Paralleling ct's of two circuits into one meter 1

[meterman3]

My question is can I meter two three phase circuits with 200:5 cts. with one form 9S meter? Will these cts be additive.

 

[Skogsgurra]

Yes. If the phase relation is 0 degrees. To add CTs means that they shall be parallelled, not seies connected.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[dpc]

What Gunnar said, but they add as phasors (vectors). If you are looking for highly accurate metering, this probably will not work very well.

David Castor

http://www.cvoes.com

 

[waross]

Actually, adding as phasors means that the kW metering should be quite accurate even if the circuits are at differing power factors.


Bill
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"Why not the best?"
Jimmy Carter

 

[electricpete]

I presume when you design/modify a CT circuit that you should do some check regarding CT saturation. If you have CT's A and B feeding the meter, then current in CT B contributes to voltage seen at CT A secondary. Since both CT circuits are similar I presume for simplicity you could assume that each individual CT circuit sees double the impedance of the meter and any shared wiring.

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(2B)+(2B)' ?

 

[electricpete]

and of course you add to that the impedance of any other non-shared wiring/devices.

=====================================
(2B)+(2B)' ?

 

[waross]

Saturation is not an issue with properly applied metering class CTs.
I believe that the spec on metering class CTs is to maintain acceptable accuracy at rated burden and 200% current.
Saturation is an issue with protection class CTs. Acceptance testing includes checking saturation voltages to verify acceptable operation at anticipated fault current levels.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

I think ePete is considering CT burden rather than saturation. Burden certainly does need to be considered, although most modern electronic meters have such a low burden that it is unlikely to be a problem, maybe more so with long leads and an electro-mechanical meter.

Metering class CTs are designed to saturate to protect the meter from high secondary currents in the event of a through-fault.


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If we learn from our mistakes I'm getting a great education!

 

[electricpete]

Yes, accuracy is certainly the relevant issue, not "saturation". My apologies for sloppy terminology.

The more direct calc (instead of doubling impedance) would of course be:
Va = Ia * Za_unshared + (Ia + Ib) * Za_shared

Since Scotty mentioned protecting the meter, it brings to mind a question what kind of thermal limit the meter input has? (and could it be challenged?)



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(2B)+(2B)' ?