Research On Optimal Control Strategy Of Photovoltaic DC Microgrid With Hybrid Energy Storage

Photovoltaic power generation technology is widely used in DC microgrid,but it is seriously affected by the environment.The power generated is random and intermittent,which leads to the fluctuation of the bus voltage of optical storage DC microgrid,and seriously affects the power quality and system stability.The hybrid energy storage system composed of battery and supercapacitor can suppress the power fluctuation caused by photovoltaic.However,due to the limited capacity of the energy storage elements,the overcharge and overdischarge of the energy storage unit during use will seriously affect their service life.Based on this,this paper proposes a hybrid energy storage system frequency division control considering the state of charge of the energy storage components to stabilize the power fluctuation and maintain the bus voltage stability with the bus voltage layered control.Firstly,the overall structure of the photovoltaic DC microgrid with hybrid energy storage is analyzed,and the working characteristics of the photovoltaic cell,storage battery and supercapacitor bidirectional DC-DC converter are analyzed;the simulation model of photovoltaic cell is built,and the output characteristic curve is obtained through simulation,which verifies the correctness of the model.Aiming at the problem that the maximum power tracking control of the traditional Perturb and Observe Method cannot consider both the tracking speed and the tracking accuracy at the same time,this paper proposes a variable step size Perturb and Observe Method based on hysteresis comparison for maximum power tracking,and the effectiveness of the algorithm is verified by simulation.Secondly,in view of the problem that the frequent charging and discharging of a single battery energy storage will seriously shorten the battery life,this paper designs a power distribution strategy based on the state of charge of the battery and supercapacitor.The low-pass filter is used to realize the power distribution between the storage battery and the supercapacitor,and at the same time,the state of charge of the energy storage element is considered.According to its available capacity,the output is reasonably shared,the charging and discharging process of the energy storage element is optimized,the stability of the DC bus voltage is maintained,and the life of the energy storage system is extended.Finally,a hierarchical control strategy of DC microgrid bus voltage based on bus voltage signal is studied.According to the bus voltage deviation value,the bus voltage is divided into three layers.Each converter under each layer adopts different control methods.The bus voltage value is used as the switching signal to realize the stable operation of DC microgrid island state.A model is built in maltable / Simulink to verify the effectiveness of the proposed strategy and achieve the purpose of maintaining the bus voltage stability.