Effect of DC bias on transient transferring characteristics of current transformer

As the DC bias caused by HVDC power transmission and geomagnetic induction becomes more serious, its effect on the transient transferring characteristics of CT(Current Transformer) should be analyzed. An equivalent analysis model is built and the effects of DC bias and initial fault angle on the transient transferring characteristics of CT, especially the initial saturation time, are deduced and analyzed in detail. The transient transferring characteristic parameters of an operating CT in an actual power grid are calculated by MATLAB programs and the quantitative relations between the DC bias current and the initial saturation time, between the DC bias current and the fundamental of secondary current and between the DC bias current and the second-order harmonic of secondary current are obtained. The conclusions of analysis provide the references for understanding the working state of CT with DC bias and for the setting of relay protections. © 2015, Electric Power Automation Equipment Press. All right reserved.