Characteristics of Potential Transformer

Characteristics of Potential Transformer Points : Characteristics of Potential Transformer (i) Effect of secondary current or VA If we increase the secondary burden, the secondary current is increased, and therefore, the primary current increase. Both primary and secondary voltage drops increase and thus for a given value of V, the value of V decreases and hence the actual ratio increases as the burden increases. The ratio error increase becoming more negative with increase in burden. This variation of ratio error is almost linear with change in burden.
With regard to phase angle, the voltage VP is more advanced in phase because of increased voltage drops with increase in secondary burden. The phasor V_s reversed is retarded in phase due to increase in secondary voltage drops. Thus with increase in burden, the phase angle between VP and V reversed increases, becoming more negative. (ii) Effect of power factor of secondary burden If the power factor of secondary burden is reduced, the phase angle of secondary load circuit is increased. This makes current I_o to shift towards current 10. The voltage V_s & V_p come more nearly into phase with E and E respectively since the voltage drops are almost constant. The result is an increase in V relative to E_p. But V_p is constant and, therefore, E_p, reduced relative to V_p,. The voltage V_s reduces relative to E. Therefore, the transformation ratio increase as the power of secondary burden reduces.
Now with the decrease in power factor V is advanced in phase and V_s retarded in phase, the phase angle (-ve) reduced with decrease in secondary power factor (lagging). (iii) Effect of frequency For a constant voltage, the flux is inversely proportional to frequency. Increase in frequency reduces the flux and therefore, I µ and I_w are decreased and, therefore, the voltage ratio decreases. The decrease is not so much, as with increase in frequency the leakage reactance increases and, therefore, leakage reactance drops are increased giving an increase in ratio. Thus changes in voltage ratio because of change in frequency are dependent upon relative values of I_o and leakage reactance’s since the effects produced by them oppose each other.
As regards phase angle error, both effects due to increase in frequency advance V_p, and the increase in secondary reactance retards V_s and, therefore, the phase angle is increased as the frequency increases. (iv) Effect of primary voltage There is no wide variation of supply voltage to which the primary winding of the P.T. is connected. Therefore the study of variation of ratio and phase angle errors with supply voltage are of no importance.