Research On Optimization Of Microgrid Line Protection Scheme

Due to the heavy use of fossil fuels,the problem of environmental pollution has been deteriorating.In recent years,large-scale development of distributed generation has made the dependence on traditional fossil fuels reduced to some extent,and the demand for electricity on the basis of clean energy has also been met consumption of some users.Micro-grid as an effective way to dissipate distributed generation,not only can improve the utilization of new energy sources such as wind power,photovoltaic,but also ensure the normal and reliable generation of distributed energy to achieve the maximum use of sustainable clean energy.And as technology advances,micro-grid technology will become more and more mature.Because micro-grids not only have multiple accesses to intermittent distributed power supplies,they also have multiple operating modes.This not only makes the single-supply radiating network in a traditional distribution network into a multi-source network.Affected by the surrounding environment and the load,the output power and load of the micro-source will change,and the micro-grid network will be affected to make the size and flow of the power flow complicated and changeable.Therefore,in this thesis,the current application of AC low-voltage micro-grid as the research object,the basic structure of the micro-grid power generation system and the main control methods of the micro power supply are studied,and the micro-grid model is constructed accordingly;Secondly,the analysis of micro-grids in the single-phase short-circuit,two-phase short-circuit and threephase short-circuit fault characteristics of the micro-source and the line under both network and island operation modes show that the fault current of the micro-grid is different under different modes.Due to the short circuit of low-voltage micro-grid,the protection based on the traditional fixed value principle can not distinguish adjacent lines,and then briefly analyzes the applicability of relay protection such as mutation protection and differential protection,and introduces the principle of direction identification of positive sequence fault components.Thirdly,This thesis constructs a micro-grid protection communication system and proposes a micro-grid line protection optimization scheme based on the communication system.Protection system consists of micro-grid protection control unit and the line branch intelligent protection unit.The main protection is started by the overcurrent protection and the positive sequence current direction and backup protection only uses overcurrent startup.All the protection along the fault branch and the protection of the non-fault branch are blocked.The fault branch between the upper and lower levels,through the communication lines to achieve the delay between the adjacent set of protection to meet the protection of the selectivity.The backup protection adopts the same starting mode as the main protection.Because the overall size of the low-voltage micro-grid is small,this paper studies a backup protection method using step delay to meet the task of fault identification and removal when the reliability of the communication system is poor.Finally,this paper establishes the micro-grid simulation model in MATLAB / Simulink,and verifies the feasibility of the line protection optimization scheme in different fault types in the grid-connected and islanding mode.