Research On Fault Identification And Reclosure Strategy Of DG Access Distribution Network

The traditional distribution network is a single power radiation structure,and the relay protection is usually equipped with three-stage current protection and reclosing,which can selectively act in case of system fault and improve the reliability of power supply in case of instantaneous fault.However,with a large number of DG access to the distribution network,the traditional single power radiation network has become a multi power structure.At the same time of improving the utilization of new energy,the complex fault characteristics of DG make the traditional direction component,fault identification and auto reclosing technology face many challenges.Based on this,this paper starts with the construction of directional elements and adaptive reclosing of DG access distribution network,focusing on the analysis of fault current of DG access distribution network and its influence on protection and reclosing,fault identification of distribution network based on current cosine similarity,reclosing strategy of distribution network based on DG impulse test closing,etc.the specific achievements and innovations are as follows:1)This paper analyzes the characteristics of short-circuit current and its influence on protection and reclosing of multi type new energy access distribution network.Combined with the inverter,doubly fed and synchronous new energy DG,this paper analyzes the different current characteristics when the system is short circuited when it is connected to the distribution network,as well as the impact on directional protection,non directional protection,reclosing,synchronization,acceleration protection,etc.The research shows that DG access to the distribution network will lead to the reduction of the protection range and the more difficult coordination of the protection settings;because of the sudden change of the positive and negative sequence impedance of DG after the short circuit,the fault direction components may be misjudged;the failure will be difficult to eliminate if DG does not exit,which will also affect the adaptability of the accelerated protection.2)Aiming at multi information acquisition and comprehensive utilization of distribution network,a fault identification method based on current cosine similarity comparison is proposed for distribution network with multi dg.Based on the network protection system using regional multi-source information,the cosine similarity theory is introduced.In the first step,the cosine similarity of upstream total input current and each output current is constructed by using multi branch edge information of distribution network,and the fault area is judged based on the comparison of similarity;in the secondstep,the fault area is judged by using the information of fault area,and the cosine similarity is compared step by step.By introducing the theory of Petri net,the fault area location method based on forward Petri net search and reverse Petri net search is proposed.The comparison based on cosine similarity solves the problem of direction misjudgment,and the scheme based on twice positioning reduces the operation dimension of complex distribution network.3)Aiming at the secondary impact problem of permanent fault,a new strategy of distribution network reclosing based on DG trial closing fault identification is proposed.Using the small capacity DG connected to the distribution network,the DG is closed after the system fault trip,and the short-time impulse voltage is obtained,and the fault nature is judged.It is found that,due to the current limiting effect of DG,the outlet voltage of DG is almost the same as the fault point voltage in the case of permanent fault.Based on this low voltage criterion,the fault property can be easily identified.Furthermore,according to the distribution network structure,the trip and reclosing strategies after system failure are proposed.Using PSCAD / EMTDC software to build the system model of multi DG access to distribution network,the simulation verified the influence of DG on distribution network protection,the identification of fault direction based on cosine similarity,the identification of fault nature based on DG impact test and closing,and the results verified the effectiveness of the research content of this paper.