Current Transformer Series Connection

[hibb1]

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.

 

[Skogsgurra]

The CT burden is probably more than a single CT kan handle. That "string of relays" may have a higher impedance than one CT can handle and then the two CT secondaries are needed to handle the relays succesfully (identical currents but double EMF available).

I really hope that the two CTs are sharing the same primary. Otherwise, it is difficult to understand.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[scottf]

Hard to say exactly without a schematic and more information, but if they wired it that way due to concerns about the burden being too high with that many relays, it would have been better to put some relays on 1 CT and others on the 2nd CT, i.e. split the relays up. Connecting 2 CTs together should be avoided if at all possible, as it just makes things more confusing.

 

[hibb1]

Thanks for the comments. Probably was a burden consideration on the original design. The relays in the series were IBC's, CHC (breaker failure), PJC, and a CPD (Pilot wire).
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.

 

[dpc]

Regarding series CT connection. As noted, this may be done due to burden but more specifically, burden and available fault current. This used to be fairly common in relaying applications and is covered in Blackburn's Protective Relaying text. You are effectively doubling the available secondary voltage.

CTs in parallel: Yes, that does change the ratio. These are CTs on the same breaker?

 

[hibb1]

Thanks, I'll go back to Blackburn to review.
The parallel CT's were on a different breaker, but still in the same yard.

 

[dpc]

Right, but the two CTs in parallel were seeing the same primary current? Just making sure it is not a bus differential connection?

 

[hibb1]

Same primary current. Only feeding an SEL 321 in this case.

 

[dpc]

The paralleling of CT secondaries for relaying is also discussed in Blackburn. Since the higher ratio CTs will generally have a higher relaying accuracy, the overall accuracy can be improved by using two matched CTs in parallel. When a multi-ratio CT is tapped down to a lower ratio, the effective relay accuracy goes down as well.

 

[safy2004]

Regarding the parallel and series connection of CT's:
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

 

[karls2]

May be also CT's work alternatively.
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.

 

[karls2]

Sorry for my explanation, I haven't see that the CT's are in series...in hibb1 1st post. In this case they can't work alternatively.
In this case i havn't see also this connection.

 

[karls2]

Anyway if you retrofit the old electromechanical relay by SEL, they will consume lower VA, additionnaly you can also increase the size mm² of the wiring between the CT's and the protective relay in order to decrease the VA consume inside wiring.
Following the distance between relay and ct's and size of the wiring most of the VA are consume inside wiring.