What are you trying to accomplish? If, for example, you are talking about bus differential, a low impedance scheme will allow you to share CT's with other devices. For line & bus protection, dissimilar ratios, types, classes of CT's can be accomodated fairly easily. This assumes the use of a modern digital relay.
High Impedance bus differential "sums" all of the CT's, preferrably to a marshalling or summation box. There are partial differential schemes where 2 sets of CT's are used with an overcurrent relay. The drawbacks of the High impedance scheme is the requirement of dedicated CT's with the same ratio and accuracy classifications. There are workarounds if multi-ratio CT's or Aux CT's are used. I recall a discussion on this forum about a "summation CT", and this may be very different from what is mentioned above. I haven't seen a "summation CT" utilized in my neck of the woods.
RITZ in Germany makes relaying and metering class summation CTs. They can be useful when summing together feeders with different CT ratios.
They have one called SKSW800 that is C200 class but couldn't find info on their website about it.
Take a look at SKSW100 and SKSW300 on page 15 of this PDF.
DTR2011 - I think that thread might have been mine. I'll see if I can find it.
<edit>> Here it is: thread238-318934
bholas,
One problem which became very apparent is the difficulty in changing an input CT to a summation scheme. I recall wanting to change a 400A circuit on an 11kV GEC switchboard to 100A or 200A when re-utilising a disused circuit breaker for another purpose and in the end it was simpler to provide stand-alone metering than try to unravel the complex and poorly-documented summation scheme.
thanks for your comments but this not what i am after.Perhaps I should rephrase my question
I am after summing currents from four 33kV feeders with CTR of 400/1 for operation metering purposes and i have following options
Option A: Use 1-1-1-1/1 Aux CT(aka as summation CT)to get summed current
Option B: Just connect CT secondaries with taking due care of CT polarities
Is one method better than other? or is there an issue/limitation with Option B?
I believe the answer will be in your output to the meter. If you parallel the secondaries of four fully loaded feeders of 400 amps each with identical pf, the meter will see four amps. The summation CT will provide 1 A for this condition.
With Option A your meter will see 1 amp when all 4 feeders are at 400 amps.
With Option B your meter will see 4 amps when all 4 feeders are at 400 amps. Make sure your meter is rated for 4 amps continuous if going with this one.
Thanks and assuming that the meter is capable of taking 4A continously; Is there any other issue ? will it create any issue if the PF of four feeders are vastly different ?
I don't believe so as current from all four feeders will add vectorially provided CT polarities are correct
Direct summation would work fine here. The difficulty would come if you needed to add another circuit with a different CT ratio, or re-rate one of the existing ones.
