Research On Coordinated Control Strategy Of Converters In DC Microgrid

With the increasing penetration of clean energy and the increasing proportion of DC loads in consumer terminals,as a distributed generation system,the DC microgrid,integrating a variety of renewable energy sources,energy storage units,the consumer load terminals and traditional power systems through power electronic conversion interfaces.Global voltage stability and proportional load distribution are two control objectives of DC microgrid coordinated control.Through the coordinated operation of multiple energy storage units,power fluctuations caused by renewable energy because of the environmental factors in the microgrid can be balanced to ensure reasonable load distribution while maintaining the stability of the DC bus voltage.Therefore,in this thesis,a bidirectional DC/DC converter is used as the power electronic interface for the energy storage unit to connect to the DC microgrid,and the in-depth research and analysis are carried out with the control method of the converter and the related problems of current sharing and bus voltage recovery in a multi-source parallel scenario.The energy storage equipment in the DC microgrid is connected to PCC via a bidirectional DC/DC converter to improve the output quality of the energy storage unit.Based on analyzing and researching the topological structure of the converter,establishes a unit model and applies current and voltage closed-loop control of the converter,which can realize the real-time tracking of the output power to the load change.The proportional load distribution in DC microgrid to ensure the reasonable output of each unit and prevent the excessive output of any source.In order to solve the problem of multi-source coordination in the DC microgrid distributed generation scenario,and to overcome the poor current sharing characteristics caused by the different line impedance between the energy storage unit and the PCC,an adaptive droop mechanism been proposed,which adapting the droop coefficient for different load conditions.Based on sparse communication network,the current regulator compares the local per-unit current of each converter with the neighbors' on a communication graph and,accordingly,provides an impedance correction term.This term is then used to update the droop coefficient and synchronize per-unit currents.Equivalently,each participating converter has equal output impedance,so that the accurate proportional load sharing can be achieved and the current sharing performance in steady and transient states can be enhanced.In order to ensure the proper functioning of the load connected to the DC bus,it is necessary to maintain the global voltage stability.For the purpose of eliminating the voltage droop caused by the droop mechanism,designing a bus voltage recover control strategy.The voltage regulator uses an observer that processes neighbors' data to estimate the average voltage across the microgrid.This estimation is further used to generate a voltage correction term to adjust the local voltage set point.The proposed upper-layer coordinated control algorithm fully considers the line impedance,and the controller on each converter exchanges data with only its neighbor converters on a sparse communication graph spanned across the microgrid.Finally,global dynamic model of the microgrid is derived with the proposed controller,and Simulink simulation is established to verify the effectiveness of the control algorithm,the link-failure resiliency and the plug-and-play capability of the system.