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Current Transformers produce an output in proportion to the current flowing through the primary winding as a result of a constant potential on the primary //

We can see above that since the secondary of the current transformer is connected across the ammeter, which has a very small resistance, the voltage drop across the secondary winding is only 1.0 volts at full primary current.

However, if the ammeter was removed, the secondary winding effectively becomes open-circuited, and thus the transformer acts as a step-up transformer. This due in part to the very large increase in magnetising flux in the secondary core as the the secondary leakage reactance influences the secondary induced voltage because there is no opposing current in the secondary winding to prevent this.

The results is a very high voltage induced in the secondary winding equal to the ratio of: Vp(Ns/Np) being developed across the secondary winding. So for example, assume our current transformer from above is used on a 480 volt to earth three-phase power line. Therefore:

current transformer secondary voltage

This high voltage is because the volts per turns ratio is almost constant in the primary and secondary windings and as Vs = Ns*Vp the values of Ns and Vp are high values, so Vs is extremely high.

For this reason a current transformer should never be left open-circuited or operated with no-load attached when the main primary current is flowing through it just as a voltage transformer should never operate into a short circuit. If the ammeter (or load) is to be removed, a short-circuit should be placed across the secondary terminals first to eliminate the risk of shock.

This high voltage is because when the secondary is open-circuited the iron core of the transformer operates at a high degree of saturation and with nothing to stop it, it produces an abnormally large secondary voltage, and in our simple example above, this was calculated at 76.8kV!. This high secondary voltage could damage the insulation or cause electric shock if the CT’s terminals are accidentally touched.

Current & voltage in power circuits

If the voltage or current in a power circuit are too high to connect measuring instruments or relays directly, coupling is made through transformers. Such measuring transformers are required to produce a scaled down replica of the input quantity to the accuracy expected for the particular measurement.

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