Current Transformer Series Connection
Current Transformer Series Connection
(OP)
I have a customer who has a 115kV breaker with two 10L800 CT's (1200/5) driving a string of electromechanical relays. These CT's are hooked up in series prior to connecting to the relays.
I've never seen this connection.
Any good explanations as to why CT's would be hooked up in series prior to driving the relays? I thought it might be to get an effective better "class" of CT (above the 800), but the fault level here is well under what an 800 class CT can drive on the secondary.
Any comments or referrals to documents on this topic would be greatly appreciated.
I've never seen this connection.
Any good explanations as to why CT's would be hooked up in series prior to driving the relays? I thought it might be to get an effective better "class" of CT (above the 800), but the fault level here is well under what an 800 class CT can drive on the secondary.
Any comments or referrals to documents on this topic would be greatly appreciated.






RE: Current Transformer Series Connection
I really hope that the two CTs are sharing the same primary. Otherwise, it is difficult to understand.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Current Transformer Series Connection
RE: Current Transformer Series Connection
The other bus in the station has similar relays on the breakers, but the CT's were not connected in series. Likely different designers, I bet, for each bus.
Doesn't matter now in the end, since I'm going to start a project to retrofit the EM relays with new SEL relays. I'll split the CT's apart and put relays on both CT's for the new install.
Speaking of CT connections, I have found in another station, that the CT's were connected in parallel on a breaker. I'm assuming that this was done to get an effective different ratio? Fixed CT ratio of 1200/5. Putting two of these CT's in parallel would give you a 600/5 ratio?
Thanks for the discussion.
RE: Current Transformer Series Connection
CTs in parallel: Yes, that does change the ratio. These are CTs on the same breaker?
RE: Current Transformer Series Connection
The parallel CT's were on a different breaker, but still in the same yard.
RE: Current Transformer Series Connection
RE: Current Transformer Series Connection
RE: Current Transformer Series Connection
RE: Current Transformer Series Connection
Secondary of the CT can be considered as CURRENT SOURCE!
Connecting two CT'S in parallel means two current sources outputs connected to the load, which means the result is the algebraic summation of the two secondaries of the CT's i.e:
If CT1 = 300/5 A
CT2 = 300/5 A
Then output result of parallel connection of the two secondaries of the CT's:
CT3 = 300/10 A, which is equivalent to 150/5 A.
NOTE: any two CT's can be connected in parallel even when they are not similar (they have different ratio)
Connecting two CT's in series is possible just in case if the two CT's are similar to avoid any current circulation between the two CT's!
This connection has an advantage to increase the burden of the output of the CT.
If CT1= 300/5, burden 15 VA
CT2= 300/5, burden 15 VA
Then:
CT3= 300/5, burden 30 VA
I hope this can simplified the idea of parallel and series connection of the CT's
RE: Current Transformer Series Connection
For example you have one relay who monitor 2 feeder with own CT'supply by the same CB.
I have already done this one time you only need to be sure that your CT's are exacly the same rating and accuracy. otherwise your relay dont see the good information for power for example if it's 300/5 or protection following the protection setting.
RE: Current Transformer Series Connection
In this case i havn't see also this connection.
RE: Current Transformer Series Connection
Following the distance between relay and ct's and size of the wiring most of the VA are consume inside wiring.