As has been seen previously, the internal voltage drop of a power transformer is causing both the ratio error and the phase angle. To reduce the errors we have to reduce the current to emptiness and the internal impedances.
The vacuum current I0 is dependent on the magnetizing power (proportional to Im) and the loss power (proportional to Ip), which is needed to maintain the flux in the core. These powers could be attenuated by lowering the magnetic induction in the nucleus. This solution is not plausible because it would be necessary to add the number of turns of the windings.
As a consequence, this would raise both the resistance and the internal reactance, increasing the internal voltage drop. Typically a TP is designed with an induction that lies approximately in the linear range of the core’s magnetizing curve, while the voltage is changing between 80º and 120º of the nominal value.
Taking into account common materials, the induction is 0.7 – 0.9 Wb / m2 and in the case of power transformers is usually much larger. With this the TP becomes more linear, that is, the dependence of the applied voltage variation can be minimized.