Research On Modeling, Analysis And Control Design Of AC Microgrids

The conventional power systems are going through a series of significant changes.Large scale power-converter-interfaced renewable energies are integrated into the grid in a centralized or distributed approach,which changes the characteristics of the conventional synchronous-generator-based power systems.Then,as an effective way to accommodate distributed generation,the concept of microgrids has raised a worldwide discussion since it was proposed.The microgrid utilizes the distributed generation technology,power electronics technology,energy storage technology,and information technology,which are also emerging in the bulk power systems.As a result,the microgrid provides an effective way to understand the future power systems.In this context,the thesis takes the microgrid as the research focus.The modeling,analysis,and control design of the microgrid are especially tackled in this thesis.Firtstly,the mathematical models of distributed generators(DGs)are derived,and their characteristics are analyzed.The DGs and the energy storage systems are basic elements of the microgrid.Two key characteristics distinguish the DGs with the conventional synchronous generators:1)the DGs are usually power-converter-interfaced and the power converters dominate the dynamics of the microgrid.2)The DGs are usually driven by the renewable energies,such as wind turbines and PV.The power fluctuation of the renewable energies makes the DGs harder to control.Therefore,we study the DGs from two aspects:1.The most commonly used three types of interface power converters,namely the droop-based voltage-controlled converters,the PLL-based current-controlled converters,and the virtual-synchronous-generator-based converters are modeled,linearized,and verified.Based on the linerazed models,the characteristics of the three types of power converters are revealed,which lays the foundation of the whole thesis.2.A new control scheme is proposed for doubly-fed induction generators based wind turbines(DFIG)in microgrids that operate in multiple modes.The DFIG is controlled as a droop-controlled voltage source by the rotor side converter,using the indirect stator flux oriented vector control technique,which is suitable in the microgrid or weak grid to suppoer the system.The designed coordinated control strategy keeps the DIFG stable when the wind speed varies.Simulations in Matlab/Simulink verify the effectiveness of the control strategy.Secondly,the microgrid system is modeled and the respective control strategies are designed.The microgrid control strategies can be categorized into three types,i.e.,the centralized control,the distributed control,and the decentralized control.This thesis mainly focuses on centralized control and decentralized control:1.A decentralized quasi-hierarchical control scheme is proposed for microgrids with heterogeneous DGs.The functions of three-level centralized hierarchical control are implemented locally in DGs without any communication,only by utilizing the local frequency measurement and a nonlinear droop curve.Consequently,the frequency restoration and economic dispatch can be realized locally.Simulations in DIgSILENT verify the effectiveness of the control strategy.2.To better understand the nonlinear droop control,we conduct further research on analysis and design of nonlinear droop control.We first extend the applications of nonlinear droop control to the grid-connected microgrid,which can support the grid in a nonlinear way.Then,the influence of nonlinear droop control on system small-signal and large-signal stability is studied.Correspondingly,an adaptive virtual inductance control and a transient enhanced control scheme are proposed to ehance the stable operation of the converters.Simulations in Matlab/Simulink verify the effectiveness of the control strategy.3.A systematic approach to modelling,analyzing,and designing a hierarchical-controlled microgrid is proposed.Microgrids with heterogeneous converter interfaced DGs are modeled,linearized and verified.Then,modified secondary and tertiary control strategies with feedforward control are proposed,providing better control performance.Finally,a nonsmooth synthesis method is applied to optimally tune the control parameters in the microgrid central controllor,which can optimize the controller in multiple dimensions.Simulations in Matlab/Simulink verify the effectiveness of the control strategy.Thirdly,we present the control design of a multi-mode microgrid on Zhairuoshan island in China.This microgrid is designed to operate in both grid-connected mode and islanded mode.The renewable-energy-based generators and energy storage systems are both taken into the design consideration.We adopt the model based design approach,which improves the design efficiency and can be easily applied in other design scenarios.