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SUMMARYA new method based on runs test is proposed to distinguish the inrush currents from the internal faults in power transformers, which is derived from the inherent differences between these waveforms. First, two statistical hypotheses that represent these differences are defined. Then by testing these hypotheses using runs test and by comparing the outputs of the hypotheses, a criterion to identify the internal fault is obtained. A total of 200 experimental cases are used to test the performance of the technique. In all of the tests, the proposed method accurately distinguished between fault and inrush currents without any errors. Also the suitable performance of the method is demonstrated by simulation of different faults and switching conditions on a power transformer. For this purpose, a small part of Iran power system involving a power transformer and the transmission lines on both sides of the transformer has been considered. To include effective factors on differential current components, the elements of this power system have been precisely modeled in PSCAD/EMTDC.

SUMMARYThis paper uses different geometrical structure of inrush current with respect to that of fault current for the identification of transformer magnetizing inrush. For this purpose, the differential current is considered as a stochastic signal, and a scheme based on stochastic analysis of differential currents is proposed. The scheme converts changes in slope of inrush and fault currents as a major feature of difference between them into a quantitative description by using the skewness concept of frequency curves. Skewness is the degree of asymmetry of a curve, usually taken relative to a normal curve. Internal faults are discriminated from inrush conditions by investigating the signs of the skewness coefficients corresponding to three-phase differential currents. Various experimental cases verify the dependability, security, and fast operation of the proposed algorithm.

This article uses a different geometrical structure of inrush current with respect to that of fault current for the identification of transformer magnetizing inrush. For this purpose, the differential current is considered as a stochastic signal, and two schemes based on stochastic analysis of differential currents are proposed. The first scheme converts changes in the slope of inrush and fault currents, which is the main difference between converting them into a quantitative description by using the skewness concept of frequency curves. The second scheme introduces a feature criterion based on the kurtosis concept of frequency curves to identify the inrush from the fault current. This criterion quantifies another major difference between inrush and fault currents, which is the peakedness of the differential currents. Simulation studies and experimental results justify that the proposed schemes are able to identify inrush currents reliably, even those with a low second harmonic component. The schemes correctly open the differential protection shortly for the internal fault in operation energized with a fault, even in the case of an internal fault current with a high second harmonic component.