Measurement of alternating current is one of the most frequent operations in electrical systems because it is necessary in determining other parameters of electrical circuits. Sample current is required for:
1. Indication instruments
2. KWh and KW meters
3. Tele metering
4. Protective relays
A current transformer is intended to operate normally with the rated current of the network flowing through the primary winding which is inserted in series in the network. The secondary winding of the current transformer is connected to measuring instruments and relays.
*CT (Current transformer) is always installed in series
*PT (Potential Transformer) is always installed in parallel
A CT is similar in construction to the single phase power transformer and also obeys the same fundamental laws. However the primary current of the CT is not controlled but is imposed on the transformer by the primary supply system.
The primary ampere turns produce a magnetic flux in the iron core which in turn induces an emf in the secondary winding. This causes a current to flow through burden connected.
For a CT,
I1N1=I2N2
or
I1/I2=N2/N1
and
V2=I2Z2
where internal drop,
E2-V2=E2-I2Z2=I2Z1 or
E2=I2(Z2+Z1)
Magnetic flux depends on E2, with variation of Z2, the flux density changes.
Where,
N1=Turns in primary winding
N2=Turns in secondary winding
I1=Current in primary winding
I2=Current in secondary winding
z2=Secondary load circuit
z1=internal impedance which consists of resistance and leakage reactance of secondary winding
E2=voltage induced by the magnetic flux
Fi=Flux common to both windings
I0=no load current which is exiting current
In case of power transformer there is always another current, I0 known as no load current for maintaining magnetic flux and iron losses which disturbs this ideal equation and therefore I1N1=I2N2 is not fully realized. In current transformers the current flows in full through the primary winding. Hence a part of this current, called no load current is consumed to produce the required flux.
The voltage induced in secondary winding and the flux density of the magnetic core will depend on the secondary current and load impedance.
