Measuring current transformers

Current transformers (CT) are used to separate (isolate) primary and secondary circuits, and also to reduce the current value to a level convenient for measurement (standard rated current of the secondary winding is 1 A or 5 A). The device and circuit diagram of current transformers are shown in Figure 4.1. The current transformer consists of a steel core C and two windings: primary (with the number of turns w1) and secondary (with the number of turns w2). Current transformers are often manufactured with two or more cores. In such designs, the primary winding is common to all cores (Fig. 4.1, b). The primary winding, made of thick wire, has several turns and is connected in series to the circuit of the element in which the current is measured or the protection of which is carried out. Relays and devices connected in series are connected to the secondary winding, made of a wire of a smaller cross-section and having a large number of turns.

Figure 4.1. The device and diagram of a current transformer:

a – with one core;

b – with two cores

Fig. 4.2. Marking of the terminals of the current transformer windings

The current passing through the primary winding of a current transformer is called primary and is designated I1, and the current in the secondary winding of a current transformer is called secondary and is designated I2. Current I1 creates a magnetic flux Ф1 in the CT core, which, crossing the turns of the secondary winding, induces a secondary current I2 in it, which also creates a magnetic flux Ф2 in the core, but directed opposite to the magnetic flux Ф1. The resulting magnetic flux in the core is equal to the difference:

Ф0 = Ф12. (4.1)

The magnetic flux depends not only on the value of the current that creates it, but also on the number of turns of the winding through which this current passes. The product of the current and the number of turns F = Iw is called the magnetomotive force and is expressed in ampere turns (A•wind.). Therefore, expression (2.1) can be replaced by the expression:

FT = F1 - F2 (4.2)

or

Inom w1 = I1w1 - I2w2 (4.3)