Direct current in transformers: effects and compensation

Due to the high magnetic conductivity of transformer cores, very small direct currents in the electric power grid are sufficient to cause half-cycle saturation of the core. Even minor DC-bias leads to a considerable increase of the noise level and to high reactive power consumption, as well as to a significant rise of no-load loss. In case of strong direct currents (e.g. GIC), the higher harmonic stray flux causes additional losses in metallic parts that can harm the transformer. Moreover, reactive power consumption can reach a level that is critical for the power grid. We demonstrate these effects with measurements on a single phase and on a three-limb transformer. Already small direct currents increase the noise level by more than 10 dB(A), even by more than 20 dB(A) in the case of a transformer with low general level of noise! With increasing awareness of the problem, the demand for power transformers increases that are less sensitive to direct current. Traditional passive measures—like increased core cross section or the introduction of air gaps in the core—have various disadvantages (costs, noise) and can only deal with DC in an extremely limited range. DC blocking devices strongly influence the power grid. For these reasons, we have developed an active compensation of the DC magnetization by the controlled injection of DC counter ampere turns. We have found sophisticated solutions to accurately measure the DC magnetization and to feed direct current at low power. The complete equipment has been successfully tested on prototypes; the first field tests have just started. With this new method, the transformer is insensitive to direct currents. DC-flux offsets in the core caused by inrush currents and switching operations in the grid can also be compensated.