Study On Suppression Technologies Of Transformer Inrush Currents Based On DC Excitation And Phase Controlled Switching

When no-load transformer is switched,inrush current of large magnitude will be generated,various harmonics will be injected into the grid,and distortion of voltage and current will be caused,which may affect operation of the power grid,such as causing HVDC blocking or overvoltage phenomenon.Therefore,it is of great significance to study limiting measures of transformer inrush current to ensure the reliable operation of the power grid and safety of related power equipment.In this thesis,new suppression technology of transformer inrush currents is studied by means of controlling transformer saturation,and suppression measures are proposed based on the additional DC excitation control and phase selection switching technology.The results have theoretical guiding significance for the development of related suppression devices and solving such engineering problems.The main research contents of the thesis are as follows:(1)Theoretically,generation mechanism and harmonic characteristics of inrush current during transformer switching process are analyzed.The influence of inrush currents on grid equipment such as HVDC is studied.Through the theoretical formulas,characteristics of transformer flux linkage and current during process of switching and harmonic components of the inrush current are analyzed qualitatively.By simulation model of a simplified power grid,switch of transformers near HVDC converter station are analyzed.Harmonics and voltage distortion of the grid voltage,as well as HVDC harmonics and commutation failures are studied.(2)A method of inrush current suppression based on DC components compensation is proposed.The key techniques and device design are studied.This method is proposed by dynamically tracking the DC components of inrush currents after switching,and generating a DC current with a rectifier circuit to compensate the DC component,thereby reducing saturation degree of the transformer and suppressing the inrush currents.Firstly,influence of the amplitude of DC components in inrush currents on the saturation characteristics is qualitatively analyzed,and the effect of reducing DC components on limiting the inrush currents is clarified.Secondly,a specific suppression method based on DC component compensation is proposed,including modes of the DC current compensation loop and methods of DC current control.And then designing method and parameters selection principle of the suppression device were studied,including basic structure of the suppression device and the controller of DC current.The basic structure of the VSC DC excitation power supply was introduced.Finally,typical equipment parameters are selected to simulate and analyze the limiting effect of the suppression devices.(3)Based on study of transformer phase control switching technology,two integrated control strategies of transformer switching are proposed.One is the strategy combination of DC component compensation and phase control,and the other is the strategy combining pre-excitation and phase control.Simulation results are used to verify the application effect of the above strategies.Firstly,transformer phase control switching is studied,including transformers with no residual flux and that with residual flux.Secondly,the switching strategy combined with DC component compensation and phase control is studied,where DC component compensation control is considered for the first phase and phase control is adopted for the latter two phases.Then the specific strategy is studied and verified by simulation.Finally,the switching strategy combined with pre-excitation and phase control is studied,that is,pre-excitation current is injected into the transformer before switching,so that flux linkage of the transformer reaches a certain value,which is equivalent to setting the residual flux value,and then phase control is used.The specific strategy was researched and verified by simulation.