Research On The Impact Of Current Transformer Saturation On Transformer Differential Protection

Current transformer(CT)is an important device to measure the primary current of power system,the distorted secondary current caused by its saturation will affect the reliability of protection relays even jeopardize the safety of power system.This paper focuses on the saturation characteristics of P-class CTs and the maloperation preventing schemes of transformer differential protection,and the research conducts around these three aspects,including the attenuation process of the remanence level in the CT core,the type-matching schemes in CTs selection process,and the methods against the misoperation of transformer differential protection.Firstly,this paper introduces the CT working principles,transient saturation characteristics and its influencing factors.Then,using the fractal theory,the computational formula of the remanence level of unloaded CTs is derivated and the behavior of the remanence level of loaded CTs is investigated via the theoretical analysis and simulation verification.It is indicated that the load current of CT primary side cannot reach a fully demagnetization and hence the remanent flux still exist in the core.Moreover,the location and shape of the final steady hysteresis loop and the reduction of the remanence level are dependent on the the CT design parameters,the CT secondary circuit impedance and the load current.Aiming at the maloperation of differential protection due to the very inconsistency of CTs’ saturation degree during transformer external faults,this paper deeply analyzes the impacts of CTs’ exciting currents on the steady and transient unbalanced currents of transformer differential protection.Based on the relationship of magnetization characteristics of each-side CT and the aim of simultaneous saturation of each-side CT,a type-matching scheme of different CTs and the corresponding assessment equation are presented.This scheme specifies the influencing factors of CTs type matching.Numerous simulation results validated the effectiveness of the proposed type-matching scheme in lowering the unbalanced currents and preventing the maloperation of transformer differential protection.On this basis,the suggestions with considering CTs’ type-matching scheme are given with regard to the design and selection of CTs.To adress the problems of maloperation of transformer differential protection due to CT saturation under external faults and the delay in enabling transformer differential protection under transferring faults(external faults to internal faults),this paper proposes a new method,which utilizes the discontinuity characteristics in the differential current waveform.This proposed method could accurately recognize external faults,internal faults,as well as the transferring faults.Simulation results validate the high speed and accuracy of this proposed method.