opposite.

As to your earlier post, the phase currents flow as you say. I was speaking of the zero sequence component, which will circulate in the delta. The two non-zero phase currents on the primary side have zero sequence components equal to zero. To keep from tripping on a through fault, differential relaying must filter the zero sequence on the wye side.

 

[waross]

Got it, Steven. Thank you for your patience.
I mistakenly assumed a two phase to ground fault.
Without ground involvement I agree that you are correct.

Bill
--------------------
"Why not the best?"
Jimmy Carter