Contents
INTERPOSING CURRENT TRANSFORMERS

There are C.Ts. which are interposed between the main C.T. and the burden so that the secondary current of the main C.T. is adjusted as required by the burden.

When used in providing transformer differential protection, these C.Ts are also used to correct the phase shift, which may result due to particular method of connection used for the transformer windings.

Reference is made to interposing C.Ts. while discussing the formation of specifications of main C.Ts. Briefly detailed below are the various applications of interposing C.Ts.

a)   When the main C.T. s having rated secondary current of 1 Amp. And the burden requires a rated current of 5 Amp., an I.C.T. is used , at the load end so that the lead burden is fed by 1 Amp. Current. The lead burden on the I.C.T. is negligible as it placed very close to the burden (figure 3).

b)   The arrangement suggested in ‘a’ above may also be used when the main C.T. is having rated secondary of 5 Amps. And the burden requires a rated current of 1 amp. In this case however, the I.C.T. of ratio 5/1 Amp will have to be placed at the main C.T. end to reduce the effect of lead burden on the main C.T. ( figure 4).

c)   In the case of transformer differential protection when the main C.T. ratios do not match. If phase shift is introduced due to the ,mode of connections of the transformer on the H.T. and L.T. side, I.C. Ts. Can be used to correct the phase shift. It is assumed here that all main C.Ts. are star connected ( figure 5).

d)   For suppressing zero sequence currents from the out of balance current. This is achieved by providing an additional secondary winding on I.C.Ts. which is connected in closed delta (figure 6).

e)   If high ratio is selected for the main C.T. against a low load current, I.C.Ts. may be used across the main C.T. to reduce the ratio effectively and satisfactorily cater ti the rated current requirement of the burden (figure 7).

f)    If it is desired to control/reduce the saturation factor of the main C.T. to restrict the maximum current flowing through the burden. Such an application may be necessary in the case of thermal protection provided to a motor and when it is desired that the thermal protection does not operate during the starting of the motor (figure 8).

g)   When, for the main C.T., it is not practicable to provide I.S.F. sufficiently low for various reasons an I.C.t., suitably designed, maybe used for limiting the I.S.f. to the desired level. (figure 9). this application, however, is not recommended as it gas already been contended that I.S.F. need not be restricted to a low value.

In all above applications it must be borne in mind that an I.C. T. imposes burden on the main C.t. if this burden is required to be restricted to a certain value, the value must be specified to enable the manufacturer to design the I.C.T. suitably. If this value of burden (expressed in VA) is too low, it may result into an uneconomical design of the I.C.T.