At what MVA does transformer differential become commonly used?

[qman5]

I'm working with a 10, 7.5 and 5 MVA transformer and am wondering if I need to specify CTs for transformer differential? The configuration is actually:

Recloser --> 10/7.5/5 MVA --> MV switchgear with 1 incomer and 3 outfeeds.

I'm not familiar with protection nor with the customer's requirements (unsolicited proposal) and am actually curious if I even need to consider CTs on the transformer.

Any thoughts or suggestions?

 

[Mbrooke]

5-10MVA is usually in the 50/50 range. Some will just protect transformers of this size with fuses/over current, while others use differential. If cost alone is a driving factor you can skip the CTs and differential, but personally I would spec them just in case.

 

[HamburgerHelper]

SEL has a relay that is priced low specifically for small two winding transformers. I think it is the SEL-787 which is listed for $2950.

https://selinc.com/products/787/

 

[stevenal]

Blackburn said 10 MVA, but that was when it was more costly to implement differential protection. We've gone as low as 5. The CTs don't necessarily need to be located at the transformer. If you use CTs located in the switchgear and /or recloser, you can extend your zone of protection.

 

[HamburgerHelper]

Does arc flash ever dictate you to use differential on small transformers?

 

[oldfieldguy]

I questioned the use of differential protection for a 1 MVA 4160-480 transformer and was told that it was for clearing times for arc-flash incident exposure. It negated the need for 'maintenance switches' and other safety-related shenanigans, according to the EE who specced it.

Before that, I think 5 MVA was about the lowest cutoff point. Most of my former clients started at 5 MVA or higher, but that was back in the day when full protection meant a door-full of electromechanical relays.

old field guy