Research On Bus Voltage Stability Control Of DC Microgrid With Constant Power Load

Compared with AC microgrid,DC microgrid has simple control structure,high flexibility in operation,and no need to consider the advantages of frequency,phase and reactive power compensation,so it has attracted the attention of scholars at home and abroad.In addition,the weak damping and low inertia characteristics of DC microgrid are further weakened with the increase of the proportion of constant power load to DC microgrid.In the DC system,the stability of bus voltage is the only standard to measure the stability of the system,therefore,its stability cannot be ignored for the safe operation of the microgrid.In this thesis,the DC microgrid with constant power load is taken as the object,and different stability criteria and control strategies are adopted to study the bus voltage stability of DC microgrid.The main contents are as follows:(1)The thesis summarizes research status of DC microgrid and the development status of constant power load are.According to the development history at home and abroad,the voltage stability control method of DC microgrid bus is briefly introduced.(2)The thesis analyzes impedance characteristics of constant power load,and analyzes the mechanism of voltage fluctuation and even instability of system bus when constant power load is connected to DC microgrid,and constant power load,its linearization model and stability criterion method are introduced.The state space average method and equivalent circuit method are used to model DC microgrid.In view of the nonlinear system caused by constant power load connected to DC microgrid,in this thesis,Lyapunov direct method and Mixed potential function method are used to verify the stability of large signal for the established model,and obtains the constraint conditions of large signal stability.(3)To solve the problem of system stability caused by constant power load,a model prediction control strategy with high order sliding mode observer is proposed.Firstly,the objective function of the Buck converter is constructed according to the model predictive control theory,the tracking equation of rolling optimization is established to solve the optimal control law.Then a high order sliding mode observer is constructed to improve the voltage control accuracy.Finally,the stability analysis of small disturbance is carried out to establish the equivalent models of the source and load sides.Compared with the traditional double closed-loop PI regulation and model predictive control,the robustness of the proposed control strategy and the correctness of accurate tracking and control of bus voltage are verified.(4)An improved droop control strategy is proposed,which is based on the line loss problem.The system objective function is established including voltage offset,current-sharing error and line loss.The solution of minimum offset is added to improve the accuracy of current sharing,improve the quality of bus voltage,and simplify the solution of line loss into the solution of bus voltage offset.Compared with the traditional droop control strategy,the proposed control strategy not only improves the bus voltage quality but also reduces the line loss.