CT TRF and Accuracy 1

[Sn00ze]

Hello all,

Can anyone suggest any good literature about the CT thermal rating vs accuracy?

Question: if you have a CT with TRF of 2. whe you select CT ratio. Do you HAVE to consider it?

meaning: if you have a nominal current of 300A can you select the tap ratio of 400:5 or do you have to go with 200:5 x 1.5 = 300A (Assuming value of 1.5 TRF from IEEE C57.13) ?

I was given this chart by a metering company : As per the Attached the Accuracy of 0.3 is only guaranteed between nominal to Rating factor. can anyone confirm this?

This would mean that if you do not use the Rating factor, you will always be operating at 0.6 accuracy.

 

[Skogsgurra]

Yes, it is correct.

The reason for the worse accuracy at low load (one would expect the opposite) is partly because percentage is calculated with respect to actual measurement and not Full Scale (like you do when peddling volt-meters and such). I am not an expert on CT:s, but similar questions are mostly due to this.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[scottf]

It is correct that for the 0.3 accuracy class, 0.3% performance is guaranteed from 100% up to the rating factor.

Below is a sheet that shows a typical actual performance and another that shows the definition of all of the accuracy classes as well as the Ritz Extended-Range Rating, which is 0.15% down to 1% Irated.

A good rule of thumb is that for metering applications, you should always use as low of a ratio as possible and use the rating factor to cover your maximum current level. You mention "HAVE to consider it"...I would say that you certainly "should" always consider the RF when applying metering CTs.

http://www.ritzusa.com/Resources/45.pdf

http://www.ritzusa.com/Resources/82.pdf

 

[Sn00ze]

Thank you very much for the clarification!


So in a windfarm scenario where sometimes they operate very close to 0. That would mean they will have to run at 0.6 or worse. How is this acceptable? what are the metering accuracy allowances?

 

[scottf]

That is why CT's are available with accuracy performance that is better than the standard 0.3 or 0.15 accuracy classes.

See below cut-sheets. These type CTs measure accurately across a very wide current range.

http://www.ritzusa.com/Resources/98.pdf

 

[Sn00ze]

Thank you for your input Scottf, very informative!

 

[bacon4life]

Scottf-
In the example in the Ritz paper on Extended Range CT's, the standard CT was not sized for load. What is the typical price adder for the extended range CTs verses a standard CT properly sized with a smaller ratio/higher rating factor?

The Extend Range CTs may be worth investigating for some of our highly varable loads, though I want to make sure I have the calulations correct. Does the Ritz paper overstate the revenue at 100A by stating improvement is (0.6%-0.15%)=0.45%. At a load of 100A, the graph of the actual stadard CT has an error of about 0.35% and the actual Extend Range CT has an error of about 0.02%, giving an actual error of 0.32%.

 

[scottf]

bacon4life-

On the accuracy improvement example, notice the term "accuracy limit improvement." Since the burden ratings of CTs and how they are applied relative to the maximum load vary widely, the example is made using the limits of the accuracy class. Also, the graph on the lower right is showing a typical CT's performance...that also varies widely by design/rating.

Also, from what I've experienced, most utility meter groups don't have great information about a metering point's load, so they tend to size the CTs based on the size of the power transformer/distribution transformer. The example of using a 200:5A CT where the normal load is 100A is very realistic in my experience. In fact, the basis for that example is based on an installation comparison a major US utility did some year back. They went to an existing primary metering install (15kV) and installed an ERCT feeding a 2nd meter. Both meters got the potential from a common VT. The resultant increase in revenue was quite high and, in fact, they used a 1000:5A ERCT where the 200:5A ERCT could have been used. They were trying to help prove to themselves that they could standardize on 1 sized ERCT for almost all of their system.

As for the price differential, I don't really feel comfortable listing that here, as I try to keep the discussions non-commercial and try not to sales promote as much as possible.

If you'll shoot a note to the general sales e-mail address of Ritz (sales@ritzusa.com), Im on that distribution list and will be happy to respond back with more $$ details.

 

[bacon4life]

Scottf- Thanks. We have 55 different ratings combinations of 600V CTs, so I'll be interested to hear back from you via the general email.