Characteristics of Current Transformer

Characteristics of Current Transformer Points : Characteristics of Current Transformer, Effect of P.F. of Secondary Burden on Errors, Effective of change of primary current, Effect of change in secondary burden, Effect of change of frequency, (1) Effect of P.F. of Secondary Burden on Errors a. Ratio Error It is observed that for all inductive burdens the secondary current. I_s lags behind the secondary induced voltage, E_s so that is +ve. Under these conditions the actual transformation ratio is always greater than the turns ratio. For burdens which are sufficiently capacitive, I loads E and so is -ve. under these conditions the actual transformation ratio decreases becoming less than the turns ratio for values of “6” approaching -900. b. Phase angle We find that for inductive burdens, phase angle “θ” is +ve for small values of “8” (high secondary p.f.) but becomes negative as the secondary burden becomes more inductive and approaches. For negative values of (sufficient comparative burdens) is always +ve. (ii) Effective of change of primary current If the primary current changes the secondary current change with proportion. At low values of current I_p (or i_s) exciting current Iµ and loss component I_s, increase, there is an increase in I and there is a decrease in ratio error and phase angle. (iii) Effect of change in secondary burden An increase in secondary burden impedance means an increase in volt-ampere rating. This necessitates an increase iii the secondary induced voltage which can be generated by an increased flux and flux density. Therefore both magnetizing component and loss component are increased.
In general, a greater burden impedance not only increases the transformation ratio, but also shifts the phase angle between primary and secondary to more positive values. (iv) Effect of change of frequency The effect of increase in frequency will result in proportionate decrease in flux density. Thus, in general, the effect of increase in frequency is similar to that produced by decrease of impedance of secondary burden.
A current transformer is seldom used at a frequency which is very different from the one or at which it is designed, and therefore, consideration of this effect is no very important.