Research On Hierarchical Cooperative Control Strategy For Isanded Microgrids

It is seen as an effective solution to manage distributed generation(DG)via microgrids,whose ability that can operate in the islanded mode is one of the key characteristics to improve the reliability of power supply and make DGs more efficient.However,the islanded microgrid,due to the lack of support of main grid,has to guarantee its stable operation and power quality by itself,which leads to more complicated situation.Therefore,cooperative control and stability analysis of islanded microgrids are the foundation and key points in order to make microgrid popularizing.In this paper,the hierarchical cooperative control strategy is investigated and the following researches are carried out:Firstly,the control strategy of inverters with synchronous generator characteristic and its impacts on microgrids are investigated in terms of the problems of low inertia and damping of power electronics interfaces.The mathematics models of different control levels for virtual synchronous generator(VSG)are introduced detailed.Various damping methods are compared to show that a simple VSG without damping factor is enough to keep stable operation of microgrid without causing oscillation due to the phase-locking loop(PLL).As VSG can highly slow down the reaction speed of DG,the eigenvalues-based method is used to analyse how VSG can influence the dynamics of microgrids.It is shown that the virtual inertia will make the DGs with quicker dynamics such as droop control more oscillatory,and however,the droop control can hardly influence VSG.An experimental platform is built to verify the dynamics of the system.Secondly,this paper built the small-signal of islanded microgrid taking the communication delay of secondary control into consideration and investigated the impact of communication delay on stability of the system using delay margin.The results showed that both quantity and distribution of virtual inertia were significant factors influencing the delay margin,where proper increasing of virtual inertia and decreasing difference among DGs would improve stable stability of the system.Two centralized secondary frequency control strategies based on Smith predictor and communication disturbance observer(CDOB)were proposed,respectively.Stability analysis was also researched.Theoretical and simulation results in PSCAD/EMTDC showed that Smith predictor could compensate impact of communication delay when the predictive model was accurate enough and otherwise,the performance would deteriorate extremely.Especially,when the predictive value was too high,the delay margin would decrease.In comparison,the CDOB based method could adapt itself to change of communication delay and had better robustness as it was not dependent on the accuracy of the predictive model.The effectiveness of the proposed methods is verified taking use of the experimental platformThirdly,in terms of distributed control of islanded microgrid,a hybrid centralized-distributed voltage control strategy based on synchronous event-triggering was proposed.Then an asynchronous event-triggered control strategy was proposed to achieve full distributed architecture and then spread to frequency control.Compared to the traditional periodical sampling control,the proposed method was based on aperiodical sampling which deceased useless signals to improve economy and decreased the dependency of secondary control on communication system.Lyapunov theory was used to deal with stability analysis of the proposed control strategy.Proof of no causing Zeno phenomenon was also carried on.Effectiveness of the proposed control strategy is verified in PSCAD/EMTDC.Finally,a distributed control strategy for voltage unbalance compensation is presented in this paper taking use of the ancillary service of DGs.The voltage unbalance compensation for critical bus was investigated in the secondary control level.And then the problem could be seen as a tracking-synchronizing problem of multi-agent system(MAS).The control target could be achieved by cooperative regulation of negative-sequence components of DGs.Effectiveness of the proposed control strategy is verified by experimental platform.