Current Transformer on both sides of the CB

[Arevaengineer]

What is the advantage of having CTs on both sides of an outgoing feeder compared to CT on outgoing only with common CTs for busbar protection on the bus section.
Thanks in advance for help.

 

[rbulsara]

They may have different functions. Do you have a specific case in mind? What voltage level?

 

[Eleceng01]

First, as long as the zones of protection overlap, it is fine. I have seen several industrial applications (5kV) that are configured with both CTs on the load side of the feeder breaker due to switchgear physical limitations.

 

[VTer]

One is prob used for relay protection and the other for metering

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[jghrist]

It is preferrable to have the CT for feeder protection on the bus side and the CT for bus protection on the line side. This way, the breaker is in both zones of protection.

If the line is not a source, then there is no real increase in protection. If there was a source on the line side and the line side protection trips the source, then a fault on the breaker would result in a trip of both sources to the fault. This could be the case if there were differential or piloted protection on the line.

If both CTs were on the line side, then a breaker fault would not be in the line protection zone and fault clearing may be slower.

 

[slavag]

Hi.
Again to OP.
What is a voltage level? It's GIS or AIS?
I suggest it's HV level.
Please attach protection diagramm and we can talk about advantage and disadvantage of every case.
Best Regards.
Slava

"If both CTs were on the line side, then a breaker fault would not be in the line protection zone and fault clearing may be slower."
Jghrist, it's depend, CB protected by BBP (87B) and second zone of distance protection from opposite side or in case of OR zone with "zero" time

 

[Arevaengineer]

Dear Slavag
Pls find attached the proposed SLD.Its a refurbishment scope with option to supply either AIS or GIS switchgear 34.5kV

 

[jghrist]

Note that on Cubicles 5A and 5B, both CTs are on the line side. The protection is not shown on the new GIS, but it doesn't look like having both CTs on the line side of the new GIS will be a problem. You could bid with an exception or figure out how much it will cost to make non-standard switchgear with CTs on each side and bid both ways. Or ask the customer whether the exception would be acceptable.

 

[slavag]

Hi.
What Jghrist saied!!!
It's seems as standrd Siemens or ABB GIS ( AIS too), standard configuartion is CT on the line side, BTW on the bus-tie on the one side too. I don't see any protection problem, it's standrd 50/51 functionality, maybe with logical bus protection (recommended).
Good Luck.
Slava

 

[Arevaengineer]

hi Jghrist
You are right, on 5A and 5B both CTs are on the line side, as per old specs. Now the new specs dictates to have CT's on both sides. I really dont have a clue what benefit it would give them.They are asking for Protection such as Directional phase overcurrent/earthfault protection and breaker failure on each breaker + high impedance busbar protection scheme with separate trip relays.
Cheers
thanks everyone for their valuable inputs

 

[rbulsara]

It is probable that one set of CTs are for Bus Differential protection and other is for the breaker overcurrent protection.

 

[slavag]

Hi.
Don't try understand, what is a benefit.
You have just provide all according spec. Don't try explain your opinion, isn't help.
Very simple logic, same was in old/exist open/air swg in another substations, it's all.

Good Luck.
Slava