Considerations on a MV/LV Transformer which Supplies a Distorting Load

The paper deals with the behavior of a medium voltage/low voltage transformer (MV/LV), used to power the excitation circuit of a power group. The transformer's load is nonlinear, therefore inducing strong non-sinusoidal regimes in the primary and the secondary windings of the transformer. Non-sinusoidal regimes are due a thyristor rectifier used to power excitation. Two distinct situations are analyzed, corresponding for two contexts in which the three-phase transformer is operating. The recorded waveform from the transformer's primary and secondary windings, along with the harmonic decompositions of three-phase voltages and currents in the two windings are presented. The power quality factors from the two windings for both analyzed cases are presented. The transformer's losses are determined for both cases. Experimental determinations have shown that the use of cells for improving the power factor (in one of the analyzed cases) increased the harmonic distortion of voltages in both windings, also increasing the active power losses in them. When using the RC cells, the additional losses increased (through eddy currents). In normal operation (no RC cells), the global power factor is reduced both in primary and in secondary winding, and the current harmonics contribute in increasing the reactive power in the transformer's windings.