Electrical Engineering ⇒ Topic : Current Transformers (CT)

Current Transformers (CT)

This transformer is used to measure large currents. The primary is usually made of a few turns or even a single turn of thick copper or brass bar. It is inserted into the core of the transformer and is connected in series with the load. The secondary current is normally rated for 5 A.

As (N₁/N₂) = (I₂/I₁), the number of turns in the secondary will be high. For 1000/5 A, the nominal turns of the secondary will be 200 times the primary. So if the CT is connected in series in a high voltage line of 11 kV and the secondary is open circuited, the secondary voltage will be excessively high causing insulation breakdown and shorting primary-secondary. Hence, the secondary of the current transformer should not be left open when the meter is removed for repairs or other purposes. It must always be shorted or connected to a relay coil.

There are two types of errors: (i) Ratio error and (ii) Phase angle error.

Ratio Error

Actual current ratio                                      R = n + (I_O + I_S)

where I₀ is the excitation current, I_S is the secondary current, and n is the turn ratio.

From the expression, it can be concluded that the ratio error is mainly due to excitation current I₀ and I_O/I_S is called the ratio error.

Phase Angle Error

When the secondary current I_s is exactly 180⁰ in phase from the primary current I_p, there is no phase angle error. In practice, there is a phase difference due to magnetizing and iron loss components of the primary current. The resultant of magnetizing and iron loss component is the excitation current I_o.

The ratio error is caused by the iron loss component I₀ of excitation current and phase angle error by magnetizing component I_m

Current Transformer (C.T.)

A current transformer (C. T.) is used to measure high alternating current in a power system. The primary of this transformer has a few turns of thick wire whereas the secondary has many turns of very fine wire as shown in Fig. (a). It is clear from the figure that a current transformer is simply a well designed step-up transformer. Since voltage is stepped up, the cun-ent is stepped down which can be measured with a low-range a.c. ammeter.

The primary of the current transformer is connected in series with the line whose current is to be measured as  shown in Fig.(a). The secondary of the transformer is connected across a low-range (0 5A) a.c. ammeter.The line current (I_p) and a.c. ammeter current (I_S) are *related as

                                                               figure (a)

Similarly, if the a.c. ammeter reads 2 A, the line current = 2 x 100 = 200 A. One most commonly used current transformer is the clamp-on or clip-on type shown in Fig. (b). It consists of a ring-shaped laminated core which carries the secondary winding. The current carrying conductor itself  acts as a one-turn primary that simply passes through the centre of the ring. The position of the primary is unimportant as long as it is more or less centred. This current transformer has the arrangement to open and close the ringshaped core so that current can be measured  without opening the line. The clamp-on current transformers are simple and inexpensive and are widely used in low-voltage (LV) and medium voltage (MV) lines in the power system.

                                                                                        figure (a)

C.T. secondary should never be left open. Every precaution must be taken to never open the secondary circuit of a current transformer (C. T.) while current is flowing in the primary circuit.Opening of the secondary under these conditions removes the *opposing m.m.f. produced in the secondary and allows the flux in the core to reach a value determined only by the primary m.m.f. The result is that the flux in the core is greatly increased. This situation leads to the following harmful effects .

• The increased flux in the core may saturate it. The effect of saturation is to leave the transformer with a large value of residual flux. This will seriously ** impair the accuracy of the current transformer.
• The flux in the core changes at a very high rate inducing a large voltage in the open secondary winding. The effect of excessive voltage may be to puncture the insulation of the transformer or to provide dangerous shock to the operator opening the circuit.

     In view of the above, if we have to remove a meter in the secondary circuit of a C. T., we must first short-circuit the secondary and then remove the meter. The short circuit across the winding may be removed after the secondary circuit is again closed.

Note. Unlike an ordinary transformer, the primary current of C.T. is not controlled by secondary but remains constant at a value determined by the current in the main circuit. Therefore, short-circuiting a current transformer does not harm because the primary current remains unchanged and the secondary current depends only upon the turns ratio.