Daisy Chained CT's

[fiberstress]

Hello,

Researching what effect piggybacking another CT onto an existing CT might have. We have 300:5 CT's for 12.47KV 3MW generators that we'd like to tap onto with an analyzer's CT's. I'd like to learn how much of an effect the analyzer's CT's would have on the signal out of the 300:5's. Any input is appreciated.

Thanks

 

[davidbeach]

What do you mean by "tap onto"? A clamp on CT around the unbroken conductor will have negligible impact.

 

[fiberstress]

Hi David, I meant clamp onto, sorry (ie to the primary CT loop). I did a little more homework and see the "burden" load is very light, but I'm just unfamiliar of how to translate that to an impedance on the primary CT loop's circuit and if we'll have to re-calibrate all the devices using that loop's signal.

 

[stevenal]

Impedance follows the square of the turns. But is your analyzer CT really a CT? Might be a transducer that applies a voltage to your analyzer.

 

[DRWeig]

I'm thinking you may be confusing the CT's primary and secondary. The primary is the conductor whose current you would like to measure. The secondary is the 0-5A part.

Let me ask without using the terms primary and secondary:

Are you putting your analyzer CT around one of the two 0-5A leads from the existing CT, or are you putting your analyzer CT around the large single conductor whose current is being measured by the existing CT?



Best to you,

Goober Dave

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[fiberstress]

DRWeig, the former. I meant "main" CT loop, which I guess would be the analyzer CT's primary. You have to watch your language here in the EE forums

stevenal, you caught me. They are transducers similar to these:
newark
tequipment

So they probably just measure potential across a known resistance, correct? Are your opinions that the impedance the "main" ct loop would see is negligible and we shouldn't have to calibrate any of our meters that use the main CT?

 

[Skogsgurra]

fiberstress
Those are current clamps. You can put them around the "heavy conductor". No sweat. We do it all the time and do not influence primary current at all. At least not to the extent that we worry about it. And probably less than a tenth of other error sources - i.e. not at all.

Gunnar Englund

http://www.gke.org

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

 

[stevenal]

The Fluke puts out 1mA/A, so it may be a CT. Take the burden of your meter and multiply it by (1/1000)*2 to see what to add to your main CT secondary impedance. The other device puts out a DC voltage so it is not simply inserting a resistor. I believe either device will have a negligible effect.

 

[fiberstress]

This bus requires an act of congress to de-energize, so installing more CT's around the bus bar isn't an easy option.
Sounds like there shouldn't be much impedance from tapping a 0-5A DC out CT on the main CT's secondary loop. We can try one and see how it goes.

Thanks for all the help!

 

[dpc]

0-5 A dc out?? That may not work as well as you think. You might want to attach a quick sketch. I really am not sure what you mean by "tapping". Or exactly what you are contemplating. I THINK I know what you are talking about, but CT circuits are not a place to make a lot of assumptions. Open circuiting an energized CT can be dangerous.

 

[davidbeach]

Open circuiting an energized CT can be dangerous.

Or, it can just kill somebody.

 

[stevenal]

The current probes linked above do not require opening the circuit.

 

[fiberstress]

Thanks guys, always important to remember. We have shorting blocks and will make sure to avoid that.
And, again, our analyzer needs a dc voltage output like the probes I linked to above. We would chose a 1:1 0-5A probe to measure the current on the secondary of the main CT. Or do you see a problem with this? If there's something out there we don't have to make ourselves that we could put in series of the main CT's secondary loop and convert to DCV out, I'm guessing that might be more accurate. But my main question is still a little uncertain: what sort of magnitude change would we expect from the added impedance to that main CT loop (either option). Sounds like no one here has run into any significant problem before. We'll probably see if we can dig up some vendor numbers and just crunch, and then do a test setup on one of the generators.
Also, I promise to try and use another term besides "tap" in future posts