Residual Flux Measurement of Power Transformer Based on Transient Current Difference

Abstract

I IntroductionThe iron core residual flux density is one of the two reasons that cause the no-load closing of the large power transformer to generate magnetizing inrush current[1]. In order to effectively weaken the residual flux density to reduce the occurrence probability of magnetizing inrush current, it is necessary to measure the magnitude and direction of the residual flux density. In the existing research, the empirical estimation method[2] and the voltage integration method[3] are mainly used to estimate the residual flux density in the core, but these methods have some limitations in the residual flux measurement of large power transformer. Therefore, the accurate measurement of the residual flux density in the core is of great theoretical significance for the efficient operation of the whole power grid. The main purpose of this paper is to study the most effective method to measure the residual flux density of transformer.II Theoretical analysis of measurement methodsWhen the transformer core structure is fixed, the excitation inductance is mainly affected by differential permeability. Residual flux will lead to the change of core differential permeability. By using the method of field-circuit coupling, the magnetic field problem with closed magnetic circuit structure is transformed into the circuit problem with equivalent inductance. Based on the electromagnetic transient characteristics of iron core materials, this paper proposes a residual flux density measurement method of externally applied voltage. The residual flux results in different transient current waveforms under positive and reverse voltage applied in the windings. The relation expression between the extreme value of transient current difference and residual flux is derived theoretically. In order to improve the accuracy of residual flux density measurement, modeling and analysis were carried out on the measured square core, and a set of appropriate test circuit parameters were optimized based on the differential permeability characteristics of the core material. It can be seen from Fig.1 that the transient current differential extremum parameter ts, Δ Imax change with residual flux changes obviously, but the parameter ts is more sensitive to the change of residual flux in the core. Therefore, ts is selected as the basis for quantitative detection of residual flux.III ConclusionBy using the finite element method, the effects of different circuit parameters on the transient process can be compared, and some valuable conclusions are obtained, which will provide a theoretical reference for the subsequent application of the measurement method. Finally, experimental verification was carried out on the measured core, and the estimated residual flux in the core was compared with the actual value. As shown in Figure 2, the maximum measurement error did not exceed 5.4%. Therefore, it was considered that the measurement method proposed in this paper was reliable. This method can be used to test the residual flux of large power transformer without any requirement for transformer, which laid a foundation for the quantitative detection of residual flux in transformer core. **