Research On Impedance Matching And Hybrid Property Of Microgrid At Autonomous Mode

As a self-control, self-management and self-protection power generation anddistribution system, Microgrid is a cluster of interconnected distributed generationsystems, energy storage systems, power conversion devices, loads, monitoring andprotective equipments, which can operate on grid-connected or autonomous mode.Based on the impedance matching and hybrid system theory, the Microgridoperating on autonomous mode is studied in the dissertation, and theoretical studyand practical experiment of modeling to inverters, operating property analysis,restraining to adverse effect of constant power loads, stability analysis of paralleledinverters, hybrid property of Microgrid, power management and its optimal controlare carried out to resolve the unstability caused by mis-matching between thedistributed generation units and loads, and multi-target optimization of the powermanagement strategies. The research results show certain theoretical significanceand practical value for the interpretation of Microgrid structure, normalized controlsystem design and the performance optimization of power management strategy ondifferent operating modes.The topologies and control methods of photovoltaic power generation unitsand energy storage unit are analyzed and revealed in details. With their small-signalmodels, the control strategies are designed. For achieving better dynamicperformance of PV units, input power feed-forward control method is proposed, andthe output power messages of PV arraies are involved into inner current-loop toimprove response speed of output current when the input power is fluctuating. Forbattery energy storage unit, based on relative analysis of different charge methods, aconstant-voltage current-limiting control method is designed to decrease the surgeof current or voltage in traditional methods by regulating the limitation of outervoltage-loop output. The above mentioned property analysises lay a foundation forstability analyzed by impedance matching condition and optimization of powermanagement strategy.Based on the impedance matching theory, the output impedance model of PVunit is set up, filter parameters impact to the output impedance and system stabilityare certified by time-domain anslysis approach, the rationality and feasibility of theimpedance specification approach are clarified theoratically. The negativeincremental impedance property and degeneration of system stability of ACconstant power load (CPL) is studied by impedance specification approach. Avirtual inductor control method, which adopts an inductor current feedback closedloop, is proposed in order to damp the inverter equivalent output impedance and improve system stability. The system dynamic response is enhanced and harmoniccomponent at switching frequency is effectively suppressed. An admittancespecification is proposed to simplify the design process of Microgrid with numbersof distribution generations (DGs) operating in parallel. And the stable quantizedrelation between inverter output and load input admittance is indicated.By analyzing the hybrid property of Microgrid detailedly, a mixed logicaldynamical (MLD) model is set up with Hybrid theory. Choosing the energy in DCbus capacitor as state variable, the system function and constraint condition withcontinuous state variable, input control variable, logical variable and auxiliaryvariable are presented. Based on the MLD model, optimum control and modelpredictive control (MPC) theory, the quadric form objective function is designedand the optimal input control variable sequence is achieved by mixed integerquadratic programming theory and Branch&Bound method to reduce the batteriesswitching times between charge and discharge. Proposed method can improve theeconomy and reliability of Microgrid, and reduce the the batteries switching timesbetween charge and discharge when distributed generation output and loadsfluctuate especially the difference is small.Based on the MPC research results of Microgrid and aimed to reduce thebatteries switching times between charge and discharge, a combination controlstrategy is designed to improve the system reliability and redundancy. Two PV unitsare selected as master units by Droop control and provide voltage and frequencyreference, energy storage unit with PQ control acting as a slave unit to ensurepower balance for the whole system. Based on the random characteristics of PVgeneration and loads, cooperative control strategy of energy storage unit and PVunits is proposed. The DC bus voltage of PV units is considered for identificationsof energy storage unit switching function, and the system power balance is ensuredby adjusting the charge current reference of batteries and output current referenceof the inverter. With proposed method, both the stable of DC bus voltage and fastresponse to PV generation and loads fluctuation can be achieved. Finally, the digitalsimulations based on Matlab/Simulink and experiment platform are carried out, andconsequently the feasibility and validity of the proposed basic theory and keytechnique are verified.