Research On The Digital Simulation Of Power Transformer Internal Faults And New Transformer Protection Principles

With development of power industry and the coming into operation constantly of large capacity, large-scale extra high voltage power transformer, put forward high demand to security, dependability, sensitivity, alternative, fast of large-and-middle-scale transformer protection. In the transformer differential protection, it is a congenital and difficult problem that can't be avoided how to distinguish the magnetizing inrush current with the internal fault current. It has become the focus studied for many years. The existing transformer protection principles all have certain defects and the main reason lies in inadequate understand of the transformer fault mechanism. So, it is very essential to promote the research of simulation of transformer internal fault which have important theory meaning and practical value to perfect the analysis and calculation of the transformer winding internal fault, improve the relay protection level of the large-and-middle-scale power transformer, reduce serious emergence of accident and develop a new generation protective relay equipments. This thesis organizes work around the analytical method of the power transformer internal fault and the research of transformer protection new principle. The full text divides into eight chapters altogether. In chapter one, the purpose and significance of this dissertation is elaborated and the comprehensive summary has been made on domestic and international relevant respects at present. Then the study content of this dissertation is also presented. In chapter two, the relevant principles of the linear simulation model of transformer internal fault have been described in detail. The computational process of the linear simulation model has been explained carefully. The calculation method of transformer model parameters based on the magnetic circuit method is presented. Considering the distribution disparity in the large-scale transformer coil, the improvement method to calculate model parameters has been proposed. All of chapter two establishes the foundation for analyzing the transformer internal fault in detail. In chapter three, the nonlinear characteristics of the iron core of transformer have been included in the simulation model. The finite element analysis approach in simulation software to calculate the parameters of equivalent circuit has been utilized. Based on these parameters, the simulation experimental circuit model of the internal fault can be set up and various kinds of electric quantity of transformer can be measured. Such nonlinear model is more accurate than the linear model. In chapter four, the software of the transformer internal fault simulation test system has been introduced. This software can be broadly applied to the analysis and protection study of transformer internal fault which have facilitated protective relay workers greatly . In chapter five, according to the primitive normal model of the transformer, the principle operation equations of transformer protection have been derived and the protection scheme have been put forward. This transformer protection scheme is immune to the magnetizing inrush current influence and does not need the accurate parameter of transformer. The scheme is simple and has been proved with the simulation test and dynamic analog test. In chapter six, two novel methods of distinguishing magnetizing inrush current have been studied. One is the method of distinguishing magnetizing inrush current based on the ratio of voltage and fluxional differential current, and the other one is the method based on attenuation of fundamental wave. The feasibility of these two methods has been proved with the simulation test and dynamic analog test. In chapter seven, fuzzy theory has been introduced to transformer protection. The transformer protection principles that studied in above chapters have been applied to the fuzzy criterion and have played good ability to distinguish the magnetizing inrush current with the light current of transformer internal turn-to-turn fault. Slave function to each kind of criterion has been provided and the overall scheme of transformer protection based on the fuzzy recognition theory has been made. In chapter eight, the summary of the full text is illustrated.