Adding currents of two or more feeders

[unclebob]

Hi all!

I have a situation where I have to add the currents of 2 different feeders while using only 1 meter. One is fed by a 800 A breaker and the other by a 150 A disconnect switch.

Is it possible to use 2 CTs in series to do that? Or in parallel (I don't think so).

Now I know this sounds like an exam question...it's not

I'm waiting for your comments. Thanks

 

[Skogsgurra]

Parallel works quite well. The CT:s are current sources ("infinite" impedance) and as such, contribute to total current in the meter.

But, you have to use same ratio in both CT:s. You cannot use 1000:5 (or 1000:1) and 200:5 (or 200:1), which would be the obvious choice. Instead, you have to use 1000 A primary for both CT:s.

There are also "summing transformers" if you feel more comfortable that way. But an extra transforme eats your error budget.

Gunnar Englund

http://www.gke.org

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

 

[JensenDrive]

Skogsgurra said it well. I might add that putting the CTs in series would be a quite bad idea. Recall if you put them in series, both CTs have to carry the same current, which does not agree with what is going on in your CT primaries.

 

[dpc]

Definitely NOT in series. Putting them in parallel will work if they have same turns ratio. If not the sames turns ratio, you could use auxiliary CT to even up the ratios.

 

[unclebob]

Very interesting!

If I had 3 feeders, 800, 400 and 200 A, I would use 3 1400:5 CTs, or a more common ratio 1600:5.

Thanks for your help all!

 

[unclebob]

...and enter 1600 A as the CT primary in the relay.

Right?

 

[slavag]

Hi Unclebob.
No, see again recommendation of Gunnar and Dpc.
You need put 3 sets of CT with ratio 1000/5A or on each feeders CT with same rato ( of course accordintly to max feeders ratio)and after will connect secondary in parallel.
It's not important number of feeders.
Regards.
Slava

 

[dpc]

You can use any ratio you want as long as thermal ratings of the CT are not exceeded and you don't exceed the maximum allowable CT current into the relay. They just have to be the same ratio.

 

[tem1234]

You can use 3X 1000:5 CT in paralell
and enter 1000A as the CT primary
The relay will see the sum of the currents in the 3 feeders

 

[slavag]

Of course, very important Dpc remarks.
For example : 800+400+200=1400A
Take in account that you have full load, that means in this case prefer use 1500/5A CT.
But in this case you have another problem: accuracy of CT
in case of load <20% of CT ratio, your meas have high errors
If it's protection relay it's another case.
Regards.
Slava

 

[slavag]

Hi.
BTW, in case of multifeeders meter, see some brunch meter
BFM136

http://www.satec.co.il/

For my pinion not bad solution.

 

[davidbeach]

slavag, before going straight to the 1500:5 or 1600:5 CTs, I'd first look at the maximum continuous current through the meter, it may be more than 5A. Next, I'd look at the expected loading of the circuits. I'd be surprised if the total maximum continuous load exceeded 1000A - remember this is metering, not relaying - and given a meter current rating in excess of 5A continuous my guess is that 800:5 CTs will probably be adequate. Use metering class CTs for accuracy and low knee point and all should be good. Of course the user will need to make the evaluations.

 

[slavag]

David, you are right.
My post,it's only example for calculation ( I'm only assume that max load is 800+400+200 and min load is 200A).
1.User need calculate max total load of all feeders and min load of smalest feeder.
2. what is a user target, only power metering or some biling data.
3.I think ( not sure), meters with 5A nominal have 10A max continuos current.
4. Of course metering class CT.
With these 4 points user can build any system.
Regards.
Slava