Control Strategy And Stability Analysis Of DC Microgrid In Island Mode

DC microgrid has the advantages of few energy conversion links and strong flexibility,and has been widely used in industrial energy saving,rail transit and new energy off-grid.The premise of practical application of DC microgrid is stable,safe and reliable operation.However,there are a large number of nonlinear elements and constant power loads in the DC microgrid,which will greatly reduce the stability of the DC microgrid and lead to the phenomenon of oscillation and instability of the DC bus.Therefore,this paper focuses on the analysis of the factors leading to the instability of DC microgrid,and studies the methods to improve its stability,which has important theoretical value and engineering significance for the long-term stable operation of DC microgrid.In order to solve the system stability problem caused by a large number of connected constant power loads,this paper studies the factors affecting the stability of DC microgrid,and proposes an active damping control method based on parallel virtual resistance.Firstly,the paper introduces the topology structure and operation mode of DC microgrid in general,and then analyzes the topology structure and control strategy of each module,including photovoltaic unit and energy storage unit.The mathematical model of photovoltaic and energy storage module was built,and the controller of energy storage converter was designed and the parameters were set according to the model.The effectiveness of the control method is verified by simulation.Then,a simplified DC microgrid model in island mode is established.The small signal modeling of energy storage unit and constant power load in DC microgrid in island mode is carried out by impedance method.Based on Nyquist criterion,the stability of DC microgrid model in isolated island mode is analyzed.The influences of constant power load power,DC side bus capacitance and droop coefficient on DC microgrid stability are analyzed.On this basis,the paper proposes an active damping control method based on parallel virtual resistance.By changing the resonant peak value of the source-side output impedance,the source-side output impedance is smaller than the load-side input impedance,and the stability margin of the system is improved.The feasibility and correctness of the proposed method to improve the stability of DC microgrid are analyzed by frequency domain analysis and impedance matching criterion.Finally,a DC microgrid simulation model in isolated island mode is built in the simulation software,and the active damping control method and stability analysis proposed in this paper are simulated and analyzed in detail.The simulation results show that the proposed control method to improve the stability of DC microgrid can still operate stably after the fluctuation of constant power load,the stability margin of DC microgrid is improved,and the voltage quality of DC bus is improved.The stability analysis is consistent with the simulation results.The proposed method is compared with existing methods through simulation,and the proposed method is more robust.