The Research Of Monitor Technology Of Ground Insulation State For Distribution Network Lines

Distribution network is an important link in the transmission of electric energy for users.It is an important link in determining the quality and reliability of power supply for users.Domestic and foreign medium voltage distribution network mostly adopts neutral indirectly grounded way,including neutral non-grounded way and petersen-coil grounded way,which is widely used in distribution power system in China.It is a small current grounding mode.In the case of low-current grounding fault,the line voltage between the three phases of the system remains basically unchanged and the system can operate for two hours with the fault to improve the reliability of power supply.Although single-phase earth fault can continue to operate,it will cause the increase of non-fault relative ground voltage,especially in the case of intermittent arc grounding or superposition of lightning strike,operation and other overvoltage,which is easy to make the non-fault line insulation weak point breakdown,cause phase to short circuit fault,and expand the accident scope and harm degree.In order to ensure the safety of the system and the reliability of power supply,it is necessary to take measures to deal with the fault points or to predict the ground insulation state of the lines in advance.At present,the technology of fault line selection and fault lacation has been basically mature,but there is little research on the monitoring technology of ground insulation state of distribution network lines.This paper analyzes the earth fault characteristics of distribution network,including steady fault characteristics and transient fault characteristics,and establishes the transient equivalent circuit model for transient fault characteristics.Compared with the traditional transient equivalent circuit,the transient equivalent circuit based on distributed parameters is more accurate and can provide more accurate theoretical basis for the analysis of transient signals.Based on the simplified transient equivalent circuit,the differential equation is established to solve the mathematical expression of the transient current.The attenuation factor and frequency variation rule of the main resonance component under different earth resistances are analyzed.This paper introduces the basic theory of Prony algorithm,expounds the selection principles of sampling frequency,data time window and model effective order in the application of Prony algorithm,and analyzes the rationality of these parameter selection for the fitting of earth transient signal.Through the establishment of the ATP simulation model and the Matlab program of the small current earth fault transient zero sequence current after a large number of simulations and Prony fitting analysis,using ATP simulation data analysis of the various operating conditions on the transient the main resonance frequency and the attenuationfactor,the influence of line insulation are analyzed from the fault wave record data before completely degradation transient changes of main resonance information trend,finally can be realized through fault wave record data to verify the distribution circuit of insulation condition monitoring.The real insulation state of the circuit can be more exposed after the transient earth fault,and the insulation state of the circuit can be monitored by the transient fault signal,so that the anti-interference ability is stronger and the prediction result is more accurate.The reason of single-phase earth fault in distribution network is analyzed from the statistical perspective.The analysis of insulation aging and fault formation process of three kinds of distribution equipment,i.e.insulator,arrester and cable line,is carried out.The key factors of transient earth fault to permanent earth fault are analyzed,and the index system and evaluation method of insulation state of distribution lines are established.Compared with the traditional insulation monitoring method that only use instantaneous grounding information,this method comprehensively considers the influence of earth fault and short circuit fault on the weak link of insulation,and combines the section positioning result to make the evaluation region more refined and the result more reliable.