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X/R Ratio for CT Saturation/Check

X/R Ratio for CT Saturation/Check

X/R Ratio for CT Saturation/Check

Good day protection and control engineers

I am busy familiarizing myself with CT specifications and saturation checks. I was recommended an IEEE CT saturation tool.

Besides burden, primary fault current, I see on the main important parameters is the X/R ratio. How do I determine this at the point I am locating the CTs for protective relaying. I do not want to guess the X/R ratio based on some random typical values I see on the internet.

How do I accurately determine the X/R ratio so that I can accuratley dimension my CTs? I see most utilities struggle to give me this

Your responses are soo appreciated

RE: X/R Ratio for CT Saturation/Check


Could you please provide a link to the IEEE CT saturation tool?

Usually, the protection relay manufacturer provides some application examples which give some standard values for the X/R ratio.


RE: X/R Ratio for CT Saturation/Check

The fault model you’re going to use for your coordination study supplies the X/R for every fault.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

RE: X/R Ratio for CT Saturation/Check

If you are interested in CT saturation you should get the correct X/R duty at the location to use IEEE tool.
Unless you do a sc study (either manual or using a software) there is no other way to get the X/R duty.

RE: X/R Ratio for CT Saturation/Check

The short circuit model also has to be populated with accurate R values. I am under the impression that sometimes folks use default estimates for transformer resistance rather than looking up actual R values from the test reports.

Some utilities are hesitant to provide specific X/R values for service transformers, since X/R varies a large amount between transformers of the same kVA rating. If you use X/R for the actual installed transformer, keep in mind that the utility can swap the transformer out without consulting with the customer.

RE: X/R Ratio for CT Saturation/Check

For the folks that perform these type protection calculations, how often does the protection class of the CT actually come into play?

Meaning, with relay input burdens on modern relays being so low that the total burden is essentially just the burden presented from the leads from the CT to the relay, are normal C200/C400/C800 protection classes (IEEE world here) commonly insufficient?

RE: X/R Ratio for CT Saturation/Check

Scott-We typically just specify C800 for all installations:
-For transmission equipment, the slight extra cost for C800 CTs is worthwhile so that the equipment can be installed anywhere on the system.
-For distribution (15 kV) switchgear, some manufactures do not have physical room to install C800 CTs. In those cases we have to do analysis as to whether the lower class CT's will be adequate. It is nice to completely avoid saturation, but sometimes it might be necessary to rely on the saturation detection algorithms in newer relays for distribution switchgear.
-I have not looked in detail at generation sites, which typically have very high X/R ratios.

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