It is opportune to calculate the errors of a power transformer taking into account the phasor representation and to indicate the phasor relation, the diagram is very distorted. Actually the voltage ΔV has orders of magnitude lower than V1 and V’2. Knowing the lag between the voltage and the secondary current, which is determined by the impedance Z’c of the load, I’2R is in phase, while I’2Xd in quadrature with the secondary current, finally, the voltage V1 will stay at the point “D “Thus completing the phasorial representation. The phasor representation promotes the calculation of errors.

As the errors are being caused by the internal voltage drops, we must treat only the two triangles of the phasor diagram and in this diagram the point “O” will be distant, so we can simulate the two voltages, V1 and V’2, in parallel.                  096002.00 leeson

In the increase of the phasor diagram we can start with the voltage V’2, which will define the point A, where the next step will be the triangle at vacuum, assuming that the angle φ0 is notorious, and the voltage drop of I0R1 is in phase with I0 (empty current) while I0Xd0 is in quadrature.