Adding currents of two or more feeders
Adding currents of two or more feeders
(OP)
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
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






RE: Adding currents of two or more feeders
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Adding currents of two or more feeders
RE: Adding currents of two or more feeders
RE: Adding currents of two or more feeders
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!
RE: Adding currents of two or more feeders
Right?
RE: Adding currents of two or more feeders
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
RE: Adding currents of two or more feeders
RE: Adding currents of two or more feeders
and enter 1000A as the CT primary
The relay will see the sum of the currents in the 3 feeders
RE: Adding currents of two or more feeders
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
RE: Adding currents of two or more feeders
BTW, in case of multifeeders meter, see some brunch meter
BFM136
http://www.satec.co.il/
For my pinion not bad solution.
RE: Adding currents of two or more feeders
RE: Adding currents of two or more feeders
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