Effect Analysis of DC Bias on the Transfer Characteristics of Current Transformers

HVDC transmission, geomagnetic induction current, and subway DC stray current cause DC bias of the current transformer, which leads to the change of transformer transmission characteristics and affects the accuracy of relay protection action. Therefore, it is necessary to analyze the influence of DC bias on the current transformer itself. First, through a reasonable equivalent of the iron core, the finite element model of the current transformer is established based on COMSOL software, and through comparison, it is verified that the model can truly reflect the working state of the current transformer. On this basis, the finite element model of the current transformer is used to simulate the changes of the secondary induced current of the current transformer and the magnetic flux density value of the core after the DC current flows through the current transformer. The simulation results show that when the CT is in normal operation, when the primary side of the CT flows a small DC current, the CT's transformer transmission characteristics will not change basically. However, in an extreme case, when the primary side of the CT flows a large DC current, the CT will become saturated, and the secondary side induced current will be distorted, and the CT cannot transmit the transformer normally.