Research On Magnetizing Inrush Current And Internal Faults Of Large Power Transformers

The power transformer is one of the indispensable electrical equipment in the power transmission process.As the power demand of the majority of power users increases,the level of power transmission voltage increases,and the level of industrial manufacturing rises,the capacity of a single transformer also becomes larger and larger.Once a failure occurs,it will inevitably affect social production,life has an impact and causes huge economic losses.It is necessary to avoid the protection malfunction caused by the inrush current and the protection refusal caused by the interturn fault.Although various protection methods based on the terminal current and/or voltage have been proposed for transformer protection,the protection malfunction and the protection refusalstill occurs occasionally.The reason is that the transformer is a complex electromagnetic coupling device.Due to the lack of scientific and powerful analysis tools,the internal characteristics of the transformer operation have not been studied in depth.As a result,the transformer protection cannot accurately calculate the change in the fault electrical quantity like the line protection.The situation is that the correct rate of action is lower than the line protection for a long time.This thesis relies on the smart grid joint fund project "Basic Research on Multiparameter Adaptive Protection and Safe Operation of Power Transformers".This project mainly studies the method of transformer fault identification based on physical field,which integrates electrical,magnetic,pressure,flow velocity,ultrasonic and other parameters into multi-parameter adaptive protection.On the basis of summarizing the relevant literature,this thesis focuses on the characteristics of magnetic flux leakage parameters in the project,aiming to distinguish the various operating conditions of transformers from the magnetic flux leakage parameters,so as to provide the criterion of magnetic flux leakage characteristics for multi-parameter transformer protection.This thesis for the power transformer leakage magnetic field of the general research situation at home and abroad are discussed,and a model for SF10-50000/110 k V transformer real size as the research object,through reasonable simplification and assumption of its structure,three-dimensional finite element calculation models are built by using finite element electromagnetic simulation software.The normal operation,excitation inrush current and turn-to-turn fault conditions of transformer are studied,from the transformer winding axial direction and the upper iron yoke amplitude to the two directions,the peak value of symmetry and other qualitative analysis.The results show that there are obvious differences in the magnitude,symmetry of change of magnetic flux leakage when operating under different operating conditions such as normal operation of transformers,inrush currents,and turn-to-turn faults.According to the above conclusions,two suggestions based on fault identification method of magnetic flux leakage parameter of transformer.