1+X/R factor for CT sizing

[stevenal]

Are any of you using this factor to account for DC offset saturation when sizing CTs for line protection? Is it necessary or desirable in your opinion? Reference:

http://www.selinc.com/techpprs/6027.pdf.

My applications are no where near generator buses per the exception given in Conclusion #4, but it's still having a huge effect compared with the 20X rated current at rated burden calc.

 

[DiscoP]

I have always used this whenever instantaneous operation is required (eg Distance, Differential).

Otherwise the CT may saturate for an Offset waveform, and the protection may not operate until the offset dies away.

Some modern numerical relays have algorithms that allow for some CT saturation, and if they do will give some guidance in their manual on how to size the CT.

 

[dpc]

I read this paper and found it interesting. But I haven't used their recommended caculation method in the past. It does require a realistic determination of the X/R ratio, something that wasn't always done in the past for utility system fault calculations.

I think all they are saying is that if you size the CT based on the equation given that the CT will not saturate either for symmetrical AC fault current or the maximum dc offset. They don't say that a smaller CT would not still work, just that it may begin to saturate and this could effect the relay's response.

 

[stevenal]

RTFM is good advice. I looked over a few manuals. One allows 7.5 times the saturation current, easing the situation considerably. Another one with a similar model number (and presumably similar algorithm) has no mention of the subject. Still another from a different manufacturer gives the straight 1 times formula, but claims their algorithm handles saturation. No universal rule, I guess.

Next question: Given an existing mutiratio CT set, do you simply connect to X1 and X5 giving the most resistance to saturation while allowing future flexibility in relay choice and also allowing system growth? Or do you find setting step sizes or other considerations limit the maximum practical ratio you can use?