Research On Inrush Current Identification Based On Optical Sensing

Power transformer plays an important role in power system.The security and stability of power system are closely related to its safe and reliable operation.Therefore,it is imperative to seek a safe,reliable and sensitive transformer protection scheme.How to avoid inrush current is one of the most difficult problems in transformer protection technology.The existing protection devices are limited by the characteristics of electromagnetic current transformer and the protection algorithm based on power frequency,which cannot fundamentally solve the problem.Based on the accurate measurement of optical current transformer,a new transformer protection technology using wavelet energy information to identify inrush current is proposed in this paper,which provides a new research scheme for transformer protection to avoid the technical difficulties of inrush current.Firstly,the basic principle of transformer differential protection and the generating mechanism of inrush current are analyzed and verified by simulation.Based on the protection principle,a simulation model of double-winding transformer system with two-side power supply is built on MATLAB/Simulink platform to simulate the excitation inrush current generated by no-load closing of transformer and various kinds of electrical fault currents occurring inside and outside the main protection area of transformer.Then the traditional electromagnetic current transformer and optical current transducer are measured and compared with each other by simulation models.Secondly,the characteristics of electromagnetic current transformer and optical current transducer and their influence on transformer differential protection are studied.On one hand,the traditional electromagnetic current transformer is easy to be saturated,which makes the measured current waveform distorted and easily causes protection rejection or maloperation,and its reliability is not high;on the other hand,it needs to extract power frequency through filtering,which is not fast enough.Optical current transducer has excellent characteristics such as no magnetic saturation and no need of filtering,which lays a foundation for accurate measurement of new protection algorithm.Thirdly,an inrush current identification method based on wavelet energy information is proposed.According to the characteristics of wavelet transform of inrush current and current under various faults,Daubechies 6 wavelet is selected to decompose inrush current and fault current in seven layers.By analyzing and comparing the coefficients of the two layers,an identification method of inrush current based on wavelet energy information is proposed.This method distinguishes inrush current from fault current by setting reasonable identification criteria according to the difference of energy ratio between high frequency and low frequency of wavelet transformed by inrush current and fault current.Finally,the validity of protection identification criterion is verified by simulation experiments,and the time window is optimized.Considering the factors affecting inrush current and fault current in practice,experiments are simulated under various conditions.The results of experiments show that this method can accurately and reliably distinguish inrush current and fault current.By improving and optimizing the time window,the identification time is 10 ms on the basis of accurate judgment,which improves the speed performance compared with the traditional method.