A method for the indication of power transformer saturation

DC currents flowing from the earth cause many Hydro-Québec transformers to saturate due to geomagnetically induced currents (GICs) or monopolar operation of HVDC systems with ground return. The resulting voltage distortion may affect the operation of control systems. This paper offers an explanation of the internal behavior of currents generated in the presence of DC currents. It also contains a harmonic analysis of these currents together with a detailed study of the magnetizing current. In saturation, a transformer core acts as a source of current generating harmonics, some of which will flow directly toward the primary and secondary windings. On the basis of these observations, the paper proposes a measurement method for obtaining the parameters needed to calculate a reliable indicator of the state of saturation of transformers. The method is based on the difference between the primary and secondary currents and on amplitude readings of the second and fourth harmonics.