Research On Fault Line Selection And Fault Location Methods For Single-phase-to-earth Faults In Neutral Ineffectively Grounded Systems

Most of China's medium voltage distribution network adopts the operation mode in which the neutral point is not grounded or grounded by the arc suppression coil.This is also called the neutral point non-effective grounding system.Among many failures,the single-phase ground fault occurs most frequently.Although it's allowed to operate with single-phase ground faults for 1 to 2 hours according to the regulations,in order to avoid the adverse effects caused by fault expansion and effectively improve power quality and power supply reliability,how to achieve fault line selection and positioning quickly and accurately has become an important research direction.Based on the research and summary of the existing research results,the two major deficiencies are improved.Firstly,the actual distribution system line parameters are complex,but usually the simulation of the existing research is based on the simple case of one kind of line;secondly,wavelet and wavelet packet technology,which are used for the processing of transient fault information,have high requirements on sampling accuracy,large amount of calculation,and are not easy to implement in hardware.In order to solve these problems and improve the accuracy and speed of the line selection and positioning technology,this paper conducted the following works:1)Starting with the construction of PSCAD simulation model,the complex system with multiple feeders and cables coexisting with overhead lines is simulated.The theoretical analysis and construction process of the simulation model are described in detail.This part mainly includes the wiring structure of the simulation system,more realistic arc model construction and the setting of various fault indicators.This simulation model provides a good experimental basis for the verification of line selection and positioning algorithms;2)A line selection algorithm combining wavelet and wavelet packet technology is proposed based on the existing research.Firstly,the modulus maximum value to determine the time when the fault occurs is obtained by the wavelet decomposition.Then,the wavelet packet decomposition coefficient and the polarity relationship between the zero-sequence current of the fault line and the non-fault line are used for line selection.Finally,to achieve the correct line selection,the fault measure is used as the line selection criterion.3)A generalized adaptive morphological filter,combined by composite structural elements and flat linear structures,is proposed to filter the line selection data.The data characteristics of the filtered data are detected by the transient detection to determine the fault line,and the fault measure is used as the line selection criterion.And taking different noise types included in the power system as examples,performing simulation verification on various fault conditions.4)Based on the characteristics analysis of transient phase current,the research of small current ground fault location is realized by mathematical morphology mutation detection.It's used to process the fault information and the fault section is used to determine the fault section.Two latter parts are the most critical technical points and innovations of this topic and ave important significance for the hardware implementation of single-phase ground fault line selection and positioning.In summary,on the basis of the previous achievements,the PSCAD simulation model is improved in this paper,and it reflects the single-phase fault condition of the real distribution network system more realistically.And an innovative combination of the existing methods and a new line selection and positioning algorithm are proposed.