Multi-source Coordinated Control Of DC Microgrid Connected To Photovoltaics

With the depletion of non renewable energy and the deterioration of living environment,distributed power has become an important way to optimize the energy structure and improve the ecological environment in the world.DC microgrid has become a research hotspot of contemporary scholars because of its local consumption of distributed energy,simple structure and easy control.In addition to distributed power,energy storage system is also an important part of DC microgrid.How to control these micro sources reasonably and efficiently is the key problem to be solved in DC microgrid.In this paper,the independent DC microgrid composed of multi PV and multi battery is taken as the research object,and the independent device level control and overall system level coordinated control are mainly studied.Firstly,this paper studies the device level control method of each micro source in DC microgrid.Firstly,the mathematical model of the battery is established,and the Bidirectional DC-DC charge and discharge control circuit of the battery is analyzed.The main role of battery in microgrid is to absorb excess power when there is surplus photovoltaic power,and make up for the deficiency power when there is insufficient photovoltaic power.Droop control does not rely on communication equipment,but only relies on local information to distribute current reasonably.Therefore,droop control is generally used for batteries in DC microgrid.Through the analysis of the traditional droop control principle,it is found that the constant droop coefficient of the traditional droop control will not only cause DC bus voltage offset,but also lead to overcharge and overdischarge of the battery.Therefore,the secondary voltage compensation is added to droop control to solve the problem of DC bus voltage offset;the power function of battery state of charge(SOC)is introduced to droop coefficient,so that the batteries with smaller SOC can discharge less and charge more,and the batteries with larger SOC can discharge more and charge less until SOC is balanced;the secondary voltage compensation is added to solve the problem of DC bus voltage offset Bus voltage offset problem.Then the mathematical model of photovoltaic is established and its output characteristics are analyzed.Due to the uncertainty of photovoltaic output power,the maximum power tracking control(MPPT)is generally used.This paper analyzes the shortcomings of the traditional MPPT method,and proposes a MPPT method based on small window rolling.This method can not only track the maximum power point faster,but also suppress unnecessary fluctuations in a stable environment.However,with the increasing number of photovoltaic connected to DC microgrid,photovoltaic needs to take more responsibility,that is,to maintain bus voltage stability.Therefore,a photovoltaic CPG control scheme is proposed in this paper.By limiting the photovoltaic output power,the DC microgrid can keep the bus voltage constant even in the case of energy storage failure.Finally,the overall system level coordinated control is studied.According to the diverse control methods of each micro source,the DC microgrid is segmented into six different working modes.The working mode of the DC microgrid is determined by collecting the power balance state and battery state of charge in the system.On the premise of stable operation of DC microgrid,the full utilization of photovoltaic system and reasonable scheduling of energy storage system are realized.The simulation model of DC microgrid is built in Matlab / Simulink to verify the proposed coordinated control strategy.