Experience with 1A rated secondary CTs in the US 1

[Distribution73]

Hi,
I am trying to figure out when the use of 1A rated secondary CTs (rather than the more standard in the US 5A CT) could be benefitial in projects, when trying to avoid saturation in situations of high short circuit currents - systems with high X/R.
So far the impression that I got was that the comparative performance of 1A vs 5A CTs, for a particular core dimension, would be dictated mostly by:
- The CT burden: As it increases the use of 1A CTs could prove to be advantageous.
- Secondary CT resistance: In the case of 1A CTs would be higher and could impair its performance compared to 5A CTs for low burdens.

However apart form this oversimplified approach (maybe even wrong!), there are surely more things to take into consideration, such as commercial availability of 1A Cs based on IEEE standard ratings, regulations, economical issues, or others. I would appreciate it very much to have your view, particularly experiences when using of 1A CTs in the US.
Thank you very much!

 

[Bill West]

So with 5A CTs and 5A relays the S/C currents are too high and the CT will saturate. If we rewind the CTs with 5x the turns / 1/5 the current we'll indeed get 5x the saturation voltage. But to get the relays to work with 1/5 the current they too will need to be rewound with 5x the turns and so need 5x the voltage to give them the same operating energy. We are no farther ahead.

An aside, if the wire size is proportionately reduced the winding resistances go up 25x but with 1/5 the current the internal voltage drops will only be 5x which keeps them at the same percentage of the overall circuit voltage.

More generous CT specs are the only answer I've seen.

Bill

 

[stevenal]

We've used slipover CTs on transformers that lacked enough bushing CTs. The one amp CTs allowed better clearance, so we've gone with one amp CTs and relays. You can even order a transformer relay with one amp inputs for one winding and five on the other.

 

[Distribution73]

Thank you Bill and Stevenal for your feedback!
From the looks of your experience the use of 1A should not be any issue, provided that we really get any gain out from it. Surely to check on a case by case basis.
For the application we were looking at we would be using numerical relays so the issues raised by Bill regarding the operating power required by the relays would be greatly minimized. Interesting also that for Stevenal´s application the 1A CTs allowed better clearace. This I understand is derived from a smaller size. I was wondering what the relative size of a 1A CT would be compared to that of a 5A CT. For the former we will need 5 times the number of turns (in principle more space), however to support the secondary voltage with the same core flux density (B), we would need 5 times less area (in principle less space). I wasnt sure how these two variables would balance out. Seems, from Stevenal´s experience that (at least in some cases)the use of 1A CT can result in smaller sizes.
Thank you very much again for the feedback!

 

[stevenal]

Five times the turns using 1/5 the copper cross sectional area (or nearest standard gauge) should nearly balance out. I think it's mainly the core size that makes the difference.

 

[scottf]

From a CT manufacturer/design point of view, there is generally very little size difference between a 1A and 5A rated secondary design. The core is sized, as a simplified explanation, based on the ampere-turns. So the core size doesn't change if you move from a 1A to 5A rated secondary, all else held constant.

The winding build is also about the same. As mentioned above, 5 times the number of turns with magnet wire 1/5 the cross section is about the same as 1/5 the turns with 5 time larger cross section. That can very slightly depending on the inner diameter of the core. on smaller cores, 1A secondary can take up more room than 5A secondary. Also, from a cost perspective, 1A secondary has a lot more labor in the winding.

In the IEEE world, I think most folks. don't consider that the protection ratings (like C800) are all based on 5A rated secondary currents. The standard instructs how to handle converting the ratings when using 1A rated secondary current.

But to the original question, taken a given type of CT...say a 2000:5A C800 vs a 2000:1A C800...applied to the same burden, the 1A rated secondary CT will not have a higher saturation primary current magnitude over the 5A rated unit. In fact, quite the opposite.

 

[Distribution73]

Thank you very much Scotff. Very interesting feedback. Based on all this information, however, I am starting to think that a 1A design should have better performance. I justify it below:
Say that we start from a 1200:5A CT design. From the comments, keeping the same core we could fit 5 times the number of turns, provided that we reduce the section by 1/5. That is, we have turned a 1200:5A CT into a 1200:1A CT keeping the core size.
The new transformer would have a secondary winding resistance 25 times larger than the original 5A design. On the other hand, since we keep the core size, the new transformer will be able to withstand secondary voltages 5 times larger than the original. Additionally we would have secondary currents five times lower for the same amount of primary current.
Applying all these assumptions to a real 1200:5A C400 sample whose magnetisation curve I happen to have:
- The saturation voltage of the original design is 445 V. Hence the new 1200:1A would have a saturation voltage of 2225V.
- The original CT has a Rsecondary = 0.45 Ohm. Hence the new transformer would have a secondary resistance of Rsecondary = 11.25 Ohm
Now, if I calculate the primary current that would drive the CTs into saturation (CT secondary voltage equals saturation voltage) considering different CT burdens, I get that the 1A sample is always able to withstand higher primary currents. That is, for the same core size the 1A has better performance.

In the curve:
- X axis: Zburden.
- Y axis: Primary Current.
Would you agree with this?

 

[scottf]

In my experience, when you factor in the increase in Rct (as you mentioned), the increase in the relay's burden between 5A and 1A rated relays, and the lead impedance between the relay and CT (most users use different size leads for 1A rated vs 5A rated CT), there is very little difference in actual primary current saturation point.