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

The resultant DC bias phenomenon in the AC power grids will emerge when a UHV or HV DC transmission system is operational in a debugging mode or a single-pole-to-ground mode, which may bring significant side effects on power equipments and the grids. The current research mostly concentrates on the impact of DC bias on power transformers as well as corresponding suppressing measures, while little has been done regarding current transformers (CT), however, the latter is most critical for a relay device to respond reliably and accurately, and is thereby very important for a secure, stable and reliable power system. In this paper the temporary characteristics of the widely used TPY type protective CT is fully studied under sensible DC bias excitation. An equivalent circuit model of the CT is established, and is utilized to analyze the excitation current and the magnetic flux through mathematical deduction. A theoretical expression is obtained to quantitatively describe the relationship between the onset saturation instant of the CT and the DC bias current, which indicates the DC bias current as well as the DC decay caused by short circuit fault will accelerate saturation of the CT, further, the bigger of the DC bias, the quicker of the onset saturation. 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, otherwise both impacts will be counteracted to each other. With regard to a real transmission system, a simulation model is established with EMTP software, as to verify the above analyzed results with DC bias and/or remnant flux. The research presents valuable reference for selection of protective CTs as well as appropriate setting of corresponding relay device in power systems.