Sometime CT secondaries are connect

[dfdt]

Sometime CT secondaries are connected in series to increase knee point voltage as each CT need to supply only half the burden voltage.
The question is what happens when there is a primary fault in between the two CTs as shown in attached diagram?

 

[davidbeach]

If you're putting two CTs in series they will be (should be) so close together that there is no exposure between them. On the other hand, CTs in series seems to be more theoretical than practical; I've run into the concept on many occasions but have never seen an implementation.

 

[stevenal]

Saturation will occur. Since the ampere turn balance in both CTs cannot be achieved simultaneously as drawn, you will need to include the excitation branches in your model. The excitation curves you included show the behavior of this non-linear branch, and I suspect you will be on the right side of both of the curves.

 

[cranky108]

With the advent of microprocessor relays, I have not seen the need to place CT secondaries in series.

So why has this come up?

 

[mgtrp]

I've seen it done once, on a 33 kV distribution line supplied from the tertiary of a grid interconnection transformer. The transformer had tripped on differential for a close-in distribution fault due to saturation, so the CTs were reconnected in series to reduce the likelihood of recurrence.

Do as David said - use two CTs that are over the same bushing or in the same CT unit so that the chances of a fault within the CT are next to nothing--probably a similar chance and outcome as an internal failure of a CT that isn't connected in series. Presumably you have upstream protection will operate.

 

[Mohamed Farghaly]

I think high voltage at both CTs secondary and it may burn or saturation.

 

[dfdt]

To answer Cranky108:the reason for connecting two secondaries in series is to increase knee point voltage i.e. effectively doubling knee point voltage.