Characteristics of Potential Transformer

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Effect of change in secondary burden:

  • With the increase in secondary burden, the secondary current is increased and consequently the primary current is increased. Both the primary and secondary voltage drops increase and that for a given value of VP there is a decrease in the value of VS, and hence the actual ratio increases as the burden increases. This variation of error is almost linear with change in burden.
  • Further, owing to increased voltage drop, VP advances while VS, retards in phase. As a result, the phase angle between Vp, and VS, reversed increases and becomes more negative as the current increases.

Effect of power factor of secondary burden:

  • When the power factor of a secondary circuit burden is reduced, angle γ increases. As a consequence, current IP, shifts towards Io. The voltages VP, and VS come more nearly into phase with EP, and ES respectively. Since the voltage drops are almost constant, the results in an increase in VP, relative to EP. But since VP, is constant, therefore, EP reduces relative to VP VS also reduces relative to ES. Thus the transformation ratio increases as the p.f. of secondary burden reduces.

Effect of change in frequency:

  • The flux is inversely proportional to frequency for a constant voltage. Thus, with increase in frequency, flux is reduced and therefore Im and Iw, are decreased leading to decrease in voltage ratio. The decrease is not so much, as with increase in frequency the leakage reactance increases and so for this, leakage reactance drops are increased giving an increase in the ratio. Thus, changes in voltage ratio are dependent upon relative values of L, and leakage reactance.

Effect of primary voltage:

  • The variations of errors due to change in primary voltage are of no importance as supply system voltage does not very widely.

Also Read- Construction and principle of operation of current transformer

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