Reactive power demand of transformers with DC bias

It has been observed that the presence of DC bias currents/fluxes in power transformers generates harmonic currents and voltages with an associated increase in reactive power (kVAr) demand resulting in a voltage drop. Such DC biases can occur: in transformers near high-voltage DC (HVDC) terminals; due to imperfect or half-controlled rectification resulting in DC, even and odd harmonics; because of geomagnetically induced currents; as a consequence of nuclear explosions changing the Earth's magnetic field; and due to rectifiers working in a normal (e.g., half-wave) mode. The actual network conditions causing such kVAr demand and its suppression, canceling, or mitigation are not very well understood, particularly for transformers with three legs. This article explains for the first time this kVAr demand and its suppression based on experimental data. Tests were performed for three-phase power transformers with three legs, with or without the influence of a magnetic tank.