Research On Transformer Differential Protection Based On Mathematical Morphology

Power transformer is the important device in part of power transmission and distribution in power systems. Its safe and stable operation decides the safety and stability of power system. Differential protection whose principle is simple and performance is reliable is the essential main protection of power devices. As the main protection of power transformer, there are two important problems which must be solved. One is that how to ensure the differential protection doesn’t act when CT is saturated in external fault. The other is that how to make sure the differential protection can correctly identify inrush current and act correctly when the transformer load switching. In order to improve the rate of correct operation of the differential protection, correct and quickly identify mutations signal CT saturation and inrush time, this article will combining mathematical morphology algorithm with the traditional identification protection methods, it will promote the developming of transformer protection principle.Firstly, Mathematical Morphological technique including its origin, development and recent application cases on various aspects of power systems are introduced in the dissertation. Mathematical Morphology(MM)is developed from set theory and integral geometry, and is concerned with the shape of a signal waveform completely in the time domain rather than the domain frequency. MM has been widely used in the areas of image processing, machine vision and pattern recognition. The mathematical calculation involved in MM includes only addition, subtraction, maximum, and minimum operations. When acting upon complex shapes of signal, MM operations are capable of decomposing a signal into meaningful parts and separating them from the background, as well as preserving the mainshape characteristics.Regarding the nonlinear characteristic of ferromagnetic materials, we study the reason of saturation of CT and saturation characteristics of CT and analyze the influencing factors to the saturation of CT. After that, we simulate the differential protection of transformer of a real 220 kV substation by using simulation software of EMTDC/PSCAD and MATLAB to study the influence of saturation of CT on differential protection of transformer when external fault happens.On one hand, we introduce a method as the development of traditional difference time method to fix the start time of fault and differential current as to judge whether external or internal fault happens by referring the use of mathematical morphology todetect singularity of signal to solve the problem that the differential protection of transformer act because of saturation of CT when external fault happens. The result of simulation refers that even if the CT is seriously saturated, the method we introduce will work correctly and has a higher stability than traditional one.According to transformer excitation inrush current identification problem, based on the basic algorithm of mathematical morphology, the paper constructs the peak detector, to correctly identify the asymmetry excitation inrush current. At the same time, basing on basic budget law, the improved morphological gradient operator is constructed correctly identify symmetry excitation inrush current. Combining the above two kinds of operators, a correct recognition based on mathematical morphology is presented of the criterion of excitation inrush current. The criterion can correctly identify is verified through simulation excitation inrush current and fault current.