Research On The Control Strategy Of Interlinking Converters For AC/DC Hybrid Microgrid In Islanded Mode

The AC/DC hybrid microgrid can be compatible with a variety of AC and DC distributed generators(DGs)and loads,which can greatly reduce the power loss caused by multiple AC/DC or DC/AC conversion links in a single bus microgrid,and provide power support for each other between AC and DC sub microgrids,which improves the power supply reliability of the system,and has the advantages of high efficiency,strong flexibility and low cost of power grid transformation.Therefore,the AC/DC hybrid microgrid has become a research hotspot in the industry fields.As the hub of the connection between AC and DC sub microgrids,interlinking converter(IC)undertakes the important tasks of power transmission and energy balance between AC and DC sub microgrids.Its control strategy is directly related to the safe and stable operation of AC/DC hybrid microgrid.Therefore,it is of great significance to study its control strategy.Aiming at the problem that IC is more easily affected by the intermittent and fluctuating output of wind power,photovoltaic and other new energy sources,unpredictable load disturbance and so on when operating in island mode,as well as the small inertia and damping,weak anti-disturbance capability of power electronic converters in the microgrid system,the control strategy of IC is studied,which can effectively enhance the inertia and damping of the system and improve the power control ability between AC and DC sub microgrids when AC/DC hybrid microgrid is in islanding operation,so as to realize the mutual support of power between sub microgrids and the power dynamic balance of the whole system,and ensure the stable and reliable operation of the system.Firstly,after the background and significance of the research project are introduced,the research status of AC/DC hybrid microgrid technologies at home and abroad are briefly described,and the research statuses of IC from two aspects of topology and control strategy are summarized.Secondly,based on the analysis of the system composition and operation principle of the AC/DC hybrid microgrid system,the system capacity is designed according to the power demand of users.At the same time,the droop characteristics of AC and DC microgrid,the per-unit idea of unifying the two droop characteristics in the same coordinate system and the operation characteristics of virtual synchronous generator(VSG)are analyzed.The mathematical model of IC is established and the parameters are designed,which lays the foundations for quantitative analysis and simulation study of the following control strategies.Thirdly,based on the analysis of the energy flow relationship between AC/DC sub microgrid and IC,the control strategy of IC with "power-voltage-current" three-closed loop control structure is designed.In this control strategy,the power outer loop unifies the droop control characteristics of AC and DC sides in the same coordinate system by using the idea of per-unit,generates the reference value of active power based on bidirectional droop control,and improves the inertia and damping effect of active power regulation by using the characteristics of VSG.The dq axis reference voltage of voltage loop is generated by voltage synthesis and coordinate transformation with the reference voltage generated by "reactive power voltage" control strategy.The modulation signal of the switch tube is generated by voltage and current double loops decouple the dq axis reference voltage,and completes the control of the IC.At the same time,the small signal modeling and stability analysis of the "active power frequency" control based on the characteristics of VSG are carried out.Fourthly,the simulation model of the AC/DC hybrid microgrid in islanding mode is established in MATLAB/Simulink,and the control strategy of IC is simulated in different operating conditions.The simulation results show that the proposed control strategy can effectively enhance the overall inertia and damping of the system in islanded operation,and realize the mutual power support between the sub microgrids and the dynamic power balance of the whole system.Finally,the main work of the thesis is summarized and the direction of further research is also pointed out.