| With the release of the "14th Five-Year Plan for Modern Energy System",how to innovate the power grid structure and promote the development of smart distribution network has become the trend of today’s power system development.The traditional grounding system has been gradually replaced by the flexible grounding system because it cannot compensate the active power and harmonic components in the fault current.But no matter what the grounding method is,when a single-phase grounding fault occurs,if the fault cannot be removed in time,the system will eventually be harmed.Therefore,in order to ensure the safety of the power grid and the reliability of the power supply,it is necessary to quickly select the fault line and locate the fault of the flexible grounding compensation device.Therefore,this paper takes the flexible grounding system as the research basis,and conducts in-depth research on the fault line selection and fault location method of the system.The specific research contents are as follows:First of all,this topic starts with the analysis of the characteristics of the small current grounding system,briefly analyzes the steady-state and transient characteristics of the ungrounded system and the resonant grounding system,and puts forward the shortcomings of the traditional grounding system.Meanwhile,this topic derives the difference between faulty lines and non-faulty lines when single-phase-to-ground fault occurs.Secondly,a flexible grounding system is proposed.Its technical characteristics and working principles are introduced,and it is modeled systematically,and the transfer function between the control objects is deduced.By comparing the advantages and disadvantages of various controllers,the double closed-loop control strategy of the outer-loop quasi-PR control and the inner-loop P control is finally adopted.At the same time,a simulation model is built in MATLAB,the size of the transition resistance is changed,and the ability of the arc suppression coil and the flexible compensation device to reduce the fault current is compared.Thirdly,aiming at the fault line selection problem of flexible grounding system,this article has proposed an innovative fault line selection method based on the combination of wavelet packet transform and discrete Fréchet distance algorithm.The method first uses wavelet packet transform to extract transient characteristic waveforms,and then applys discrete Fréchet distance algorithm to calculate the distance between the above extracted waveforms.The distance between curves is used to measure the similarity between curves,and then the fault line selection is realized.At the same time,the accuracy of the line selection method is verified by building a simulation model in MATLAB and changing the size of the transition resistance.Finally,aiming at the fault location problem of flexible grounding system,this paper has innovatively proposed a fault location method based on the combination of transient abrupt change and dynamic time warping.The method firstly introduces the principle of the method based on transient abrupt change to find out the distinguishing features between the upstream and downstream of the fault point of the fault line and the non-fault line.Then,a dynamic time warping algorithm is systematically introduced to compare the differences between individual curves by calculating the cumulative shortest path between them.Combining the above two methods,the method based on the transient mutation is used to initially select the fault interval,and then the current waveform measured in each interval above is utilized to calculate the cumulative distance using the dynamic time warping algorithm,thereby realizing the location of the fault interval.At the same time,the accuracy of the positioning method is verified by building a simulation model in MATLAB and changing the size of the transition resistance.Through the above simulation verification,this paper draws the following conclusions: the simulation results show that the ability of the flexible compensation device to reduce the fault current is much greater than that of the arc suppression coil;the fault line selection method based on the combination of the wavelet packet transform and the discrete Fréchet distance algorithm can achieve 100% fault line selection,and is not affected by the influence of the size of the transition resistance;the fault location method based on the combination of transient abrupt change and dynamic time warping can achieve 100% fault location,and is not affected by the size of the transition resistance. |
