Theoretical Analysis And Study On Magnetizing Inrush Current Of Three-Phase Power Transformers Terminals Planning Model

The present schemes of differential relay protection for power transformers havea low percentage of correct actions and high possibility of maloperation ratio, whichthreatens the safe operation of the whole power system. The main reason lies in thenonlinear core in the protection scope, which may cause many nonlinear phenomenasuch as magnetizing inrush current. Therefore, it is an important problem to identifythe inrush current from the internal fault current. Based on three novel criterions—2ndharmonic phase, flux locus and difference of main flux criterion, this paper focus onproblems of varying leak inductance and symmetric inrush current to analyze theelectromagnetic mechanism of three-phase transformers, which may lay the theoreticfoundation for the improvement of these criterions.At first, a novel online identification method is proposed to measure theparameters of the series connected nonlinear resistance and inductance (RL) circuitexcited by a mixing frequencies supply. The parameters are obtained by extractingthe geometric features, namely the equivalent slopes, defined by least square method(LSM) on the characteristics of voltage-current and generalized flux-current curves ofthe circuit. The physical meaning of the proposed method is discussed from theviewpoint of electric power and magnetic energy. The algorithm of the new methodhas a simple and clear form, and is especially suitable to recurrence computation. Itavoids the shortcomings of conventional algorithms based on fast Fourier transformer(FFT), e.g. large amount of computation and dependency upon the spectrum offrequency in the measured signals. The measured results of applying the new methodto an inductance and a transformer are given.The second part of the paper analyzes the impact of various factors on differenceof main flux criterion and studies the experimental phenomenon of singularity insome points. Moreover, this paper discusses the problem of the varying rule of leakinductance during the inrush course. Compared with the value obtained by shortcircuit test, it will decrease because of the magnetic saturation of the core. The lowestlimit of leak inductance of transformer equals to that of air transformer while thehighest limit equals to the steady state value. The varying rule is similar to that ofmagnetizing inductance.The symmetric inrush current, a special phenomenon of three-phase transformerswith delta-connected windings, is researched in the third part. The circulating currentin delta-connected windings leads that the differential current cannot really reflect themagnetizing current so that the feint symmetric magnetizing inrush current occurs,which may cause 2nd harmonic phase criterion and flux locus criterion to maloperate.In fact, the symmetric inrush just exists in one phase, the saturation level of which islow. Utilizing such a characteristic, the paper constructs an observer for circulatingcurrent by the induced potential, resistance and leak inductance of the delta-connectedwindings. Thus, the magnetizing currents of three-phase transformer can be obtainedand the bad influence of symmetric inrush current on the criterions may beeliminated.