Equivalent circuit.

 Taking winding resistances and leakage flux into account

The assumption that all the flux produced by the primary links the secondary is far from

true. In fact a small portion of the flux only links primary only and completes its path mostly

through air. The total flux produced by the primary is the sum of the mutual and the leakage

fluxes.

While the mutual flux alone takes part in the energy transfer from the primary to the

secondary, the effect of the leakage flux causes additional voltage drop. This drop can be

represented by a small reactance drop called the leakage reactance drop. The effect of

winding resistances are taken into account by way of small lumped resistances as shown in

the figure.

Per unit representation of impedance.

Transformers used in commercial and industrial applications typically show impedances

marked in percentages.

The per-unit impedance describes that percentage of the rated voltage required to produce

full load current in both the sides while the transformer output is shorted. The lower the

impedance, the lower the voltage required to produce full load current. Lower impedance

transformers allow higher fault currents to flow at a particular voltage.

Voltage Regulation at upf, lagging pf & leading pf.

The output voltage in a transformer will not be maintained constant from no load to the full

load condition, for a fixed input voltage in the primary. This is because there will be internal

voltage drop in the series leakage impedance of the transformer the magnitude of which

will depend upon the degree of loading as well as on the power factor of the load. The

knowledge of regulation gives us idea about change in the magnitude of the secondary

voltage from no load to full load condition at a given power factor.

The change of no load to full load voltage is expressed as a percentage of the no load

secondary voltage is expressed as voltage regulation. Lower value of regulation will ensure

lesser fluctuation of the voltage across the loads. If the transformer were ideal regulation

would have been zero.