differential protection on 3 phas TXR

[rg1]

Here is the text:

In our example, the transformer has the Δ-Y connection. Traditionally, CTs on the Wye connected transformer winding (winding 2) are connected in a delta arrangement, which compensates for the phase angle lag introduced in the Delta connected winding (winding 1), so that line currents from both windings can be compared at the relay. The Delta connection of CTs, however, inherently has the effect of removing the zero sequence components of the phase currents. If there is a grounding bank on the Delta winding of the power transformer within the zone of protection, (IMHO this situation can be imagined same as star connected wdg with ground, on main Delta wdg side too- So the situation becomes like this - Both sides, primary and secondary of the Xmer are star with grounded neutral- and the CTs on one side are star connected (because main wdg is delta) and other side it is delta connected. In such a case the zero sequence currents on star connected CTs will flow through the Operating coil of the relay.  In this case you will have diff relay tripping on through L-G faults because they give rise to zero sequence currents in the star connected CTs. To avoid this the traditional method is to connect both side CTs in delta connection, attached is the diagram for connection. This is called marketing strategy of T60,  nothing else- as if till digital relays were not there, we were without electricity).  a ground fault results in differential (zero sequence) current and false trips. In such a case, it is necessary to insert a zero sequence current trap with the Wye connected CTs on the Delta winding of the transformer.

This is a very special case. If there weren't so many special cases we wouldn't need engineers.

I hope the explanation makes sense. I have never dealt with grounding winding of a transformer because I have never worked in a ungrounded system, I mean it is really funny to have undergrounded system and then ground it by a separate wdg. However the concept is no rocket science, I must have understood it rightly. Discussion welcome. 

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[rg1]

I hope the explanation makes sense. I have never dealt with grounding winding of a transformer because I have never worked in a ungrounded system, I mean it is really funny to have undergrounded system and then ground it by a separate wdg. However the concept is no rocket science, I must have understood it rightly. Discussion welcome. 

View attachment 9987

When you have delta winding and a star connected neutral grounded winding on same side of transformer, you have two parallel paths for Zero sequence currents, one is to circulate with in delta and the other in the neutral. How much will go where will depend on the  resistances of the paths. I have taken everything going  into neutral (extreme case) just for better appreciation.

 

[TNPE]

Per the original drawing, no zero sequence current flows to the relay coils.  Zero sequence circulates in delta CTs and no zero sequence present on wye CTs due to the absence of a neutral.  X2 bushings are bonded together and floating.  Am I missing something you're trying to say?

The primary of the XFMR winding is wye grounded, thus zero sequence currents will flow between that point and it's upline source.  This is a rather odd connection.  I'm used to the whole configuration being flipped.

 

[rg1]

Per the original drawing, no zero sequence current flows to the relay coils.  Zero sequence circulates in delta CTs and no zero sequence present on wye CTs due to the absence of a neutral.  X2 bushings are bonded together and floating.  Am I missing something you're trying to say?

The primary of the XFMR winding is wye grounded, thus zero sequence currents will flow between that point and it's upline source.  This is a rather odd connection.  I'm used to the whole configuration being flipped.

The discussion started with Cos90 brought  a scenario from t60 manual which states like this

" In our example, the transformer has the Δ-Y connection. Traditionally, CTs on the Wye connected transformer winding (winding 2) are connected in a delta arrangement, which compensates for the phase angle lag introduced in the Delta connected winding (winding 1), so that line currents from both windings can be compared at the relay. The Delta connection of CTs, however, inherently has the effect of removing the zero sequence components of the phase currents. If there is a grounding bank on the Delta winding of the power transformer within the zone of protection, a ground fault results in differential (zero sequence) current and false trips. In such a case, it is necessary to insert a zero sequence current trap with the Wye connected CTs on the Delta winding of the transformer." 

So this is a situation in which there are three windings on Xmer, Delta, Star, and one more star on delta side to provide ground on delta side. The relay manufacturer (t60, page 372) while pointing out to better characteristics of his relay states that there will be spurious tripping in case of the conventional methods of relaying. I tried to analyse  why there be spurious tripping in the event of through L-G   fault and how it could be avoided even in conventional methods. Now if the problem statement is clear you can go through my explanation whether it is true.

 

[TNPE]

So, this sounds like a zigzag has been installed near the delta so that a ground reference can be made for the secondary side.  In this case, yes, I can see where you would have to protect in a seemingly unconventional manner to offset the introduction of zero sequence from the zigzag.

That said, can anyone explain to me where a XFMR arrangement like the one presented in the OP would be used, or even practical, in a utility environment?  Personally, from my experience and the applications I'm accustomed to, I would never use that arrangement at the sub-transmission or distribution levels.

 

[cos90]

My apologies for starting this discussion. Here's how you would solve this problem before microprocessor relays came onto the scene:

@TNPE  you would see a wye-delta transformer applied for Generator Step Up (GSU), the tertiary of a large autotransformer, and at interconnection sites where two systems are operating at a different phase angle. For distribution you typically see a Wye-Wye or Delta-Wye transformer.

 

[cos90]

A GSU is a delta-wye. I misspoke.

The LV winding is the primary on a GSU transformer.

 

[cos90]

You could also see a wye-delta used as a grounding bank, used like a zigzag transformer.

 

[TNPE]

That's my point, it is almost impractical to use this winding configuration on the utility scale when other more practical, existential options are available.  Most transmission step-downs (e.g. 161-69kV or similar) are wye-wye grounded with a delta tertiary.  The tertiary serves two purposes, trapping zero sequence and providing station service to a 13.2 kV bus (typically). Generally speaking, to pick up ground faults, you want your down line sections to be looking back at a grounded secondary.  Without this, you have to use a more expensive alternate approach to detect "ground" faults.  Whether you accomplish this by beefing up the relays or adding a zigzag.  Why even go that route when it's not necessary?

 

[rg1]

My apologies for starting this discussion. Here's how you would solve this problem before microprocessor relays came onto the scene:

@TNPE  you would see a wye-delta transformer applied for Generator Step Up (GSU), the tertiary of a large autotransformer, and at interconnection sites where two systems are operating at a different phase angle. For distribution you typically see a Wye-Wye or Delta-Wye transformer.

Yes it will. Thanks Cos90 for sharing this, you seem to have good stuff. When I look back the reason of zero sequence - I was right and then solution is different from what I suggested. In my suggestion 30 degree phase angle  problem, will again appear which need to be resolved by ICTs ( Intermediate CTs connected in delta). Here they are using a delta connected trap for resolving  zero sequence currents. But as mentioned by the t60 manual- it is not true that the conventional relaying system were not able to tackle this problem.

Still I fail to understand when you chose and  need an ungrounded system why you need grounding. Should have gone for star-star Xmer. To be honest my practical knowledge of an ungrounded system is nil.

 

[TNPE]

Dodge the problem entirely by doing what I mentioned in other posts.  No need to complicate it and make it more expensive at the same time.   

I have yet to see a scenario where this configuration would be needed, at least with regards to HV, MV and distribution utility applications.