Transformer differential relaying

[alpha75]

hi to everyone. this is my first post. and i have found the posts in this forum very informative.

i am trying to learn more about differential protection in general and i have not read much in depth about this. one particular question that i have in mind for now is about how to dimension the CTs for transformer differential protection.

Say we have a 2500kva, 13.8kv:480Y/277v transformer. The primary full load current Ip = 104.59A and the secondary full load current is Is = 3007.12A.

what should be the CT ratings be in this case? do these CTs matched in terms of the relaying class?

thanks.

 

[RalphChristie]

You should use next ratio bigger than the full load current value. Remember to use same CT-secondary ratio (1A or 5A)Another thing is that the CT's used in a delta connected trf must be connected in star and in a star connected trf the CT's must be connected in delta.

see following site:

http://www.selinc.com/techpprs/6025.pdf

(page 14 - 16) for example how to choose CT's

"Class X" CTs are used in diff. protection, this is special CTs used mainly in balanced protection systems (including restricted earth fault) where the system is sensitively dependent on CT accuracy.

RCC

 

[busbar]

A good laundry list of application considerations for harmonic-restrained transformer diffs is a basic single-function relay booklet like:

http://www.geindustrial.com/products/manuals/bdd/bdd.pdf

[Er, that would be kind of a small transformer to apply differential protection to. IEEE C37.91 suggests 10MVA.]

 

[james001]

make sure you use the microprocessor relay such as SEL587 because it can accept any CT connections as well as any range of CT ratio. If you're planning to use the old mech relays such as HU type etc.. then you need to connect the CT as Ralph indicated and be sure that the CT ratio mismatch etc is within the relay range.

james

 

[alpha75]

thanks to all. i guess i need to read more about transformer differential protection in particular.

busbar - you are right. as for myself, i have never been seen any differential protection for this not so large power transformer - 2.5MVA. anyway, this is just an example that i have thought of as i could not relate this to my experience.

 

[alehman]

Differential protection is very senstive to differences among CT's. Be sure to use the same mfg, type and accuracy class on the primary and secondary. Make sure the accuracy class is adquate the burden (CT secondary circuit imperdance) so the CT's don't saturate during fault. They must be rated for protection application (not metering) - typically class C in the U.S.