Research On Coordinated Control Strategy Based On Improved Droop Method In Independent DC Microgrid

With the deterioration of energy crisis and ecological environment,because distributed generation has the advantage of being able to dissipate electricity locally,it has gradually become an important way to replace traditional fossil energy and realize efficient and flexible use of new energy.DC microgrid has become a hotspot of current research because of its high efficiency,simple structure and easy control in the utilization of distributed power.Hybrid energy storage and distributed power are important components of DC microgrid,whose appropriate control method is the basic condition for safe and stable operation of DC microgrid.In this paper,an independent DC microgrid composed of PV cell and hybrid energy storage system is taken as the research object.This research is carried out in two aspects: device-level control strategy and system-level coordinated control.This paper firstly studies the device-level control strategy in the system.The mathematical model of photovoltaic,battery and supercapacitor was established,and the output characteristics of the PV cell and the working principle of the bidirectional DC/DC converter were analyzed.According to the different working conditions,the control of the PV cell is set to maximum power tracking control(MPPT)and constant voltage control.In the energy storage system,the battery bear the surplus or the lack of power in the system for a long time,so ensuring the service life of batteries is the prerequisite for the safe and stable operation of the system.In general,the load current distribution between multiple sets of batteries uses droop control.Through the principle of the traditional droop control,it is found that the detect of fixed droop coefficient,which may lead to overcharge and overdischarge of the battery and DC bus voltage deviation.Therefore,the mathematical model of the real-time droop coefficient is established by using state of charge(SOC)of the battery as an intermediate variable.The appropriate parameters were selected by analyzing the model,so that the bigger SOC of battery discharges more and charges less,the smaller one is the opposite.Then the effect of equalizing the load current is achieved.At the same time,the secondary voltage compensation is performed in the droop control for the defect of the DC bus voltage deviation,and the actual voltage is closer to the given reference voltage.The supercapacitor is mainly responsible for the abrupt power in the system.After the power distribution of the low-pass filter,constant current control is used to achieve the absorption and release of the abrupt power in the system.The simulation verification of all the above strategies is implemented in Matlab/Simulink.Then the overall system level coordinated control strategy is studied.System-level control is to realize the scheduling function by collecting the distributed power data in the system to issue commands to the controllers of each unit in the equipment level.According to the power balance in the system and the SOC state of the battery,a coordinated control strategy for multi-mode operation management of independent DC microgrid is studied.According to different control modes of each power generation unit,the system is divided into six working modes,which achieves the goal of rational allocation of system energy and realizes high efficiency and economical operation of the system.Finally,the simulation model is built in the Matlab/Simulink environment,and the multimodal operation management strategy above is simulated and verified.In order to verify the practicability of the strategy,an independent PV energy storage DC microgrid experimental platform was built in the laboratory,and the hardware circuit and software program were designed with TMS320F2812 as the control core.The hardware circuit in the platform was debugged.The constant current discharge experiments for two groups of battery and the comparison experiments of traditional droop control and improved SOC-based droop control were completed.The experimental results further verified the feasibility of the proposed strategy.