Distributed Frequency And Voltage Control Of Microgrid Clusters Under Islanded Opweraion Mode

As an efficient way to consume renewable energy sources(RESs),microgrid technology has also been widely studied and discussed by various countries in recent years.However,due to the randomness and volatility of RESs,their increasing penetration into microgrids not only poses a challenge to the safe and stable operation but also deteriorates the dynamic performance of the system,resulting in rapid fluctuation and overshoot under disturbances.Therefore,based on the background of the high proportion of RESs,discussing stable and efficient control methods of microgrids has always been the focus of the attention of scholars at home and abroad.As a new control mode for microgrids,distributed control has the advantages of reliability,fast convergence rate,and high scalability compared with traditional centralized control.However,existing distributed control studies mostly focus on fast state recovery,delay compensation,uncertainty suppression,and fault tolerance while ignoring the problems of the cooperation of different DGs,the reduced stability caused by the integration of RESs,and the voltage demands of interconnection loads.Therefore,to improve the dynamic stability and realize efficient consumption of multiple types of resources,this paper forms an integrated distributed control scheme of microgrid clusters under isolated island operation mode.The main work of this paper is summarized as follows:(1)A state-observer-based standard modeling method is proposed.Since model heterogeneity of different control units is the main factor that hinders cooperative control,a standard modeling approach that combines state estimation and power coupling theory is proposed in this paper.This method unifies the control states of different models by state equivalence,simplifies the model order by adopting input-to-output feedback,and separates the nonlinear states by adopting state observation.The proposed modeling method can finally simplify grid-forming DGs and gridfollowing DGs into a standard second-order linear model and simplify the microgrids under centralized and distributed control framework into a standard first-order linear model.(2)A distributed control strategy for the stand-alone microgrid level considering low inertia operation characteristics is proposed.Since microgrid is the main operation level of low-inertia DGs,this dissertation regards improving dynamic stability as the main objective at this level.The proposed control strategy contains two components:distributed frequency control based on equivalent inertia enhancement and distributed voltage control based on dynamic performance customization.For the proposed distributed frequency control,this dissertation realizes the equivalent inertia compensation through the constraints of the rate of change of frequency(RoCoF),which effectively suppresses the frequency oscillation and overshoots under disturbances.For the designed distributed voltage control,an additional feedback loop is introduced to the above inertiaenhanced distributed control algorithm.Thus,the expected dynamic performance,such as overshoot suppression and fast recovery,can be obtained by adjusting the parameters according to the timedomain analysis of the entire control system.(3)A distributed frequency and voltage control method for the microgrid cluster level based on multiple loads pinning is proposed.Different from the control objective of a single microgrid,the main goals of the microgrid cluster are to achieve stable coordination among microgrids and to ensure a reliable power supply for critical buses.Therefore,to address the cooperation between static loads and microgrids,a containment-based distributed control method for the microgrid cluster is proposed in this dissertation.This method incorporates static loads into the distributed control framework by designing a multi-leader containment framework,allowing the static loads can participate in distributed cooperation.Then,to avoid the long waiting time and data asynchronization,an adaptive compensation mechanism based on differential delays is introduced for the containment-based distributed algorithm.This treatment efficiently improves the robustness of the entire distributed control system under heterogeneous and long-time communication delays.In summary,based on the limitations of the existing distributed control studies of microgrids,this dissertation further discusses the differentiated control requirements of different operation levels in microgrid clusters and forms an entire distributed frequency and voltage control scheme of microgrid clusters under island operation mode.The issues of heterogeneous unit coordination,dynamic performance degradation,and long-distance communication delay are addressed in the proposed distributed control scheme.The research in this dissertation can provide theoretical and technical support for the distributed control of the high proportion of RESs and has practical significance for further improving the consumption of RESs in the microgrid and the collaboration of different types of microgrids.