Research On Active Grounding Device And Grounding Fault Handling In Distribution Networks

With the construction of a new power system,the distribution network not only bears the access of large-scale distributed energy,but also directly supplies power to multiple loads and a wide range of users.Under the special nature of integrating power generation and power consumption,higher requirements are put forward for the safety and reliability of power supply.Single-phase ground faults in distribution network account for nearly 80%of all fault types,and are closely related to fault power outages,safety accidents and personal safety.Therefore,the development of intelligent ground fault safety disposal technology is of great significance to improve the reliability and safety level of power supply in power distribution system and reduce the incidence of power accidents.Under the traditional neutral point grounding method of distribution network,it is often difficult to deal with ground faults taking into account the safety and reliability of power supply,which leads to the spread of faults and subsequent major economic losses when operating with faults.The Active Grounding Device(AGD)composed of power electronic equipment has the characteristics of flexibility and controllability.It can adjust the zero-sequence voltage arbitrarily and then control the fault voltage.It provides a new direction and idea for ground fault compensation and protection in distribution networks.Especially in high-risk areas such as forests and grasslands where high-resistance ground faults are likely to cause fires,AGD has broad application prospects.Therefore,this paper carried out research on the AGD voltage-type arc suppression control,as well as the research on ground fault handling process with AGD as the core of fault disposal.The main research contents include:(1)In view of the lack of detailed modeling and comprehensive analysis of the traditional voltage-type arc suppression control of distribution network AGD,three feedback voltage control implementation methods are proposed and compared from multiple perspectives.Firstly,based on the principle of voltage type arc suppression and combined with the characteristics of substation voltage transformers,three different realization methods of voltage feedback control are proposed.Secondly,the transfer function model of the voltage feedback control system is established,the fault voltage and current expressions before and after AGD voltage control are obtained,and the difference between before and after AGD control under two extreme cases of ground fault is analyzed.On this basis,using the analysis method of amplitude-frequency characteristic curve and root locus,different voltage feedback controls are compared from four aspects:arc suppression compensation effect,control system stability,control parameter influence and implementation difficulty.Finally,it is verified by simulation and experiments that the control of the fault phase voltage feedback by the PT in the station is simple to implement and has a good compensation effect,which lays a foundation for the subsequent implementation of arc suppression control and line selection protection by AGD.(2)The arc suppression scheme of AGD adapted to uncertain fault resistance has been proposed,including fault detection algorithm,fault phase selection process,and adaptive parameter arc suppression control method.Firstly,a high resistance fault model based on the principle of solid breakdown was established for arc high resistance faults that are difficult to detect and handle.The wavelet coefficients of fault zero sequence voltage are extracted by wavelet decomposition,and the energy value is constructed by wavelet soft threshold de-noising.Based on this,a fault detection algorithm is designed.The accuracy of fault detection and starting speed are verified by simulation and experimental data validation.Secondly,because the AGD voltage type arc suppression control depends on accurate fault phase selection,a fault phase selection process is proposed to distinguish low resistance and high resistance faults.Field experimental data validation verify the effectiveness of the proposed phase selection process for low resistance,high resistance and continuous short-time arc faults.Finally,the uncertainty of the fault resistance leads to a contradiction between the compensation effect under low resistance faults and the control stability under high resistance faults,making it difficult to design parameters for AGD arc suppression control.Arc suppression control method with adaptive control parameters is proposed,which is not only easy to implement but also has operational stability and arc suppression effects that can adapt to various ground faults.Simulation and experiments have verified the effectiveness of the proposed control method.(3)Aiming at the compatibility between AGD and existing grounding fault handling requirements,a distribution network grounding fault handling process centered around AGD has been designed and proposed.Firstly,a handling process including fault detection,arc suppression,judgment,and isolation has been designed for the handling of grounding faults after AGD is connected to the distribution network,which is fully compatible with the current grounding fault handling methods and requirements.Focusing on the fault line selection after AGD arc suppression control,two implementation technical routes are proposed,namely the passive line selection device installed independently in the station and the active line selection method integrated in AGD based on voltage disturbance.Based on this,two line selection methods are proposed.a)The traditional passive line selection method has limited fault protection range,which can easily cause misjudgment when high resistance grounding faults occur and cannot be used in conjunction with AGD.Therefore,an improved zero sequence admittance fault line selection method is proposed.Considering the problem of zero sequence three-phase asymmetry of distribution lines,the calculation formula of zero sequence admittance under the condition of three-phase asymmetry of two grounding modes of small current earthing system is derived.Aiming at the problem that the line selection criteria of resonant grounding system are not obvious,the concept of phase angle difference coefficient based on zero sequence admittance is proposed.Through simulation and test data,the effectiveness of the improved admittance method is verified,and the defect that the traditional zero sequence admittance is not accurate for high resistance grounding fault line selection is overcome.b)For the active line selection method using AGD for voltage disturbance,the characteristics of the zero sequence current of the faulty line and the normal line before and after AGD arc suppression control were first analyzed.Secondly,the expressions for the zero sequence current of the fault line and the normal line after AGD voltage disturbance were derived,and based on this,permanent fault criteria and fault line selection criteria were constructed based on the sudden variable of zero sequence active power.Finally,the reliability of the active line selection method with AGD voltage disturbance for fault line selection within 10k Ω resistance was verified through simulation.Finally,based on the research content of this paper,AGD prototype applied to the simulation laboratory of 10kV distribution network is designed and developed.The main circuit of AGD is completely designed,the hardware and software of AGD control system are developed,and the laboratory test is completed.Through the research and development of AGD,a more comprehensive,active and safe protection process is provided for single-phase-to-ground faults in distribution networks,especially high-resistance faults,which has practical significance and application value.