CT Specification for Transformer Differential Protection

[Parkpower]

Hi everyone,

We need to implement differential protection on a 30/6kV 15MVA transformer. Are the CTs for differential protection different from CTs for overcurrent protection? Can we use 5P20 xxx/5A CTs for differential protection? In which case shall we use PX or TP series CTs?

How to decide the knee value of secondary voltage and burden of CTs for differential protection? We need to know the Rct etc. according to the formula. However, I can not find the data in manufacturers' literature.

Thanks in advance.

Parkpower

 

[slavag]

Hi Parkpower.
Please provide more info:
1. What is a CT you have?
2. What is distance between CT's up to protective relay?
3. What is diff protection? Usually you are have info in the relay manuals about CT performence.
4. Are you have AR on this 30kV side?
5. Are you have near to your trafo generators?

Best Regards.
Slava

 

[oldfieldguy]

At the most basic and simplest level, CT's for transformer differential protection shouldn't go into saturation for through faults.

The manufacturer should be able to supply you with excitation curves for your prospective choices. Plug in numbers for your expected current levels (that nasty through fault for both sides, short circuit available current for the input side), your relay burden (very low for modern relays) and impedance for the CT wiring, and you can see whether your choice will work.

This simple thing will get you through 98% of the 87T applications. Working around when it doesn't work is the fun part.

As for using diff CT's for overcurrent protection, this is not uncommon. Some users want their overcurrent protection to be totally separate from the 87T equipment and require a second set of overcurrent CT's. Other users are using multifunction digital relays where overcurrent is one of many functions available within the relay, and therefore no second set of CT's is needed. Amongst protection engineers, this is a philosophical choice bordering on religion.

old field guy

 

[alehman]

NXTPhase has a nice CT saturation calculator spreadsheet. I don't see it on their web site right off, so you have have to ask for it.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[ProtectionEngineer]

Here is the NXTPhase CT Saturation Calculator.

Thanks
Alex

 

[jghrist]

Instruction manuals for differential relays usually include calculations for CT performance requirements.