Dynamic Characteristic Analysis And Stabilization Control Of Microgrids

As an important way to realize the smart grid,microgrid is an effective solution to solve the large-scale utilization of renewable energy power generation,improve the reliability of power supply and meet the flexible requirements of users at the low and medium voltage level.Most of the renewable energy power generations are connected to the microgrids by means of power electronic converters.Power-electronic-converter(PEC)based distributed generations(DGs)differ significantly from the conventional synchronous generators in power conversion,control schemes and dynamic characteristics.The diversity of DG control methods and the increasing penetration of PEC DGs are leading to serious challenges in coordinated control and stable operation of low-inertia microgrids.Moreover,the coexistence of heterogeneous microsources and loads within the microgrid may result in interactions among the DGs and loads.The coupling among the devices with different characteristics reshape the dynamic responses of the microgrids and induce stability issues.The traditional stability analysis and control method based on synchronous generator cannot meet the requirements of large-scale development of microgrids.Combined with the status quo and development trends of power electronic domination,decentralization and interconnection in future and power systems,the clue of this thesis can be concluded as follows: from the the coexistence of PEC DGs and traditional power generation unit to high penetration of PEC DGs,from the combination of centralized and decentralized control to decentralized control,and from single microgrid to multi-microgrids.Aimming at the existing problems and challenges,this thesis deeply studies the dynamic characteristics and stabilization control of microgrids through the means of mathematical modeling,theoretical analysis,simulation and experimental verification,thus to slove a series of basic theory and key technical problems for the microgrid in dynamic modeling,operating characteristics,interaction mechanism and control strategy design.The results of this thesis provide theoretical basis and useful reference for the system design,analysis and operation of microgrids.The main contents and innovations of the paper are summarized as follows:(1)This thesis summarizes the new features such as power flow control methods for microsources,and stability issues of the microgrids with continuous increasing penetrations of renewable energy resources for the first time.Based on the classification of microgrid stability issues,state of the art in microgrid stability studies are reviewed,evaluated and discussed comprehensively from the viewpoints of small disturbance stability,transient stability and stability improvement methods.Detailed review from the system point of view clarifies the research context of microgrid stability and control in recent years for the first time.(2)Based on the significant demand of practical engineering,the mixed-source microgrids in which PEC DGs and traditional power generation unit coexist,is selected as the typical object of study in this thesis.Firstly,delay-dependent dynamic modelling method is proposed for the first time to establish the model of autonomous medium voltage mixed-source microgrid,which includes grid-supporting controlled inverter-interfaced DG units,induction generator wind turbines,diesel generator set,loads and network.Based on the rigorous mathematical modeling,the interaction issue among the microsources caused by the coexistence of heterogeneous DGs is investigated for the first time by analyzing the participation factor and sensitivity,and the dynamic stability mechanism of mixed-source microgrid in Dongao Island is analyzed.Moreover,the delay-dependent stability of the mixed-source microgrid is analyzed by using the Chebyshev discretization method for the first time.Finally,the simulation and experimental results show that the dynamic model can predict the dynamic characteristics of the microgrid system successfully.The proposed mixed-source microgrid shows robust performance against under various disturbance scenarios such as communication delay,gust wind speed and grid connection of wind turbine.(3)Aiming at the shortcomings of the existing microgrid control strategy which cannot guarantee the stability under large disturbance,and also with poor applicability,to ensure power balance and frequency/voltage stability in multiple time scales for mixed-source microgrid,this thesis introduces a hierarchical stability control architecture with the capability to operate stably and reliably at different conditions.The proposed control architecture divides the system frequency and voltage in three zones:(A)stable zone,(B)precautionary zone and(C)emergency zone.In this way,dynamic stability control in short-time scale is implemented by microgrid central controller(MGCC)within Zone B and Zone C.Meanwhile,steady-state stability control in long-time scale is executed by microgrid energy management system(MEMS)within Zone A.Furthermore,based on the developed state-space model of the microgrid,the dynamic stability margin of mixed-source microgrid system is analyzed.The critical control parameters of the hierarchical stability control strategy are designed in a way that both system stability margin and dynamic performance constraints are met,thus to provide the basis for the optimal setting of the global and local control strategies of the mixed-source microgrid.Comprehensive simulation and experimental results shows that the coordination between dynamic stability control based on the millisecond level and steady-state stability control based on the second level is able to realize the stable and optimal operation of the mixedsource microgrid.(4)Based on the study of the power voltage characteristics of the double fed Induction Generator(DFIG)in the microgrid,to improve the steady-state voltage stability of microgrid with DFIG,steady-state voltage stability enhancement control strategy is proposed considering the constraint of microgrid operation modes.Secondly,the transient operating characteristics of DFIG and dynamic loads in isolated medium-voltage(MV)microgrid during voltage sag are studied,thus to reveal the mechanism of transient voltage stability of mixed-source microgrid.On this basis,to ensure the transient voltage stability of mixed-source microgrid,a multivariable coordinated transient voltage stability cooperative control strategies based on energy storage system(ESS),fast pitch angle control of variable-speed wind turbine and dynamic load shedding in local control layer are proposed.The simulation results are presented to reveal the influence factor of transient voltage stability of mixed-source microgrid,and verify the feasibility and effectiveness of proposed voltage stability control strategy.(5)Aimming at the PEC DGs dominated multi-inverter microgrid,comprehensive modeling,analysis design and transient stability control of low inertia multi-inverter microgrid are studied in detail,considering composite loads as dynamic element.The global dynamic model of multi-inverter microgrid with dynamic load is deduced and the contribution of induction motor load to the oscillation behavior of multi-inverter microgrid system is evaluated and analyzed for the first time.It was found that using only static representation of loads in microgrid stability studies may lead to misleading analytical results.Furthermore,in order to improve the transient stability of multi-inverter microgrid considering the start-up constraint of induction motor,transient virtual impedance strategy is introduced in the decentralized control architecture inverter to optimize the motor starting process for the first time.Finally,detailed simulations and the experiment based on the research platform of multi-inverter microgrid are conducted to verify the validity of theoretical analysis results of multi-inverter microgrids with dynamic load.The proposed transient virtual impedance strategy effectively avoids the instability issue of conventional current limiting strategy.(6)Based on the analysis of the dynamic stability mechanism of single microgrid,the dynamic characteristics and wide-area stability of PV-based multi-microgrids are investigated.Dynamic model for PV-based multi-microgrids considering local adaptive dynamic droop control mechanism of the voltage-source PV system is developed.And then the dynamic stability characteristics of the single microgrid and interconnected microgrids are assessed and compared for the first time.It is revealed that interconnecting neighboring PV microgrids yields ultra-low frequency interarea oscillations between sub-microgrids and mid-low frequency local oscillations behaviors within sub-microgrids.The interactions between PV microgrids significantly reduces system stability margin and leads to system instability.In order to enhance system wide-area stability,a tie-line flow and stabilization strategy is proposed to suppress the introduced interarea and local oscillatory modes for the first time.Moreover,the problem of robustly choosing the key control parameters is then transformed to an eigenvalue-based multiobjective optimization problem.Finally,rigorous theoretical analysis,simulation results under various scenarios have been presented to verify the finding of interactions and instability phenomena in PV multi-microgrids.The proposed stabilization strategy can effectively damp the power oscillations and provide robust control performance.