Page 857 - Basic Electrical Engineering

Figure 11.30 Measurement of line current using a CT and a low-range ammeter

Calculation of ratio error and phase angle error of a CT: In Fig. 11.30 is
shown a CT connected with the circuit where high current, I is flowing
1
through the load. The primary circuit current of the CT is equal to I . The
1

secondary circuit current of the CT is I The turn ratio
2

or,

Thus, by multiplying the reading of the ammeter connected to the

secondary circuit of the CT by the turn ratio, i.e., , we can get the value of

current flowing through the load circuit. The ammeter is rated at 0–5 A. With
this arrangement, i.e., having a CT and low-value ammeter, we are able to

measure high current flowing through a circuit.
However, due to the no-load component of the primary current of a

transformer, the actual transformation ratio deviates somewhat from the rated
or nominal ratio of currents of the primary and secondary and also of their

turn ratio. As we know, a transformer will draw some small amount of
current at no load which has two components, viz the magnetizing
component of the no-load current, I which produces the flux in the core and
m