Wiring Current Transformers in Series

Why you want to connect them in series?

Are they on same phase ? If not (3 in 3 pghases) then of course you are on residual current.

But if they are on the same phase then why at all?

The current will not add up exactly, a lot of accuracy loss likely.

why connect cts in series?

do you mean that the cts are on the same primary conductor and you wish to connect the secondaries in series.

if the cts are identical then the effect will be increased volt drop.

question remains - why connect cts in series?

idea not good. :(

Why don't you go straight for a single CT with 4000:5 ?

The only situation I can think of that uses CT secondaries wired in series is for center-tapped single phase metering, which would effect the ratio of 2000:5+5 (2000:10). This method is older tech and was used to account for load usage to earth.

In your question, you have referred to a relay, and at the 4000 amp range of possible currents, I hope this is a homework question. If not, please offer a greater explanation for the reason for this question posed to us, and as many details as you can think of.

You mentioned "have to", indicating you are possibly following an engineering diagram or instructions supplied with the CT's or from some authority. If this relay is the ground fault detection provided with a switchgear of sizable capacity, you really need to have the manufacturer or a qualified engineer or technician assisting you.

We can only guess and offer assumptions with the tiny bits of information we have been given so far.

Regards, CJM

Another possibility is the differential delta, DAB & DAC relay schemes may look to you to be wired in series.

OK, lets say we connect two 2000/5 CT secondaries in series serving an ammeter. The primary is a 1 phase bus running through both donuts. Each CT has 400 turns and the secondaries are in phase.

Therefore wouldn't we see an effective turns ratio of 800/1 or 4000/5. So, for the same 2000 amps primary on a single CT, where we would see 5 amps secondary. Now at our ammeter (with the 2 CT secondaries in series) we will see 2.5 amps.

No - Effective 2000/5 -

The addition of a second identical CT in series results in the following changes (see Figure 16):
a) The bm'den (voltampere) requirement is divided between the two CTs.
b) The burden voltage capability is doubled.
c) The burden impedance placed on each CT is one-half of the external connected burden impedance.
Therefore when two CTs with similar excitation characteristics are connected in series, the excitation
voltage of each CT is reduced by 50% and the excitation cnn'ent is also reduced. The burden voltage
capability is doubled. (IEEE I EEE Std C37.110-2007)

2000/5

No - Effective 2000/5 -

The addition of a second identical CT in series results in the following changes (see Figure 16):
a) The bm'den (voltampere) requirement is divided between the two CTs.
b) The burden voltage capability is doubled.
c) The burden impedance placed on each CT is one-half of the external connected burden impedance.
Therefore when two CTs with similar excitation characteristics are connected in series, the excitation
voltage of each CT is reduced by 50% and the excitation cnn'ent is also reduced. The burden voltage
capability is doubled. (IEEE I EEE Std C37.110-2007)

Absolutely no idea; try asking the <...we...> instead of this forum.