Investigation of transformer behavior under DC influence with simulation models and laboratory measurements

Abstract

The behavior of transformers under DC components has been since long studied. One of the principal sources of these components is the AC-DC energy conversion, in which unsymmetrical direct current components can arise. Another important phenomenon are the Geomagnetically Induced Currents (GIC), caused by an increased solar activity, which introduce an off-set in the grounded neutral terminals of power transformers. Among the literature there can also be found different investigations about the harmful effects that DC components may cause in power transformers. The presence of a DC component induces an offset on the flux flowing through the transformer core, which can lead the transformer into saturation. An increase on the magnetic field is immediate, and high associated magnetization currents appear. As a consequence of this, harmful harmonics and a distorted secondary voltage can also be observed. Furthermore, the increased magnetizing currents caused by the DC bias can lead to high leakage fluxes (increasing the losses) and localized hot spots on the tank walls. Experimental setups and different measurements like the ones shown in this thesis can be useful in order to know in detail how the behavior of the transformer under DC influence is and which are the most harmful effects that may occur during transformer operation. Towards this purpose, a MATLAB-Simulink model for a three-phase three-limb transformer is introduced, and different simulations are carried out for a verification task.