Restricted Earth Fault Protection
Restricted Earth Fault Protection
(OP)
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 center tapped winding?
The TX is a delta/wye 33/11kV with the star point on the secondary solidly earthed.
The TX is a delta/wye 33/11kV with the star point on the secondary solidly earthed.






RE: Restricted Earth Fault Protection
RE: Restricted Earth Fault Protection
RE: Restricted Earth Fault Protection
RE: Restricted Earth Fault Protection
RE: Restricted Earth Fault Protection
RE: Restricted Earth Fault Protection
RE: Restricted Earth Fault Protection
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Why do you think that current through a set of CTs wouldn't change depending on which side of the CTs has the fault?
RE: Restricted Earth Fault Protection
RE: Restricted Earth Fault Protection
RE: Restricted Earth Fault Protection
RE: Restricted Earth Fault Protection
So turn the arrows around to stay with convention, and then draw it again for an out of zone fault. Also try it with a remote A phase LV source that also feeds both an in zone and out of zone fault.
RE: Restricted Earth Fault Protection
RE: Restricted Earth Fault Protection
RE: Restricted Earth Fault Protection
RE: Restricted Earth Fault Protection
I think I have seen the light!
RE: Restricted Earth Fault Protection
For an example, consider the CT to the left of the terminal. The CT will now see all neutral current, both line to neutral and line to ground.
Now consider a variable load connected line to neutral on one phase. The neutral CT (now placed to the left of the terminal) will see this load current. Now increase the load. The current seen by the CT will increase.
Continue to increase the load until the load current reaches fault level. At no point will the current reverse.
Now reconnect the load from line to ground. Apply and increase the load. The results will be the same, no current reversal.
Now consider a winding fault on an unloaded transformer.
If we consider a fault at the 95% point on one winding, then we may simulate this by connecting our test load from a -5% tap to ground.
Same result. No reversal of current.
I accept that the OP saw a reversal of current.
I believe that the question is:
Why was the test set showing a current in reverse of the normal current before the test started??
Concerning delta connections:
If the transforme under test was feeding a wye:delta transformer with a four wire primary connection, it may be possible for the fault current to reverse.
CT position:
If the CT is to the right of the terminal as shown, the CT will see only ground currents.
If the CT is to the left of the terminal, the CT will see both ground and and line to neutral currents.
Another possible cause is if one of the test connections was inadvertently made to the neutral terminal rather than to the proper ground position. Such a connection may allow the fault current to split and part of the current to return via the grounding jumper (and CT) in the reverse direction.
Bill
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"Why not the best?"
Jimmy Carter
RE: Restricted Earth Fault Protection
Recap:
Load curent and fault current are in the same direction.
Why were we seeing a reverse current before the application of the fault?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Restricted Earth Fault Protection
RE: Restricted Earth Fault Protection
RE: Restricted Earth Fault Protection
I have had another look at the manual at figure 4.11 which is the enable logic for the REF element, and if I understand it correctly, requires 0.8 x the pick up setting in the polarising winding to enable it.
Please correct me if Im wrong, but wouldnt that mean that there needs to be zero sequence current flowing in the phase CTs to enable the element?
This is the crux of why i started this thread. So if you guys think you could see where Im going wrong here perhaps I might be able to cut down on the headache tablets. :)
RE: Restricted Earth Fault Protection
RE: Restricted Earth Fault Protection
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 phase CT of the faulted phase on the transmission transformer?
Please see sketch in the next post.
RE: Restricted Earth Fault Protection
RE: Restricted Earth Fault Protection
RE: Restricted Earth Fault Protection
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 phase of the faulted transmission transformer?
RE: Restricted Earth Fault Protection
Why on earth would you disable the bypass logic?
RE: Restricted Earth Fault Protection
There is no way to disable the bypass logic in the relay. You can, however, map any relay word bit you like into a trip equation. Did you choose to map something other than REF1F to a trip equation? I would suggest you stick with REF1F so the bypass logic is used as intended.
I also suggest rotating the windings in your diagram so that winding currents will show as properly in phase. Then show your fault currents on the other sides of the transformers as well.