Research On Seamless Transfer Strategy Of Master-slave Microgrid

Microgrids integrated independent distributed generation systems can operate in grid-connected or island mode,provide uninterrupted power supply for loads,and improve the utilization rate of new energies as well as the reliability of power supply.Inverters are the core device of microgrid power generation systems,which can not only convert the electric energy,but also can control the operation mode of microgrids according to the requirements.When the utility grid failure accident occurs due to accidental factors,inverters need to be disconnected from the utility grid in time to ensure the stable operation of microgrid systems.In this process,owing to the difference of the control structure under different operating modes,the instability of microgrids may be caused,and the system protection may be triggered,which leads to the system shutdown.Therefore,seamless transferring technologies are important for the stable operation of microgrids.Firstly,this paper analyzes and summarizes the common control structures of microgrids,and introduces control strategies under the different operation modes of microgrids with peer-peer control structure and master-slave control structure.Combined with engineering applications,the master-slave control is determined as the basic control structure after analyzing the advantages and disadvantages of different control structures.This paper introduces the seamless transferring control strategy of master inverters and analyzes the existing problems.Secondly,the topology of the master inverter is selected and its mathematical model is built.The voltage and current control loops are designed after decoupling the control loop,the d axis and q axis components are controlled by PI controllers,respectively.The reactive droop loop is compensated to improve the control precision of reactive power.To study the stability of the system,the small signal model of the inverter is built by using dynamic phasors method,the parameters of droop control loops are designed,and the stability of the system is analyzed.Then,this paper analyzes the transient change of the bus voltage when the utility grid fails,aiming at the problem that the traditional mode transferring is easy to cause bus voltage fluctuation,unified control strategy and control time sequence are proposed to realize seamless transferring,thus simplifies the control structure of the system and realizes the seamless transferring between operation modes.After the design of the component parameters of the system,the control and transferring strategies of different operation modes of the master inverter are verified by the Simulink simulation tool and hardware-in-loop simulation platform.Finally,this paper builds an experimental platform of the master inverter,and introduces the selection of power switching device as well as the design of hardware circuit.In the aspect of software,the programs of each function module for the master inverter control system are designed,and the work flow diagrams of each program are analyzed.The effectiveness of the proposed seamless transferring control strategy for the master inverter is verified by experiments,experimental results show that the bus voltage of the master inverter remains stable during switching processes,and the seamless transferring between different operational modes is realized.