Improved Droop Control And Coordinated Control Of Microgrid

With the rapid development of economy and society,the scale of traditional large grid is constantly expanding,its drawbacks of security and reliability are gradually revealed.Meanwhile,environmental pollution and energy crisis are increasingly serious,the microgrid which can effectively solve these problems emerges as the times require.A microgrid combines distributed generation DG,load,energy storage unit and control unit into a single controllable system,it can not only run in parallel with the large power grid,but also in the islanding operation when the large grid is in failure.There are many important problems to be solved in the process of microgrid operation and control.Therefore,the control and stability of microgrid are studied in this paper.In this paper,the purpose and significance of microgrid research,the basic concepts and research status at home and abroad,the main control methods are reviewed.Then,the mathematical models of the inverter in various coordinate systems are analyzed,and three basic control strategies of the DG interface inverters are introduced.Secondly,the basic principle of droop control and power transmission characteristics of the inverter are analyzed,the relationship between inverter output impedance and line impedance and inverter control method is demonstrated.The droop control is improved by virtual impedance method,so that the traditional P/f,Q/V droop control can be used in low-voltage microgrid,and this realizes the decoupling control of active power and reactive power.The influence of the virtual impedance control loop on the output characteristics of the system is analyzed by frequency domain method,and the validity of the droop control method based on virtual impedance is verified by simulation analysis of microgrid operation.Finally,the small-signal state space model is established for each component of the microgrid,the eigenvalue method is used to analyze its state matrix.Then,the principle of the optimal decentralized coordinated control is analyzed theoretically,and the specific method of solving Levince-Athans equations is given.The optimal decentralized coordinated control is applied to the state space model of the microgrid as a whole,and it is proved to improve the stability of the system by an example.