Restricted Earth Fault Protection 3

[RichCarnell]

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.

 

[waross]

I should have used the word "bushing" rather than the word "terminal".
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

 

[stevenal]

Read the logic diagram a bit more carefully. If insufficient current is available at the line CTs to provide polarization for a directional determination, the relay will still operate non-directionally based on summation alone. Had a 387 trip properly on REF just recently. Main breaker on the low side was open at the time, and the phase CTs were on the load side of the breaker.

 

[stevenal]

Here's the event I spoke of. 1NW2 is the neutral current. Winding 2 is the wye winding that is part of the REF protection.

 

[RichCarnell]

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 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.

 

[davidbeach]

In the SEL-787 manual (the one I'm looking at has a date code of 20140714) look at the paragraph above Figure 4.14. This explains how the REF function operates when there is no current at the terminals CTs. If you are dealing with a typical distribution transformer, that's about the only way that the REF will ever operate. If you're dealing with a transformer in the transmission system then you will probably have currents into the transformer through the terminals for an internal fault.

 

[RichCarnell]

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 phase CT of the faulted phase on the transmission transformer?
Please see sketch in the next post.

 

[RichCarnell]

Sketch

 

[davidbeach]

The relay WILL operate without zero sequence current in the phase CTs. It will operate with an open breaker, that's what the bypass logic in the cited paragraph is all about. In your diagram, where does the current in the wye side of the distribution transformer go? If it can't flow in the wye side then it can't flow in the delta side - the amp-turn balance between the two sides must be maintained.

 

[RichCarnell]

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 phase of the faulted transmission transformer?

 

[davidbeach]

Transformer laws, no current in one coupled winding without a corresponding current in another winding in that coupled set. In the case of a delta-wye transformer there will be currents on both sides, the delta and the wye, and those currents will be related by the turns ratio of the transformer; in per unit the two sides will have the same current. If it can't flow on the wye side it simply won't flow on the delta side, no matter what.

Why on earth would you disable the bypass logic?

 

[stevenal]

Your latest sketch shows an ungrounded remote delta winding feeding back toward the ground fault. I see no way for this occur short of a simultaneous line to line fault on the secondary of the distribution transformer. If this truly represents your system, you still have no I0 at the phase CTs since the delta winding can only supply positive and negative sequence current. All the more reason not to bypass the bypass logic. If set as you say, your 787 REF element will remain secure for your in-zone winding to ground fault and you might as well leave the element disabled.

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.