Identification Method For Transformer Inrush Current Using Least-Square Matrix Pencil Algorithm

Along with the construction of high-voltage grid in China,higher performance is required in protective relays for the power system.As the important electrical equipment of system,power transformers are highly responsible for the electrical energy exchanges,and the reliable operation of transformer play a key role in the security of power supply.Differential protection,as one of the most popular protective relays in the power system providing reliable discrimination,has been selected as the primary protection in transformer protection areas.But differential protection also meets great challenge.Magnetizing inrush current is a critical factor for affecting the performance of transformer differential protection.As a result,it is necessary to do intense research into the different characteristics between inrush current and fault current and to design reliable lock strategy to guarantee the reliability of the transformer differential protection.This thesis mainly includes the following parts:(1)The equivalent circuit model of transformer in different working condition are summed up on the basis of existing research.The basic principle of differential protection of transformer is analyzed and the mechanism,characteristics,and influences on differential protection of single phase and three phases transformer magnetizing inrush current are discussed.It focuses on the analysis of the characteristics of inrush current in time and frequency domain and different distribution properties between inrush current and fault current,as the basis for the identification method.(2)In order to improve the performance of extraction algorithm,this paper puts forward a modified least-square matrix pencil algorithm which can avoid the errors caused by the numerical differentiation of high order derivative in time-domain algorithm and greatly upgrade anti-jamming ability.Then the characteristics of the least square matrix pencil algorithm are analyzed.Comparing with traditional Fourier algorithm,the matrix pencil algorithm is more fit for the transient signal analysis in power system.(3)By considering the different waveform and attenuation characteristic between inrush current and fault current,the algorithm for identifying magnetizing inrush current based on the least-square matrix pencil algorithm has been proposed.Typical fault currents are sinusoidal,while the inrush currents usually have vertex sharp waves with decaying.Magnetizing inrush has high entropy and a small standard deviation of current energy as it is not a pure periodic wave,while fault current has relatively smaller entropy and larger standard deviation.The matrix pencil algorithm was used to calculate the component of transformer differential current.And then characteristics of current energy distribution with decay factor was computed to discriminate the inrush current.(4)Current sample values were the only needed information.Simulation results in PSCAD/EMTDC software showed this scheme reliability and accurate,can recognize the magnetizing inrush current in many working conditions effectively.The proposed scheme was also verified by data of new input transformer in a substation.