YD interposing CTs for Trafo Plain Balance (Differential) protection 3

[EPEngineer]

Hi,

I am involved in protection scheme design for substation in UK. We have a plain balance scheme similar to a high impedance differential scheme for trafo protection but as a back up so thats why 5P20 CTs and interposing CTs are used. 3 Wdg trafo vector group is Yyn0yn0. It is a common O/C relay. We have to install interposing CTs for ratio change anyay. My question is do we need to install delta connected CTs for zero sequence blocking? An old UK standard shows yd and dy set of interposing CTs and the relay is sonnected in delta. The main CTs are connected in Y, then YD interposing CT, relay, DY iterposing CT and then main Y connected CT for both LV legs. I was wondering what is the logic behind that. Your expert opinion is requested.

 

[jghrist]

There is some confusion. Is this a high impedance differential scheme? Probably not for a transformer. Is this a "common O/C relay"?

If this is a normal current differential relay for a tranformer, and it is a modern microprocessor type relay, then interposing CTs are not required. Zero-sequence blocking will be done in the relay.

 

[slavag]

Hi.
Yes, it's some confusion. Not so clear.
Maybe you try install old version of trafo protection ( electromechanical)?
1. 5:1 interposing CT you need for the LV side B. CT is 2000/5, according to your drw CT on the site A is 2000/1A
2. Only after this CT you connect in parallel LV side A and B.
3. on LV side you have Yn trafo connection, that means, you must use next set of interposing relays with delta connection for zero current elemination.
and this set will be multitap output interposing CT's.
4. From my point of view ( I don't remeber exactly, not sure) you also connect HV to set of multitap output inerposing CT's (will be connected in delta) .
5. Now you calculate range of tap according to the :
I HV nom/ I HV CT nom and I LV nom (sum)/ I LV CT nom.
Best Regards.
BTW, more cheaper and simple, bay new digital relay with
3 set's of CT inputs, instead all.....

 

[EPEngineer]

jghirst / slavag,

Many thanks for your responses. I must confess the scheme is not correct, excuse my drafting techniques. I have uploaded another one. The relay used is P122 Micom series relay. 5:1 interposing CTs shall be used to balance 2000/1 to 2000/5 LV sides as one is existing. The only issue is the vector group of main and interposing CTs. Are the vector groups shown on the new sketch correct and if not why and what is the correct answer?

Many thanks in anticipation

 

[slavag]

Hi.
one thing not seems .
interposing CT 1:2.4.
it's calculated according to CT 2000A.
But actually you have 4000A. Think about two winding trafo.

Anaway. Why this solution? With simple OC relay.
Please explain to your customer, isn't right solution.
You haven't all 87T ( trafo diff relay ) benefits.
2-nd harmonic blocks, 5-th harmonic block deblock,
restrained curves.

Best Regards.
Slava

 

[EPEngineer]

slavag,
I used to work in IEEE world earlier and this concept is also new to me but as I mentioned earlier, I am now in UK and this is a backup protection. There is a already a combined diff biased and HV/LV REF protection (Duobias Relay) as main protection. Plain balance is a standard back up protection in UK. Hasto be there.

So what about the vector group of CTs? Can anyone help?

 

[slavag]

OK, I see.
Vector group is O.K.
you need only re-calculate 1:2.4A interposing CT.
60000/1.73/11/4000=0.788
60000/1.73/66/800=0.657
tap will 0.788/0.657=1.2 instead 2.4
it's all.
Best Regards.
Slava

 

[EPEngineer]

slavag,

Many thanks

 

[slavag]

Hi EPEngineer.
Sorry, forgot wrote about it.
Please pay attention on YD interposing CT vector group.
That means, for example: both of them YD1 or ( depend on main CT connection) one is YD1 and second YD7.
or you can play with star point of main CT on the HV side.
And of course interposing CT sould be 5P20 with calculated VA's.
Good Luck.
Slava

 

[slavag]

Please see interposing CT cataloge:

http://library.abb.com/global/scot/...nt_transformer_1_or_5A_for_RADSB__SLCE_12.pdf

 

[ScottyUK]

Slight aside:

Be aware of the CT grounding conventions sometimes seen in the UK: the 'B' phase is sometimes grounded, rather than the star point. Lots of scope for confusion if you mix the two schemes.


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If we learn from our mistakes I'm getting a great education!

 

[slavag]

Hello Scotty.
We understand why VT phase "B" is grounded, but why with CT same.
tradition or some technical issue.
Best Regards.
Slava.

 

[davidbeach]

Grounding of 'B' phase of the CT secondary is common around high resistance grounded generators as a ground fault on the secondary of a solidly grounded set of CTs can draw as much current as ground fault on the generator. One would rather not trip off a generator for a CT secondary fault. With the CT secondary grounded at the wye point there are three potential faults that can trip the generator; A-G, B-G, C-G. With the CT secondary 'B' phase grounded that is reduced to only one potential fault, N-B (N-G).

 

[slavag]

You are great David!!!!
Thanks a lot.
learn, learn and learn, always.
Best Regards.
Slava