Modeling and simulation, and their validation of three-phase transformers with three legs under DC bias

This paper proposes a new model for three-phase transformers with three legs with and without tank under DC bias based on electric and magnetic circuit theory. For the calculation of the nonsinusoidal no-load currents, a combination of time and frequency domains is used. The analysis shows that 1) asymmetric three-phase transformers with three legs generate magnetizing currents with triplen harmonics not being of the zero-sequence type. 2) The wave shapes of the three magnetizing currents of (asymmetric) transformers are dependent on the phase sequence. 3) The magnetic history of transformer magnetization - due to residual magnetization and hysteresis of the tank - cannot be ignored if a DC bias is present and the magnetic influence of the tank is relatively strong, e.g., for oil-cooled transformers. 4) Symmetric three-phase transformers with three legs generate no-load currents without triplen harmonics. 5) The effects of DC bias currents (e.g., reactive power demand, harmonic distortion) can be suppressed employing symmetric three-phase transformers with three legs including tank. Measurements corroborate computational results; thus this nonlinear model is valid and accurate.