Study Of The Influence Of DC Bias On Current Transformer

At present，power system is affected by geomagnetically induced current becauseof the construction of transmission lines from the east to the West，the development oflong distance transmission system and the implementation of strategy of national powersystem，it makes the DC component problem become more prominent on AC powernetwork. Electromagnetic current transformer is one of the important equipment in thepower system，the influence of DC bias on current transformer cannot be ignored.Thus，Study of the influence of DC bias on current transformer has important theoreticaland engineering significance.This paper analyzes the working principle of current transformer by the Tequivalent model，and the relationship between error of current transformer andmagnetic permeability is derived. The ferromagnetic performance test is designed by thepositive relationship between inductance and magnetic permeability，and the variationlaw between inductance and magnetic permeability obtained from this test on DC bias.Besides，the error experiment of current transformer on AC/DC hybrid circuit isdesigned in this paper，and it is applicable to the current transformer whose terminallead on the outside under complete package. Research indicates that，DC bias does notchange the core saturation characteristics. With the increase of the DC offset，themagnetic permeability will decrease which resulting in the error of current transformer.Besides，the ratio error will overall increase towards negative direction and the phaseerror will overall increase towards positive direction. This experiment validates that，theincrease of the magnetic permeability with the increase of the DC offset is the source oferror of current transformer. When the DC offset over3%of theAC power，theharmonic component of the secondary side of CT will increase，especially the secondharmonic. Influence of DC bias on the error of current transformer is similar to theremanence of current transformer.This paper simulates the saturation hysteresis loop of current transformer usingBasic magnetizing curve，and it also analyzes the saturable characteristics of currenttransformer on DC bias by the model established on MATLAB. Through the theoreticalanalysis and simulation，the positive DC bias will accelerate saturation of the protectivecurrent transformer，and the negative DC bias will slower saturation of the protectivecurrent transformer. This paper analyzes the value range of braking criterion of the sampled valuedifferential protection when the sampled value differential protection is better thanphasor differential protection，and it analyzes the influence of braking criterion on DCbias. It discuss the fuzzy region of action criterion of value differential protection underdifferent initial phase angle of differential current，and the action criterion of valuedifferential protection is determined by avoidind fuzzy region. The simulation ofsampled value differential protection was performed through external fault model andinternal fault model established by MATLAB，and the scheme of differential protectionwas Proposed in the end.