Research on the temporary performance of the protective current transformers with DC bias excitation

Operating in a debugging mode or a single-pole-to-ground mode of a UHV or HV DC transmission system will cause DC bias phenomenon in the AC power grids, resulting in severe effects on power equipments and the grids. However, under this condition little has so far been done regarding current transformers (CT), which is rather vulnerable to the DC bias impact. The transient performance of the widely used TPY type protective CT is investigated in the paper with DC bias excitation. Based on an equivalent circuit model, the excitation current and the magnetic flux are attained through theoretical deduction, and a mathematical expression is then obtained to quantitatively describe the scenario between the time-to-saturation of the CT and the DC bias current, which indicates the DC bias current will accelerate saturation of the CT to a considerable extent. Further, the interactive impact of both DC bias and remnant flux on the transient behavior of the CT is also studied, which show that they may execute significant effect on the CT if the DC bias flux is in the same orientation as the remnant flux. A simulation model is established with EMTP software as to verify the above analyzed results with DC bias and/or remnant flux. The proposed research sets up referential basis for selection of protective CTs as well as reliable setting of corresponding relay device in power systems.