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






RE: Why 2x3 CT-s on LV side of transformer for diff . prot?
RE: Why 2x3 CT-s on LV side of transformer for diff . prot?
RE: Why 2x3 CT-s on LV side of transformer for diff . prot?
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
RE: Why 2x3 CT-s on LV side of transformer for diff . prot?
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
RE: Why 2x3 CT-s on LV side of transformer for diff . prot?
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.
RE: Why 2x3 CT-s on LV side of transformer for diff . prot?
"..... 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.
RE: Why 2x3 CT-s on LV side of transformer for diff . prot?
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.
RE: Why 2x3 CT-s on LV side of transformer for diff . prot?
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
RE: Why 2x3 CT-s on LV side of transformer for diff . prot?
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.
RE: Why 2x3 CT-s on LV side of transformer for diff . prot?
Your drawing does'nt show precisely how the CTs are wired(series, parallel).
RE: Why 2x3 CT-s on LV side of transformer for diff . prot?
RE: Why 2x3 CT-s on LV side of transformer for diff . prot?
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.
RE: Why 2x3 CT-s on LV side of transformer for diff . prot?
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.
RE: Why 2x3 CT-s on LV side of transformer for diff . prot?
RE: Why 2x3 CT-s on LV side of transformer for diff . prot?
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
RE: Why 2x3 CT-s on LV side of transformer for diff . prot?
As indicated by others, there are multiple reasons that two sets of CTs may be required.
RE: Why 2x3 CT-s on LV side of transformer for diff . prot?
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.
RE: Why 2x3 CT-s on LV side of transformer for diff . prot?