Distributed Cooperative Control In Microgrids With Consideration Of Asynchronous Information Interaction

Under the background of increasingly severe issue regarding energy and environment,the development and utilization of renewable energy resources have undoubtedly provided a novel solution for the energy crisis.Nevertheless,the large-scale access of renewable energy resources could bring about a significant impact on the power grid.Confronted with this challenge,the concept of microgrid emerges.As a small autonomous generation system,a unique superiority of microgrid is that it is capable of islanded operation,whereas in this case,the design of the associated control system is more complex.Hierarchical control is the most widely adopted control method for microgrids,where the primary control generally employs droop control to maintain system stability,on this basis,secondary control is introduced to compensate for the defects of primary control.Among the existing secondary control structures,distributed architecture has attracted the most interest due to its high reliability and control accuracy.However,in view of its characteristic that all centralized devices are excluded,each unit will execute sampling and communication actions independently according to their local clocks.Further considering that the local clocks of heterogeneous and geographically dispersed distributed generations(DGs)would differ from each other inevitably,the secondary control instants decided by them would also be misaligned.Therefore,the process of information interaction between DGs is asynchronous in practice,thus,the neighboring information used for control decision is likely to be outdated,which would in turn affect the accuracy of secondary control,or even increase the risk of system instability.In terms of this problem,this paper intends to investigate the approach to improving system performance in the case of asynchronous information exchange caused by diverse clocks,the main contents involved are listed as follows:(1).The influencing mechanism of asynchronous information interaction on system performance is studied.The variation law of offsets between control instants of different DGs is analyzed.Thus,the discrete small-signal model of the whole system is established via integrating the individual modules with diverse sampling rates.On this basis,the calculation method for the stability region with respect to each independent control period is proposed.Then,in terms of the asynchronous cases where the stability condition is satisfied,the asynchronous consensus protocol is established based on the probability distribution of the offsets of control instants,and the expression of convergency deviation is further derived,with its influencing factors discussed.Ultimately,the validity of the proposed theoretical analysis methods is verified by simulation.(2).Two kinds of clock synchronization strategies for secondary control in microgrids are designed based on pinning-based consensus algorithm and maximum consensus algorithm respectively.The model of local clock is established first,and the adjusted time reading is acquired via introducing relevant correction coefficients.Then,the pinning clock is selected optimally and the event-triggered mechanism is designed for communication.Further combing with the idea of pinning-based consensus,the distributed clock synchronization method for secondary control is proposed.On this basis,the selection principle for algorithm parameters is discussed.In addition,the impact of stochastic transmission delay on synchronization performance is further taken into account,with corresponding compensation scheme designed,and another clock synchronization strategy is raised employing the maximum consensus protocol.Meanwhile,the event-triggered mechanism is designed to reduce the update frequency.Then,the convergence issue of the proposed algorithm is discussed to guide the selection of triggering parameters.Eventually,simulation test is conducted to demonstrate that the distributed secondary control performance in asynchronous cases can be improved with excellent economical efficiency under both clock synchronization strategies proposed above,and the second scheme enjoys better robustness to stochastic delay.(3).An innovative distributed secondary control scheme is proposed based on the fast convergence algorithm.The estimation of average state is implemented using the fast convergence algorithm,and the loop removal strategy is introduced to expand the application scope of the algorithm.Then,based on the consideration of transmission accuracy of all data involved during communication,the event-triggered communication mechanism is designed.On this basis,the distributed event-triggered secondary control strategy using fast convergence algorithm is proposed,and the sufficient condition to guarantee the algorithm convergence is further investigated for the design of triggering threshold.Ultimately,the comparative simulation cases are designed to show the superiority of the proposed control strategy over conventional consensus-based control method in terms of convergence performance and asynchronous robustness,and confirm that the proposed event-triggered mechanism can effectively reduce the communication cost in the premise of ensuring required control precision compared with conventional time-driven mechanism.