Primary winding harmonics due to nonsymmetric power transformer structure in high current ANSI 45-46 configuration

In high current transformer-rectifier designed according ANSI 45 or ANSI 46 configuration, secondary windings have very high current and parameters present some differences due to the structure nonsymmetry. As an example, a 6 pulses rectifier with a 35 kA DC load current have a secondary phase current given by 17.5 kA/√3 = 10.1 kA. These parameter differences produce a DC unbalance current in the interphase reactor. The effect of this is a saturation risk on the reactor and in the power transformer. As a consequence of this, noncharacteristic harmonics on AC primary winding appear. This phenomenon is analyzed in this paper. Conclusions concern how to determine the order of harmonics in primary winding due to interphase reactor unbalance. This concept of harmonic analysis of primary current is used as a diagnostic method to determine how is the interphase reactor operating. Finally, an independent current control of each secondary load currents is proposed in order to mitigate the reactor saturation.