Why 2x3 CT-s on LV side of transformer for diff . prot? 1

[pisimatza]

Hi
I learned at school and never met since then with this problem: for differential protection of transformers 3 CT-s are needed on LV and 3 CT-s on HV side. I recently saw a diagram with 6 CT-s for LV side. No info-s about HV side, anyway i am concerned only about LV switchboard.
So, why 6? Redundancy?
Thanks.

 

[DanDel]

The zone of protection for a differential relay is between the two sets of CTs. Each set is connected to a different input on the relay. Are you saying that there are 6 CTs at the same place connected to one input side of the differential relay?

 

[Zogzog]

Are you sure all 6 Ct's on the LV side are for the 87 relay?

 

[oldfieldguy]

Check your wiring. It is not uncommon for two or three sets of CT's to be found on a transformer secondary. One set might be used for the differential scheme, another set for the secondary overcurrent relaying, and a third set for metering.

You will likely find that another set of CT's feed into the differential relays from the primary side of things. Commonly these are on the incoming line side of the circuit breaker feeding the high voltage side of the transformer. This puts the circuit breaker inside the protection zone of the transformer differential relay.

old field guy

 

[slavag]

Maybe its two CBs on the low side.
In this case you can connect two CT sets in parallel ( of course if its same size of CTs). Please attached this drw.
Regards.
Slava

 

[dpc]

Sometimes CTs are wired in series to improve their performance during short circuits. With two sets of matched CTs in series, each CT only has to produce 1/2 the voltage to drive the secondary current. So two sets of C400 CTs become effectively a C800 CT.

The operating principle of the differential relay is unchanged - the relay operates in exactly the same way.

This is discussed in J.L. Blackburn's Protective Relaying book.

 

[521AB]

Slavag:
"..... Maybe its two CBs on the low side.
In this case you can connect two CT sets in parallel ( of course if its same size of CTs). Please attached this drw...."

this is dangerous, but sometimes it is the only solution.
It would be better to enter with the 6 CTs in the relay separately, (from the 1 1/2 CB side) and with the 3 CTs from th eother power transformer side (totally 9 analog inputs in the relay) and use a "3 tap" differential protection which handles the situation. This way in case o through fault in the 1 1/2 side, you keep the bias current high, and increaqse stability for the protection.

 

[pisimatza]

I am sure that all 6 CT-s are on LV. In fact there are:
3 CT-s for measuring, 3 CT-s for some signal to a Generator, and 3+3 CT-s for transformer differential protection. I am interested in the last 3+3 of them.
Why not only 3? I reproduced essential part of drawing on Paint Brush and attached.

 

[521AB]

3 CTs for Transformer Difefrential.
maybe other 3 CTs for Restricted Earth Fault (zero sequence differential protection between 3 line CTs in holmgreen connection and neutral CC).
Maybe?
Probably it is also a high impedance restricted earth fault, that needs different (and dedicated) CTs

 

[521AB]

Sorry, I'll post once again, too many typing errors

3 CTs for Transformer Differential.
Maybe other 3 CTs for Restricted Earth Fault (zero sequence differential protection between 3 line CTs in holmgreen connection and neutral CT).
Maybe?
Probably it is also a high impedance restricted earth fault, that needs different (and dedicated) CTs.

 

[unclebob]

What is the secondary current of the xformer?

Your drawing does'nt show precisely how the CTs are wired(series, parallel).

 

[DanDel]

Possibly the CTs are used for two separate relays protecting two separate zones? We can't see the other CTs or the relay itself in the drawing.

 

[pisimatza]

DanDel (Electrical) wrote:
11 Mar 08 10:52
"Possibly the CTs are used for two separate relays protecting two separate zones? We can't see the other CTs or the relay itself in the drawing."


Well, nice thinking, but even if it is so, wouldn't be enough one CT for both zones?
And this specification is all i have. Stage is quotation, and we already moved on proposing only one (one set= 3 pcs) CT.
Thank you all for opinions.

 

[RRaghunath]

I agree with 521AB. Recently I have come across a scheme using separate set of CTs for high impedance REF protection. Since pisimatza says that the CTs are on the Trafo secondary (and it is likely that the windings are Y-connected), the second set of CTs could be for REF. Single relay but separate CTs.
I understand AREVA and SIEMENS confirm that one set of CTs can be used for Differential and REF protection, though I haven't seen any factory test results with the said connection. ABB, I am not sure. The RET manual is also silent on the subject.

 

[DanDel]

pisimatza, each differential relay would need its own set of CTs; three at one end of its zone, and three at the other end.

 

[slavag]

Hi.
Are possible about 1000 and 1 options.
1. As DanDel saied, two diff protection on one relay. for example: one for the trafo diff , one for the generator protection.
2. As 521AB and Raghun saied one for diff protection bias and one for HI REF protection.
3. One for the bias diff protection and one for the HI dif protection .
4. One for the bias diff protection and one for the OC LV back-up protection.
5. One for the bias trafo diff protection and one for the feeder dist protection .
6. One for the bias diff protection, one for the metering.
All for the one box solution.

Raghun. What ABB RET silent this information? I work with four types of RET:
RET316*4 Swiss.
RET670 Sweden
RET521 Sweden
RET54_ Finland.
Please send type, I'll try help you.
Regards.
Slava

 

[jghrist]

...anyway i am concerned only about LV switchboard.

And this specification is all i have. Stage is quotation, and we already moved on proposing only one (one set= 3 pcs) CT.

Are you quoting on the switchboard with CTs? If so, and you have no more information than what you have shared, I think you should quote on the two sets of CTs as requested.

As indicated by others, there are multiple reasons that two sets of CTs may be required.

 

[521AB]

"...ABB, I am not sure. The RET manual is also silent on the subject. .."

Regarding RET 521 and I think RET 670:
you can use one set of CT, from transformer diff and for Restricted earth fault and for overcurrent back-up and for integrated distance protection etc.
As long as they are not based on High Impedance principle.
For high impedance restricted earth fault, as it is based on the saturation of the CTs in case of external faults, you have to use a different set of CT.

 

[Rodmcm]

Perhaps Pisimatza you could tell us the voltage of the primary and secondary? It is not uncommon on large transformers to have two totally independent protection systems using two different manufactures of diff relays. In Australia this is a standard for both generator and transformer protection