Distributed Adaptive Overcurrent Protection For Microgrids

Microgrid technology has a great prospect to solve the current energy crisis and environmental problems,but the structure of microgrid is very different from the traditional distribution network,and the traditional protection methods are no longer applicable to the microgrid system.The protection of microgrid is a key issue in the development of microgrid technology.In view of this,this thesis makes an in-depth study on the protection of microgrid.The main research contents of this thesis are as follows:(1)Firstly,this thesis studies and analyzes the microgrid structure and its typical fault characteristics.The Distributed Generator(DG)is an important part of the micro-grid.In this thesis,the types,topological structure and control mode of DG are studied in detail,and then the control strategy of the micro-grid is introduced.Based on the above introduction and research analysis,a microgrid simulation model was built by using PSCAD simulation software to simulate and analyze the fault output characteristics of the microgrid under different DG access positions,operation modes and fault positions.(2)Aiming at the problem that the traditional adaptive overcurrent protection does not consider the contribution of DG to the fault current,this thesis proposes a decentralized adaptive overcurrent protection.In the calculation of adaptive current setting value,the contribution of DG to the fault current is considered.The instantaneous overcurrent relay is adopted as the only proxy,and the protection method is distributed among the instantaneous overcurrent relays,which can make decisions according to local data without the need of communication between media.The proposed decentralized adaptive overcurrent protection method solves the problem that the traditional adaptive overcurrent protection is not applicable to the microgrid,and improves the reliability of the protection.Finally,the proposed method is verified on the PSCAD simulation software,and the results show that the proposed protection method can reliably protect the micro-grid,and the reliability of protection is improved.(3)Aiming at the problem that it is difficult to calculate the equivalent impedance value in the traditional adaptive overcurrent protection,Rudin Osher and Fatemi(Ro F)optimization algorithms are proposed to calculate the equivalent impedance value of the system.Through the research and analysis of the fault characteristics of microgrid and the traditional adaptive overcurrent protection,it is found that the equivalent impedance value of the system changes in real time when the fault occurs,and the equivalent impedance value of the traditional adaptive overcurrent protection is difficult to calculate.Therefore,this thesis studies the Thevenin equivalent impedance calculation method based on the concept of covariance.By analyzing the error of this method,the Thevenin equivalent parameter estimation problem is transformed into a mathematical optimization denoising problem.In order to reduce the errors existing in the above methods,this thesis built a Ro F optimization denoising model based on Total Variation(TV).The Primal-Dual Hybrid Gradient Method(PDHG)is used to solve the equivalent impedance of the system,which improves the accuracy and stability of the equivalent impedance of Theyannin.Finally,MATLAB and PSCAD software were used for simulation verification,and the results show that the proposed method can adapt to the two operating modes of the microgrid simultaneously,and is superior to the existing Thevenin equivalent parameter value calculation method.