| The microgrid,is a small autonomous power generation and distribution system with self-control,self-protection and self-management functions,which is combined with generators,loads,energy storage units and control devices.As a complete controllable unit that can support the power grid,the microgrid plays an important part in the smart grid.From the perspectives of the modeling and control of the microgrid inverter and the coordination control of the microgrid system,many key issues such as the operating characteristics of the inverter,the design and control of the virtual impedance,the parameter selection method of the inverter under different operation conditions,the seamless switching algorithm of the inverter between different operating mode,the control strategy adapting to the multi-operation mode,the microgrid frequency regulation algorithm based on controller parameter adaptation and the power scheduling strategy of microgrid in islanding mode are studied.The specific research work and innovations are as follows:(1)Introduce the background,the definition and the characteristics of the microgrid.According to the current research status of the microgrid,the key techniques are summarized which cover the structure design of the microgrid,the introduction and comparison of control strategies for microgrid inverters,the optimization and coordinated scheduling algorithms,the system planning and energy management algorithms.(2)The output impedance of the microgrid inverter affects the stability during operation and the power distribution of multiple inverters in parallel.In this paper,an inverter impedance model based on V2C virtual impedance algorithm is established.Through the zero-pole estimation,root locus and Bode diagram,the effects of different virtual impedance parameters,different network characteristic parameters,different controller parameters on the operating characterstics and the response of output variables in both grid-connected mode and islanding mode are analyzed.According to the constraints of the two mode and the characteristics of the impedance model function,the reasonable range of the virtual impedance is determinded,and the controller parameter optimization formula coefficients are corrected.Both the simulation and the experiment verify the accuracy of the model.(3)A low voltage ride-through strategy based on VSG power angle limiting is proposed.The VSG power angle is limited while limiting the current command to ensure fast and stable response during fault recovery.A VSG-based microgrid multi-operation mode smooth switching control strategy is proposed,wich can realize seamless handover between islanding mode and grid-connected mode and stable operation in different modes of the microgrid.(4)The flexible characteristics of the microgrid put forward higer requirements on the controller parameter design of the secondary frequency regulation algorithm.When the system operating conditions such as power distribution characteristics,output impedance,and access conditions change,controller parameters should be adjusted online in time.In this paper,through the modeling analysis,the transfer function is effectively equivalent and simplified,and the parameter correction formula of the frequency regulation controller is derived.Thus,a parameter adaptive algorithm of the frequency regulation controller is proposed.When the operation of the microgrid changes,the system frequency can quickly recover and stabilize.(5)In the islanding state,the microgrid runs independently and has two scheduling tasks:frequency regulation and peak shaving.Frequency regulation is a top priority controlled by the MGCC while peaking is a secondary task adjusted by the EMS.The coordination between MGCC and EMS is an important part of the safe and efficient operation of the microgrid.This paper focus on the characteristics of VF mode and droop mode microgrid and point out the shortcomings of both the modes.The analysis and examples show that the coordination control mode affects the boundary conditions of the EMS algorithm,the complexity of the algorithm and the cooperation with the MGCC instructions,which can provide an important reference for the host computer scheduling strategy and data interface design.(6)A complete microgrid experiment platform including microgrid inverters,energy storage devices,photovoltaic power generation system,EMS and MGCC was built.Based on this microgrid experimental platform,the experimental results of various researches in this paper were verified. |
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