Current transformers ratio error

The error in the secondary current from the rated caused by the excitation current / or the variation in the actual transformation ratio is expressed by ... 

Ratio and phase angle errors also occur due to the need for a portion of the primary current to magnetize the core and the requirement for a finite voltage to drive the current through the burden. These errors must be small. Current transformers are classified (in descending order of accuracy) into types AT, AM, BM, C and D. Ratio errors in class AT must be within the limits of ( 0.1 per cent for AT and ( 5 per cent for D, while the phase error limit on class A1 is ( 5 minutes to ( 2 minutes in types CM and C. 

The current transformer (CT) overcomes the power loss and circuit isolation problems of the current shunt, bnt like the VT, it introduces ratio and phase-displacement errors. The constraction of a current transformer (Fig. 26) is different from that of a power transformer, although the basic theory of all transformers is the same, that is, (a) the voltage induced in a transfonner winding is proportional to N mf (where N is the number of turns in the winding, is the flux in the core, and / is the frequency of operation) and (b) the ampere-tums of the windings balance. 

To investigate the current error (ratio error) and phase displacement for a current transformer, consider the simplified phasor diagram in Fig. 27. It can be seen that the difference in magnitude between the primary current 7p and the secondaiy current multiplied by the rated transformation ratio ihK ) is dependent on the amount of the primary current used to energize the core, which must therefore be kept to a minimum. It should be noted that a CT is operated with a load consisting of an ammeter that almost short-circuits the secondaiy terminals should this... 

NTPs constructed for the case of EAC containing two coordinated cyanide ions are presented in Figure 8.8. Current densities = i /r were normahzed with respect to the ratio r = [Au(CN) j/Au(CN) ] at different and the array of the transformed data was approximated by a single line with an acceptable accuracy. The value of = 0.41 obtained from its slope is in satisfactory agreement with the value found earlier in a different way. By extrapolating the data presented to = 0, one can obtain ig = 1.1 x 10 A cm . Considering the experimental errors and a possible departure of the properties of the real system from the model concepts, the quantities obtained could reasonably be rounded offto = 0.4 and in = 10" Acm . ... 

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