CTs in series

[Noway2]

I was having a conversation with my boss the other day and he alluded to a concept that I wasn't familiar with: using two CTs in a series combination where the secondary of one CT sources the primary of another.

Now, I can understand where one might do this in really high current applications to avoid using ridiculous turns ratios, but I was wondering if this is a common practice.

The other question I have, also related to CTs, is how common are center tapped CTs?

The application I was reviewing used the two CT approach I mentioned above, with the secondary CT center tapped. The center tap was then tied to analog ground. From looking at the circuit this would seem to have a few advantages over the traditional (?) approach of tying one end of all three CTs to ground. First off, it would also eliminate, or all but eliminate the common mode signal on the burden ressitor while still referencing the AC signal to analog common. This benefit is of practical use as it would allow you to put a difference amp across the burden resistor with the CT providing the DC path to ground for the input offset currents (a very low DC resistance too). I am also wondering if it would avoid or limit the "virtual (vector sum) neutral" that can cause ground currents or other disruptions when tied to analog common.

I am curious is anyone has run across these technique before as I am not drawing much information in my google searches.

 

[ScottyUK]

Interposing CTs are reasonably common in protection systems to either provide ratio change or vector shift when a CT group is shared by more than one function, e.g. in transformer protection using electromechanical relays a combined differential & REF scheme applied to a star-delta transformer may share one set of main CTs on the star side if ICTs are used to provide phase and ratio correction.



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If we learn from our mistakes I'm getting a great education!

 

[cranky108]

Placing CT's in series is much less common in protection, mainly because relay burdens are now much lower. But the main advantage of the series CT's is to develop a higher circuit voltage across the load.

 

[dpc]

What you are describing is not generally referred to as putting two CT in series - that would be putting two secondary windings in series.

What you are talking about is having the secondary of one CT connected to the primary of a second CT. In the US, we would refer to the second CT as an auxiliary CT. As mentioned, this used to be fairly common in order to match up CT ratios and secondary current to what the relay or meter was expecting.

If I understand what you are wanting to do, I think it should work. Most modern relays take the CT input wiring through a small toroidal CT inside the relay to convert the current to something they want to deal with.

 

[Noway2]

Thank You to everybody for your input. I have been continuing to give this approach some thought, reviewing previous artwork, etc and I do agree that this approach looks promising. Adding up all the estimated costs, it doesn't seem to be any more expensive than the other approaches I have been considering.