Research On Hierarchical Cooperative Frequency Control Of Island Microgrid

As an important application mode of microgrid,islanding microgrid has been widely used in power grid systems far away from the power supply center,solving the problem of difficult power supply in remote areas.However,the distributed power supply in the island microgrid generally uses power electronic equipment to be connected to the grid.The system has small inertia and poor overload capacity,which increases the difficulty of frequency adjustment.Therefore,starting from the control objectives of the equipment layer and system layer,this thesis proposes a layered cooperative frequency control strategy including primary control and secondary control,and realizes the frequency stability control of the island microgrid.The main research contents are as follows:Starting from the representative distributed power supply model in the microgrid,a mathematical model with priority complexity and real-time control is established.The simulation of the output characteristics of each distributed power supply is performed to study the frequency of the island microgrid system.Control provides the basis.Taking the control system of DC converter and inverter in DC microgrid as the research object,the control strategies of DC converter and inverter are analyzed,and the corresponding real-time control mathematical model is established,and the simulation research is carried out.Aiming at the disadvantages of low impedance and strong coupling of power control in low-voltage microgrid,an improved droop control scheme with virtual inertia and virtual impedance is proposed as a primary control strategy to solve the problem of underdamping and low inertia of island microgrid system,and correspondingly improves the stability of the system.The simulation results show that the proposed primary control strategy of island microgrid has better dynamic and steady state characteristics.Aiming at the defect that the primary control has a steady-state frequency difference,the fastness and economical efficiency of islanding microgrid is taken as the goal,by considering the difference of source,load,and storage frequency,a secondary frequency control strategy for islanding microgrid based on distributed control is proposed.This strategy accomplishes the frequency optimal control process through sparse communication of neighboring control nodes.Each distributed controller obtains the optimal reference values of the source,load,and storage transformers through distributed computing,and optimizes source,load,and storage to participate in island microgrid.The active power of frequency modulation achieves the goal of synergy optimization of frequency modulation while minimizing the total adjustment cost of frequency in the dynamic recovery process.Simulation analysis verifies the correctness and effectiveness of the proposed two-stage frequency modulation strategy.