Research On Identification Method Of Power Transformer Inrush Current

As the key electrical equipment of power system,the safe and reliable operation of power transformer is directly related to the reliability of power supply.However,the operation experience shows that the accuracy of transformer protective action is low for a long time,the accident of mal-operation and failure of the protection happens time and again.Inrush current is one of the main reasons affecting the correct action of transformer differential protection,and the current identification methods of inrush current still have many shortcomings.Therefore,further study the characteristics of inrush current to seek new methods for the identification of inrush current have great theoretical significance and engineering application value.First of all,the magnetic saturation characteristic of transformer is discussed,and the mechanism,characteristics and influencing factors of excitation inrush current in single phase and three-phase transformer are analyzed.The traditional identification methods of inrush current is compared,and the typical methods are analyzed and evaluated.The basic principle of differential protection of transformer and the measures to prevent mal-operation of protection are studied.Secondly,on the basis of analyzing the characteristic of inrush current,the difference between inrush current and fault current is extracted,and the sampling value distribution difference characteristics of inrush current and fault current are compared.Kurtosis and skewness coefficient are introduced to represent signal sampling value distribution characteristics.The waveform of the inrush current is obviously deviated from the sine wave,it is not symmetric about the mean and its kurtosis and skewness are larger.While the transformer under the normal operation,external short circuit,and internal short circuit,the differential current is a sine wave,whose the skewness and kurtosis are smaller.Based on this principle,the method of inrush current identification based on current sampling value distribution is proposed.At last,the relationship between the second harmonic of the differential current and the instantaneous reactive power fundamental frequency component is analyzed,and the characteristic difference of active loss in different running states is combined.Then,an identification method of inrush current based on instantaneous power is proposed,and the identification criterion of inrush current is constructed based on the ratio of the fundamental frequency component of instantaneous reactive power loss and the dc component of active power loss.The above two methods,from different views,which utilize the different characteristics of the inrush current and fault current can distinguish the inrush current and fault current quickly and accurately.A lot of three-phase transformer running states such as no-load closing,external fault removal,turn-to-turn short circuit,no-load closing with internal fault,port failure and sympathetic inrush current are simulated with MATLAB.The simulation results verify the feasibility and effectiveness of the proposed methods.