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Current Transformers (C.T.)

12.2. Current Transformers (C.T.)

  • Just as a shunt extends the range of a D.C. ammeter, so does the current transformer perform the same function in A.C. circuits. Thus a high magnitude alternating current can be easily measured by a combination of a current transformer and a low range ammeter.
  • The primary of a current transformer (C.T.) consists of a few turns of thick cross-section connected in series with the high current line. Very often the primary is just one turn, formed by taking the line conductor through the secondary winding (Fig. 37). The secondary winding consists of a large number of turns of fine wire designed for either 5A or 1 A rating. Thus a current transformer is step-up transformer. The current transformer
    has the secondary effectively short-circuited through the low impedance of the ammeter. Fig. 38 shows the current transformer connections.9

    Fig. 37. Line conductor acting as primary.10

    Fig. 38. Current transformer connections.

    • The current transformer ratio is not equal to the ratio of secondary to primary turns, mainly because of the effect of the magnetising current. The primary current can be thought of as the sum of two currents, the first to balance secondary current so that primary and secondary m.m.fs. may balance and the second being the no-load current I0, The components besides being responsible for a slight error in the current ratio, is also responsible for a phase angle error. The transformer must be carefully designed to minimise the ratio and phase angle error.

    It may be noted that current transformer must never be operated on open-circuit for the following two reasons:

    (i)                           There will be no secondary m.m.f. and since the primary current (and m.m.f.) is fixed, the core flux will increase enormously. This will cause large eddy current and hysteresis losses and the resulting high temperature may damage the insulation or even the core.

    (ii)                        A very high voltage will be induced in the multi-turn secondary and this high voltage may be dangerous both to life and to the insulation.

    Fig. 39 shows the wiring diagrams for potential and current instrument transformers.11

    Fig. 39. Wiring diagrams for potential and current instrument transformers.