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Podemos destacar três razões para o uso de transformadores em paralelo.

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If this condition is not exactly met, that is, assuming two transformers A and B that present a difference in their voltage or transformation ratios, parallel operation is still possible.

By applying the same primary voltage to the transformers in parallel and, since the transformation ratios are unequal, the EMFs in the secondary windings will not be equal. Due to this inequality of EMFs induced in the secondary windings, there will be, even without load, a circulating current flowing from one secondary winding (with higher induced EMF) to the other secondary windings (with lower induced EMF). In other words, there will be circulating currents between the secondary windings and, consequently, also between the primary windings when the secondary terminals are connected in parallel.

The impedance of transformers is small, so that a small percentage difference in voltage may be sufficient to circulate a considerable current and cause additional loss of I²R. When the load is applied to the secondary side of such a transformer combination, the circulating current will tend to produce unequal loading conditions on the transformers. Therefore, it may be impossible to supply the load at the output of the parallel-connected group without one of the transformers becoming excessively hot. Ignoring this fact may result in problems such as short circuits, overloads and inefficient operation, compromising the reliability and safety of the system.

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The requirements of the above conditions are known as ideal condition or ideal case. Figure 97-01 shows the equivalent circuit of the two transformers in parallel.

In this case, when there is no load, the voltage in both secondaries has the same module and the same phase, that is, the phase difference between E₁ and E₂ is zero. However, this is only possible if the values ​​of the magnetizing currents of the two transformers are very close. Under these conditions, it is possible to connect the two transformers in parallel, since, in the absence of load, no current will flow between the windings. Figure 97-03 shows the diagram of the equivalent circuit illustrated in Figure 97-01, in which the transformers connected in parallel are sharing the load current, I_L, representing two impedances in parallel.

The voltage triangle over impedances is represented by two triangles: one is formed by the legs I₁ R₁ and I₁ X₁, with the hypotenuse being I₁ Z₁; the other is formed by the legs I₂ R₂ and I₂ X₂, with the hypotenuse being I₂ Z₂.

Note that in this case, the currents I₁ and I₂ are not in phase with each other or with I_L, since Z₁ is different from Z₂. Also, I_L is the phasor sum of I₁ and I₂, according to eq. 97-05 below.