Research On Fixed-Frequency Control Of Islanded Microgrid

Microgrids are active power distribution systems aggregating distributed power sources and various types of loads at the distribution voltage level,which are capable of self-control and management,and therefore can largely improve the reliability and flexibility of the integration of renewable energy sources such as wind and solar energy.Compared to masterslave control,droop control has received widespread attention from both domestic and abroad scholars since it greatly reduces the dependence of microgrid operations on high-bandwidth communication networks.The miniaturization and widespread development of the global positioning system(GPS)synchronization devices have made it possible to realize fixed frequency microgrid control and consequently completely solve the microgrid frequency problems.In this paper,the frequency stability and fixed frequency control strategy of isolated microgrids are studied.The problems of synchronization,power sharing and harmonics attenuation to achieve fixed frequency operation of microgrids are investigated based on the establishment and analysis of detailed state-space models of microgrids.The main contents are as follows1.Modeling and stability analysis of a droop-based islanded microgrid.A detailed statespace model of a droop-based islanded microgrid is developed,which includes modeling of the inner-loop controllers and filters of the inverter,as well as detailed modeling of line impedance and load.The root locus of the dominant eigenvalues of the overall system are obtained with respect to different control parameters,in order to analyze the influence of different droop coefficients on the dynamic characteristics of the system and facilitate the selection of control parameters.Finally,the differences between the state-space model and the physical simulation model under step load are compared,and the accuracy of the proposed state-space model is further verified by observing and analyzing the relationship between the eigenvalues and timedomain response of the dynamic model with different parameters.2.Control and synchronization of the fixed-frequency operation of microgrid based on the GPS system.The assumptions and derivations of GPS-based fixed-frequency angle droop control and traditional frequency droop control are analyzed and compared.Unlike conventional frequency droop control which adjusts the inverter output voltage frequency according to the output power to achieve load sharing and inverter synchronization,angle droop control directly manipulates the angle of the output voltage according to the output power to achieve the same goal,and therefore has faster dynamic characteristics.An auxiliary frequency droop loop is proposed to ensure synchronization during GPS offline events.The viability of the proposed control strategy is validated by simulation case studies.3.A robust fixed-frequency control scheme for microgrid under complex load situation.A novel robust voltage control algorithm with angle droop scheme is proposed.The voltage controller design is specifically formulated on rotating reference frames in order to minimize the adverse impact of harmonics and negative-sequence disturbance caused by nonlinear and unbalanced loads.The adverse effects caused by parameter variation,model uncertainty and unmodelled dynamics are evaluated by transfer functions and considered as a constraint in the optimization problem to improve the robustness of the overall system.The controller optimization is then translated to form a set of linear matrix inequality(LMI)conditions which can be conveniently solved with LMI theories.Simulation results are presented to verify the efficacy of the proposed control algorithm.Hardware-in-the-Loop(HIL)experiments are conducted to further test and analyze the performance of the proposed controller in practical application situations.4.Analysis and implementation of virtual impedance for fixed-frequency control strategy in microgrid.The similarities and the correlations between angle droop control and V-I control are studied and it is revealed that they can both be regarded as different forms of virtual impedance control.A novel adaptive virtual impedance design method is proposed accordingly,which considers both stability constraints and power quality requirements based on the smallsignal model of GPS-based microgrids.An adaptive transient resistance concept is adopted to enhance the system stability during large disturbances and grid faults.Case studies are presented to validate the system performance and fault ride-through abilities of the proposed control scheme.5.Improvement of the power sharing performance among DG units based on the consensus protocol.A consensus-based virtual impedance control approach is proposed.The virtual resistance concept is implemented comprising a basic local implementation for output impedance shaping,and a sparse resistance tuning network for the compensation of mismatched feeder impedance which demands no knowledge of the actual output impedance.The resistance tuning network only requires neighboring interactions among DG units.A complete tuning of output resistance for a given load condition results in accurate active and reactive power sharing even after communication is interrupted and will still outperform conventional droop control methods if load changes during the interruption.Small-signal analysis based on delay differential equations(DDE)model of the overall microgrid is performed to investigate the adverse impact of communication delays on system stability.The efficacy of the proposed approach is validated by both simulation and experimentation.