Characteristics Analysis Of Inrush Currents Under Multiple Factors And Identification In Power Transformers

Power transformer is an important transformation equipment in power plants and substation and it plays a role of changing the applied voltage,thus it is of great significance to ensure its safe operation for the whole power system.However,its main protection—the differential protection is usually affected by inrush currents caused when transformer no-load switching-in,which is also the main reason why its correct operation rate is not high.Therefore,it is important to develop a new correct and reliable method to detect inrush currents,which is useful to improve the performance and optimization of transformer differential protection.This paper gives a systemic summary of the existing methods to detect inrush currents at home and abroad and the produce mechanism and characteristics of inrush current is further analyzed.Combined with waveform characteristics of inrush currents and inner-fault currents,this paper puts forward two kinds of effective methods to identify inrush current from inner faults.One is a novel algorithm of identifying inrush current based on waveform cross-correlation coefficient.A short sampling data-window is used to construct a normal sine wave as reference,either in the case of no-load switching or internal faults.And then analyze the cross-correlation between the reference and the practical sampled waveform.In the situation of inrush current,it is deviated from 1 and waves notably.But In the condition of internal faults,the normalized cross-correlation coefficient is close to 1.Finally the PSCAD/EMTDC simulation results verified that the developed algorithm with clear principle,simple method and small calculation can detect inrush currents from internal faults correctly and quickly.The other is a method to detect inrush currents using current derivation characteristics.The difference of the time span determined by the extreme values within a power frequency cycle is used to detect inrush currents.In the case of internal faults,its derivation rate curve is close to sine waveform and the time span between the maximum value and the minimum one is approximately near to half a cycle.While In the condition of inrush currents,the current waveforms have the characteristics of discontinuity and steeple wave and the time span between the maximum value and the minimum one of the derivation rate in a power frequency cycle is less than half a cycle.Thus,a criterion using the current derivation rate feature is developed and the corresponding realization flow is presented.At last,the dynamic tests under different conditions verified that the proposed algorithm is of favorable performance for identifying inrush currents from internal faults.And for the transformer internal faults,especially mild inter-turn faults,it also shows good performance and provides a good theoretical basis for the engineering application of transformer protection.