Research On Fault Line Selection In Small Current Grounding System Based On Injection Principle

As an important part of the power system,distribution network is directly connected with users.Its operation safety,reliability and economy are related to social production and people's lives.Small current grounding is mainly used in medium and low voltage distribution network in China.According to statistics,more than 80% of the faults in small current grounding system are single-phase grounding faults.With the distribution network becoming more and more complex and there are more and more lines,if failures can't be found in time,it is very easy to make the fault worsen and expand,resulting in more serious short-circuit fault,even seriously affecting the normal operation of the power system.From the practical application,the problem of fault line selection for small current grounding has not been solved thoroughly up to now.Therefore,it is of great significance to further study the technology of fault line selection for small current grounding.In order to solve the problem of low sensitivity of single-phase grounding fault line selection in small current grounding system,this paper propose two methods of fault line selection based on AC injection method and busbar separation,analyses the operation mode of neutral point and transition resistance of two methods.Aiming at the defects of the "S injection method",it is proposed to use the stochastic resonance theory to detect the weak S injection current signal,and then complete the fault line selection work.The main research contents are as follows:(1)For the neutral ungrounded system,the variation characteristics of zero sequence voltage and zero sequence current of each line of the neutral grounding fault are analyzed by using the symmetrical component method.Based on this feature,a method of fault line selection based on AC injection is proposed to enhance the steady-state feature of fault.After single-phase grounding fault,the zero-sequence current of the fault line is increased for a short time by the power capacitor is connected relative to the bus,so as to expand the gap of circuit criterion between the fault line and the non-fault line,and improve the selection criterion based on fault steady-state characteristics.Based on the principle of not affecting the operation of distribution network,the strength of the increased current is determined.The resistance to transition resistance of AC injection method is analyzed,and the specific line selection process is designed.The proposed AC injection method is simulated and verified by PSCAD simulation software.(2)According to the operation provision of grounding fault line stipulated in the rules for dealing with electrical accidents," Using the busbar separation to determine the grounding faulty busbar system",a small current ground fault line selection method based on busbar separated operation is proposed in this paper.For neutral grounding system and neutral grounding system via arc suppression coil,the change rule of zero sequence voltage before and after bus line separation,as well as the electrical quantity of fault line and non-fault line are analyzed.And a criterion for fault line selection of small current grounding based on bus-bar splitting operation under two neutral-point operation modes is proposed.The criterion can distinguish between metallic and non-metallic faults.The fault line can be selected accurately,and the bus fault can be identified at the same time.The flow chart of line selection is designed and the validity of the criterion is verified by PSCAD simulation and field test.(3)On the basis of introducing the basic principle of "S injection method" for line selection,this paper points out the reasons why the detection of S injection signal is difficult.The classical model of stochastic resonance theory and the mechanism of enhancing the output of the system are analyzed.A method of detecting S-injection signal using stochastic resonance theory is proposed.The frequency of S-injection signal is processed by normalization theory to meet the frequency requirement of weak input signal when solving stochastic resonance theory.The S injection signal detection process based on stochastic resonance theory is designed.PSCAD and MATLAB software simulation verify the validity of the signal detection method.