AC Micro-Grid Line Protection Method Based On Differential Energy

With the rapid development of distributed generation technology,as an effective way to maximize the efficiency of distributed generation,and due to its intelligence,flexibility and high efficiency,microgrid not only improves the efficiency of distributed generation,effectively relieves energy pressure,but also improves power supply reliability of the distribution network.However,due to the large number of distributed power sources connected to the microgrid and the network structure of multi-terminal power supply,the two-way of power flow and the short-circuit current varies greatly in grid connection and island operation,which makes the traditional electrical protection method is no longer applicable.For this reason,this paper proposes a microgrid line protection method based on differential energy.First,by analyzing the fault output characteristics of the inverter interfaced distributed generation when a short-circuit fault occurs at different positions of the line in the microgrid system.Secondly,establishes the equivalent circuit diagram of the short-circuit fault of the line under the two operation modes of microgrid grid-connected and isolated island,and theoretical analysis on the fault characteristics at both ends of the line.Theoretically,the fault characteristics under different operation modes are explained,the energy difference of the short-circuit current at the first and the end of the fault line is obvious,while the energy of the short-circuit current at the first and the end of the non-fault line is the same,and the difference is zero.This characteristic law exists in both grid-connected and isolated island operation modes,which provides the theoretical basis for the study of microgrid line protection methods.In view of the large difference between grid-connected and islanding fault currents of microgrid systems and the multi-directional flow of power flow,a differential energy-based AC microgrid line protection method is proposed.First,use the Variational Modal Decomposirion(VMD)algorithm to decompose the fault current at both ends of the line.Secondly,select the IMF1 mode and perform Hilbert spectrum analysis on it to obtain its instantaneous amplitude function and calculate the current of spectral energy at both ends of the line.Finally,by calculating the differential energy at both ends of the line to construct the protection criterion,and judging whether the differential energy exceeds the protection threshold of grid-connected or islanding operation,the faulty within and outside the line can be determined.In order to verify the correctness and reliability of the microgrid line protection method based on differential energy proposed in this paper,a typical microgrid system model was established in PSCAD/EMTDC software,and the two different operation modes of grid-connected and islanding were simulated respectively.The data is processed by MATLAB to simulate and verify the method.A large number of simulation results show that the protection method in this paper is applicable in both grid-connected and island situations,and the protection method is accurate and reliable.