Research On Key Technologies Of Inverters In Microgrid Based On Virtual Synchronous Generator Control

Microgrid is a kind of small scale generating and distribution system.Through interacting and supplying the grid,microgrid can remit the impact of a mass of distributed generators access to the grid.However,the traditional grid-connected inverters in the microgrid can just generate active current.It cannot satisfy the requirement of the microgrid on the high reliability and flexibility.The main focus of this study is based on the research of the inverter control of the microgrid and multi-machine coordinated control.Through simulating the performance characteristics of the traditional synchronous generator,the mathematical model of virtual synchronous generator(VSG)algorithm is constructed and borrowing from the hierarchical control thoughts of the power system,the issues of the power decoupling control and the frequency and voltage regulation of the microgrid under different operation mode are analyzes.This thesis firstly researches the control principle and ontology algorithm of VSG.The mathematical model consists of the main circuit topology,the power/frequency controller,the excitation controller and the voltage and current controllers is established.Based on the small signal approach,the equivalent mathematical model of the power loop is set up.The selection of related parameters is also confirmed based on the system design requirement,which can guarantee the stability and performance of the system and implement the request of the decoupling between the internal and external loop of the controller.All of above design establishes the foundation for further research of virtual synchronous generator.The line impedance in a microgrid generally presents resistance-inductance or resistive property,which can result in a strong coupling between the control of the active and reactive power.For the power decoupling control,this thesis analyzes the mechanism of the power coupling and points out that in the microgrid,the power decoupling control must accomplish the power approximate decoupling and the sharing control of the reactive power on the premise of the system stability.Then based on the output power model of the microgrid,a combination of the diagonal decoupling matrix and the estimated voltage of the point of common coupling is proposed.It can implement the power approximate decoupling and improve the accuracy of the power control.Furthermore,it can also implement the sharing control of the reactive power and avoid the generation of the reactive circulating current.Up to now,only primary frequency modulation and primary voltage modulation of VSG can be realized.However,the primary modulation incurs static error and cannot guarantee the power supply quality under the islanded mode of the microgrid.Hence,the distributed generators(DGs)in the microgrid are required to implement the secondary modulation.In this thesis,an improved control structure of virtual synchronous generator is proposed to realize the function of secondary frequency and voltage modulation for the islanded microgrid.In the power/frequency controller,an integrator is introduced to the damping element of VSG,which enables the realization of non-error frequency adjustment and secondary frequency with multiple inverters.It uses local information of the inverter and improves the dependability of the system.In addition,multi-inverter participation facilitates the allocation of power shortage of microgrid according to the capacity each inverter.In this manner,the extension of the total frequency regulating capacity is possible.For the excitation controller,the distributed control thought is introduced to implement the secondary voltage control.With the help of the nearest-neighbor communication,a new method based on reactive power consensus of the multi-agent model is proposed.Through evaluating the average reactive power of the whole network,it can regulate the microgrid voltage to its nominal value while maintaining the reactive power sharing among the VSGs.The distributed architecture allows for flexibility and redundancy and eliminates the need for a central microgrid controller.To verify the effectiveness and correctness of the above control strategies,some simulated and experimental results are carried out.