Distributed Voltage Control Of Islanded Microgrids In The Peer-to-peer Mode

With the propulsion of techniques of microgrid(MG)and smart grid,the islanded microgrid as an effective integration of distributed generations(DGs)not only enables the uninterrupted power supplies for critical loads in case of the failure of main grid,but can also work as the black start power.In contrast to the grid-connected microgrids,islanded microgrids lose the frequency and voltage support of main grid with the small equivalent inertia and are susceptible to the disturbances of intermittent DGs.How to maintain the proper power sharing and stability of voltage and frequency has become the focus of current researches.The peer-to-peer control mode enables the power sharing and the regulation of voltage and frequency among DGs without communication links,which has received extensive attentions in the field of microgrid control.As opposed to the frequency droop capable of accurate active power sharing,voltage droop typically demonstrates a poor reactive power sharing performance or even a power reversal due to the inconformity of output impendences of individual DGs.Therefore,it is necessary to design a reliable,effective and cooperative voltage control scheme for islanded microgrids,to improve the MG stability and renewable energy utilization.Based on the disadvantages of centralized and decentralized control structures,this paper focuses on the issue of reactive power sharing and voltage regulation in islanded microgrds by utilizing the peer-to-peer and distributed control framework.Furthermore,the optimization of distributed control structure and distributed controller gains as well as the distributed model predictive control-based consensus algorithm are studied from the perspectives of the cooperativity mechanism,strategy formulation,implementation and simulation.Research achievements of this study can lay a theoretical foundation and technical guidance for the distributed cooperative voltage control in islanded microgrids.The main contents of this thesis include:(1)Control objective and control structure of the secondary voltage control in islanded microgrids: Referring to the contradiction between the reactive power sharing and voltage regulation,the necessity of information exchange among DGs for secondary voltage control is illustrated.According to the characteristics of centralized,decentralized and distributed manners,the peer-to-peer and distributed control framework is deeply studied for the cooperative secondary voltage control in islanded microgrids,where each DG exchanges information with its neighbors to realize the global information sharing.(2)Optimal design for the control structure of distributed cooperative voltage control: The distributed cooperative voltage control strategy in islanded microgrids is derived,which reveals that the distributed control structure plays an important role on the system performance.By judging the reachability and heterogeneity of communication topologies,the connected and heterogeneous network range is acquired;then the Pareto optimal networks can be obtained based on the multi-objective optimization model with the consideration of the convergence performance,robustness to time-delays and link number;eventually,the explicit optimal topology is determined from the offline optimal network set according to the practical network requirement.(3)Optimal design for the controller gains of distributed cooperative voltage control: Under the premise of specific distributed control structure,the optimization of distributed cooperative voltage controller gains is investigated.Firstly,the optimal design of distributed voltage controller is transformed to that of decentralized controller,then,the controller parameters are designed based on the principle of linear quadratic regulator to improve the system dynamics;considering that communication delays are inevitable during the data transmission,the delay-relevant robustness can be taken as an additional index for the controller design besides dynamic performance,that is,PI gains can be selected to arrive at a larger delay margin with comparable or even a relatively small degradation of dynamic performance.The optimal design for distributed secondary voltage control scheme consists of the optimal distributed communication topology and optimal controller parameters.(4)Distributed model predictive control-based cooperative voltage control scheme: For the conventional distributed cooperative voltage control which is based on the current information interaction and dynamic feedback,there may exist over-control,under-control or mis-control during the operation procedure.By incorporating the predictive mechanisms into distributed generations,the future operational status of MG can be predicted based on the current and historical data of the local and neighboring DGs,and then the optimal decision is obtained.Firstly,an auxiliary prediction term is introduced to the distributed secondary voltage control and the distributed model predictive control(DMPC)strategy is proposed.By adjusting the prediction coefficient,the convergence of MG voltages to the reference value is drastically accelerated and the robustness against information update interval is improved.Additionally,as the non-linear dynamic characteristics of inverters are not considered in the auxiliary prediction term-based DMPC,an advanced voltage control algorithm based on input-output feedback linearization is proposed,through which the non-linear dynamics can be partially linearized and then the linear DMPC is utilized.Therefore,the dynamic performance of secondary voltage control and the robustness to time delays in islanded MGs are improved.