Research On Protection Strategy Of AC Microgrid

As an effective networking method for distributed generations and loads,Microgrid has been widely accepted,because it has lots of advantages,such as being able to make efficient use of distributed generations and can switch seamlessly in both grid-connectting and islanding operation modes.As a control unit,Microgrid has the advantages of easy control and management,but its flexible operation mode and complex failure characteristics present new challenges to the existing protection configuration,the traditional distribution grid protection is no longer applicable.Therefore,it is necessary to design a new protection scheme.The main work and achievements of this paper are as follows:(1)Analyzed the fault characteristics of microgrid which is composed of inverter distributed generations.The microgrid model composed of three protected areas was taken as the research object,and the fault simulation was respectively carried out in grid-connectting and islanding operation modes.The failure characteristics of the microgrid were analyzed in detail,which laid the foundation for the follow-up work.(2)The low voltage protection scheme based on Park transform is studied.In order to adapt to the characteristics of small fault current in islanding operation mode,voltage drop in fault is used as the basis of protection,which can improve the reliability of protection.The protection scheme uses the Park transform to convert the three-phase voltage to the dq-axis direct current,to better describe the disturbance.At the same time,the protection theory is analyzed theoretically,and the threshold value of the disturbance quantity is setted to realize the protection action.(3)The low-voltage protection scheme based on Park transform is improved.Considering the cooperation between the protection and the fastness of the action,a microgrid protection scheme based on low-voltage inverse-time is proposed.Through the analysis of the previous fault characteristics,the protection action equation is proposed,and the principle of protection is explained.In order to achieve the coordination between the protection,the coordination between the upper protection and lower protection were setted.Aiming at the microgrid with rotary generation,the idea of improved protection which based on fault phase selection is put forward.Considering the problem of transition resistance in grounding,the conjecture to improve the protection by using the reactance proportion coefficient is put forward.(4)Finally,a microgrid system is built on MATLAB / Simulink platform to validate the proposed scheme.The simulation's results show that the fault protection scheme based on low-voltage and inverse-time can cut off all kinds of faults in the grid-connectting and islanding operation modes,and the fault protection between the upper and lower levels can be effective.Solve the problem that over-current protection can not be adapt to microgrid fault characteristics.