Coordination Control Strategy Of DC Microgrid With Two DC Buses Based On Power Pool

As DC microgrid has the advantages of simple control strategy and no need to consider reactive power,frequency and phase synchronization,DC microgrid has been recognized by domestic and foreign experts.At present,most of the researches on DC microgrid are focused on the single bus structure in which there exists only one voltage level.As a result,DC microsources and DC loads of other voltage levels should be connected to DC bus through DC/DC converters,which increases the loss of energy conversion and reduces the efficiency of the whole system.To deal with this problem,a DC microgrid with two DC buses based on power pool is designed in this thesis.Its coordination control strategy is also proposed to ensure the normal operation of the DC microgrid.The specific contents of this thesis are as follows:Firstly,this thesis introduces the development of DC microgrid at home and abroad.The concept,structure and characteristics of DC microgrid are introduced in detail.Then,this thesis gives a summary of the present coordination control technology of DC microgrid.The working princ iples,control characteristics and application occasions of centralized control,distributed control,master-slave control and peer-to-peer control are introduced respectively.Secondly,the basic components of DC microgrid are introduced in detail.The simulation model of photovoltaic cell is established,based on which,the output characteristic curves of PV cell are analyzed and the control strategy of photovoltaic power unit is put forward.The proposed control strategy is verified by simulations then.Taking lithium battery and super capacitor as examples,this thesis introduces the energy storage unit of DC microgrid.The working princ iples,energy storage characteristics and equivalent circuit models of lithium battery and super capacitor are analyzed.Then,the energy storage characteristics of lithium battery and super capacitor are compared.Three kinds of DC/DC converters commonly used in DC microgrid are introduced: Boost converter,Buck converter and DC/DC bi-directional converter.Their structures,working principles and voltage quantity relations are analyzed in detail.Thirdly,a DC microgrid with two DC buses based on power pool is designed in this thesis.Its coordination control strategy is also proposed.The DC microgrid is composed of two sub-microgrids with different voltage levels and a power pool in connection.A lithium battery pack is set as energy backup of power pool.They constitute an energy storage system for bus voltage regulation in cascade structure.Power exchange mechanisms between the two DC sub-microgrids and the power pool,the battery pack and the power pool are designed.The power exchange mechanisms can stabilize DC bus voltages and realize the two sub-microgrids spare to each other.Low frequency power exchanges between the two sub-microgrids and battery pack are thus realized.In this way,mutual influences of high frequency power fluctuations between the two sub-microgrids are eliminated and service life of the battery pack is prolonged.Regulation mechanism of state of charge of the battery pack is designed.It is regulated by the sub-microgrids when bus voltages are stable,thus improving ability of DC microgrid to operate continuously.The effectiveness of the proposed control strategy is verified by simulations.Finally,a set of DC microgrid experimental platform based on DSP TMS320F28335 is developed.The selection methods of the key components in main circuit are introduced.The voltage sampling circuit,current sampling circuit and voltage current signal conditioning circuit are designed.Then,the thesis introduces the schematic diagram,internal wiring,phys ical appearance,man-machine interface of the experimental platform.Bas ing on the experimental platform,the thes is develops coordination control programs of the proposed DC microgrid.The effectiveness and feasibility of the proposed coordination control strategy are verified by experiments.