Complex Grounding Fault Diagnosis Technology For Medium-voltage Distribution Network

Electric power is an important basic industry.With the rapid development of the national economy,electric energy,as a clean and efficient energy,is widely used in every corner of life,and the demand for electric energy is also increasing.As a bridge between users and transmission networks,the safe and stable operation of distribution networks is related to the overall situation of national economy,people’s livelihood,and economic development.With the continuous development of the power system in China,the distribution network covers an increasingly wide range,densely distributed in cities,rural areas,and mountainous areas.The equipment is numerous and complex,and is located outdoors.It is greatly affected by the weather,environment,and external forces.Therefore,faults often occur in the distribution network.How to transmit power safely and efficiently has become a research topic for many scholars.China’s medium-voltage distribution network generally adopts the small-current grounding mode with the neutral point grounded through the arc suppression coil or the neutral point ungrounded.Single-phase grounding fault is the most common and frequent grounding fault.At present,the line selection technology for this fault has been mature,and the accuracy of the line selection device is high.The fault can be removed by a variety of methods including detection of transient and steady-state quantities.However,due to the increasingly complex distribution lines,the probability of complex ground faults such as two-point ground fault and broken line ground fault is becoming higher and higher.The complex faults can be affected by many factors such as transition resistance,fault sequence,fault phase,grounding condition,neutral point grounding mode,etc.At present,it is difficult to select the line,and it is urgent to solve the problem of such fault line selection on site.In order to solve the above problems,this paper establishes a two-point grounding fault and a complex broken line grounding fault model for the small current grounding system,and proposes a complex fault line selection technology suitable for the neutral point grounded through arc suppression coil and the neutral point ungrounded system.The specific research work is as follows:1.The fault analysis of two-point grounding fault is carried out.According to the three-sequence voltage balance equation of fault and the boundary condition of grounding point,the zero-sequence voltage of neutral point and the zero-sequence current at the outlet of each line are calculated from the theoretical level in the case of two-point in-phase grounding fault and two-point out-of-phase grounding fault,and the unique fault characteristics compared with single-phase grounding fault are studied.According to its unique characteristics,the line selection method for successively judging the fault line is proposed,The first fault line with obvious transient characteristics is selected by using the existing line selection method,and the second fault line is selected by conversion according to the zero-sequence voltage of neutral point and zero-sequence current data of each line before and after disconnection.2.The fault analysis of complex broken line grounding fault is carried out.When a single phase grounding fault occurs in one line,the fault type of another fault line is divided into three cases:broken line ungrounded,broken line load side grounding and broken line power side grounding.The fault characteristics of each case are obtained from the formula,and then the line selection technology applicable to complex broken line grounding fault is proposed according to the characteristics of each case.When a broken line is not grounded,due to the unbalanced three-phase voltage at the broken line,a method of selecting the fault line by using the combination of FTU and TTU to monitor the line voltage is proposed.When the grounding fault occurs at the power supply side and the load side of the broken line,the line selection method that can also judge the fault line in succession is proposed.3.Developed a complex grounding fault line selection device for small current grounding systems,and designed its software and hardware.In ATP and MATLAB simulation software,all cases of two point ground fault and complex broken line ground fault are simulated and verified,and fault characteristics that are fully consistent with theoretical analysis are obtained.After disconnecting the first fault line,it is obtained that the second fault line is indeed the line with the largest change in the calculated value of zero sequence current.At the same time,these complex ground faults were also simulated and verified on the physical experimental platform,and all the faulty lines were successfully selected,proving the correctness and practicality of the proposed line selection method of successively judging the faulty lines.