| As an effective way to absorb renewable energy,the DC microgrid integrates distributed energy such as photovoltaics and wind power,energy storage devices and loads through a simple topology.At the same time,it overcomes the problems of AC system frequency transients and grid harmonics,which possesses broad development prospects.However,the kinetic energy on the distributed power source side or the energy stored in other forms cannot participate in voltage regulation because it is isolated by power electronic converters.So,the DC microgrid presents the characteristics of small inertia and weak damping.In order to make reasonable and effective use of converters to provide inertial support for DC system,and to prevent excessive inertia from causing slow response time or over-limiting of output power,it is necessary to study the coordinated control between the converters at each end participating in inertial control and its voltage adjustment effect.This thesis studies the virtual capacitance control of DC microgrid considering multiple constraints and its multi-terminal coordination method.The aim is to improve the ability of DC microgrid to suppress DC bus voltage fluctuations and enhance power quality under power disturbances.At the same time,the purpose is to provide reasonable inertial support,which improves the stability of the DC microgrid system.The main tasks completed are as follows:(1)On the basis of studying the topology of a typical DC microgrid,the mathematical model of the DC/DC converter on the side of the energy storage unit is established and its control principle is briefly described.For the multiple energy storage units in the DC microgrid,its basic control strategy is studied.Besides,the primary/secondary/tertiary voltage regulation and inertial control are analyzed from different time scales.And a voltage regulation system is established on different time scales,which lay the foundation for subsequent research on virtual capacitor cooperative control technology.(2)In order to ameliorate the extreme operating state of the equipment under single-ended virtual capacitance control,a virtual capacitance control method considering multiple constraints is proposed.Based on the analysis of flexible virtual capacitance control,some influencing factors such as the battery state of charge,DC voltage change rate,instantaneous output power and output power per unit time of the converter,etc.are considered comprehensively.The indexes are constrained and the value range of the virtual capacitance is discussed.(3)In order to realize the cooperative distribution under the control of multi-terminal virtual capacitance,and to ustilize converters at each end reasonably and effectively,a virtual capacitance cooperative control strategy based on gray correlation analysis is proposed.The magnitute of adjustable virtual capacitance of the multi-energy storage units is analyzed.The distribution coefficient of each end converter is obtained applying the gray correlation analysis,and its weight is gained with the variation coefficient method.That provides effective inertial support for the DC microgrid.(4)The small signal analysis is used to reveal the influence law of the main control parameters in the proposed control on the system stability,which provides a basis for the value of key parameters.By building a DC microgrid hardware-in-the-loop simulation test platform,the effectiveness of the proposed control methods and the validity of stability analysis are verified. |
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