| In order to effectively relax energy crisis and reduce the environmental footprint,the distributed generation(DG)based on the renewable energy has been greatly promoted.The integration of DGs in the power grid has accelerated the necessity of energy management in a multi-generation source network to ensure a reliable and continuous power supply,which results in the emergence of microgrid(MG).An MG is defined as a coordinated group of DGs,storage systems and loads,which is connected to the main grid at the point of common coupling(PCC).The MG brings sufficient generation close to loads and maintains the power supply in the event of main grid faults.In addition,the MG allows DGs to present themselves to the main grid as controllable units that meet technical requirements for the connection to the main grid.Therefore,the MG is considered as an effective solution for flexible and reliable integration of renewable energy resources into the main grid.The MG operates in either islanded mode or grid-connected mode,and it is able to perform seamless transitions between these two modes.In the islanded mode of operation,the real and reactive power generated within the MG,including the temporary power transfer from/to storage units,should be in balance with the demand of local loads.However,the islanded MG is a lowinertia system because DGs are interfaced with the MG through converters.Moreover,the fast dynamic responses and the intermittency of DGs may possibly result in the instability and increased operation cost of the islanded MG.Therefore,how to maintain the power balance,stability of the voltage and frequency and economic operation of the islanded MG requires further research.Conventionally,the centralized control is implemented for the islanded MG,where the control decisions are performed based on global information.The centralized control method requires all DGs to communicate with an MG central controller(MGCC)first,and control actions are then broadcasted back to each unit,being highly dependent on this central controller.Once failure of the central controller occurs,the MG may fail,which decreases the reliability of the system.Moreover,the computational burden of the central controller in the centralized control method is high and data sharing is not easy due to its complexity and cost,when the number of DGs in an MG reaches a certain level.Alternatively,the distributed control with a spare communication network does not need a central controller and each unit is controlled by its local control system,which allows the control action to be simply made based on local information rather than global information.Thus,large quantities of information manipulated by the MGCC can be distributed among those local control systems that only need to communicate with their neighbors.Furthermore,the interactions among DGs are negligible and the requirement for the communication network among DGs is totally obviated in decentralized control.Therefore,the distributed and decentralized control methods are more practical for a large sized MG than centralized control,due to their flexibilities in plug and play,as well as sparse communication channels.In this thesis,based on multi-agent system(MAS)theory and droop control,a distributed and decentralized control framework for the islanded MG is proposed.The main research contents of this thesis are shown as follows:(1)MAS based distributed control model for the power balance of the islanded MG.In order to ensure the power balance and stability of voltage and frequency in the islanded MG,an agent-based distributed control model for the islanded MG is proposed.The control model consists of a two-layer control structure.The bottom layer is the electrical distribution MG,while the top layer is the communication network composed of agents.Moreover,a systematic method is presented,which can be used to derive a set of control laws for agents from any given communication network.Further,the information is processed according to the control laws and agents adjust the power outputs of DGs to which they connect.Finally,it has been seen that the output power supplied by DGs satisfies the load demand in the MG when agents use the proposed control laws,and the frequency and voltage fluctuations are small and meet the requirements.(2)MAS based distributed reactive power sharing control model for the islanded MG.The reactive power cannot be shared proportionally among DGs with the conventional droop control,due to the mismatch in feeder impedances,etc.Therefore,a distributed control model for reactive power sharing is established based on the two-layer control model.First,a theorem is demonstrated,which yields a systematic method to derive the control laws from a given communication network.Further,agents exchange the information acquired from DGs with neighbors,and calculate set points for DGs they connect to according to the control laws.Finally,three cases are carried out to test the performance of the control model,in which the uncertainty of intermittent DGs,variations in load demands,as well as impacts of time delays are considered.(3)Distributed economic dispatch strategy for the islanded MG considering the communication noise.The existing distributed economic dispatch methods assume noisefree communication links among DGs in the MG.As the communication noise is non-negligible,a distributed noise resilient economic dispatch strategy for the islanded MG is proposed.Firstly,a novel consensus-based economic dispatch strategy is proposed.The strategy is fully distributed such that the optimal operation of the MG can be implemented in a distributed manner.Further,a post-iterate averaging step is employed to smooth out the noise effect and reduces the variance of the noise.Finally,the simulation results are presented to demonstrate the effectiveness of the proposed control method.(4)Cost-based droop schemes for economic dispatch in the islanded microgrid.The malfunction of the communication links may possibly result in invalid distributed control method.Therefore,in this part,an incremental cost based droop scheme is proposed,to minimize the total active power generation cost in an islanded MG,while the simplicity and decentralized nature of the droop control are retained.First,the incremental cost of DG is embedded into the droop scheme.Further,in the steady state,DGs share a single common frequency and the proposed droop scheme equates incremental costs of DGs,thus minimizing the total active power generation cost,in terms of the equal incremental cost principle.Finally,simulation results are presented to demonstrate the effectiveness,as well as plug and play capability of the proposed droop scheme. |
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