Study On The Capacity Configuration And Control Strategy Of Dual Battery System Considering Charging And Discharging Unbalanced Energy

The wind output power shows obvious fluctuation characteristics,and direct grid connection will bring great challenges to the safe and stable operation of power system.Due to the chargeable and dischargeable characteristic,the battery can huff and puff power flexibly and rapidly to smooth the fluctuation of wind output power.However,when the wind power changes rapidly,minimal Depth of Discharge(DOD)and frequent charge-discharge conversion will seriously affect the service life of battery.The Hybrid Energy Storage System(HESS)which contains battery and power-type energy storage device,can reduce the battery damage to some extent,but the control process is complicated and investment cost is high.Therefore,it is necessary to consider the important factors that affect the battery life,so as to choose the reasonable and economical mode of Battery Energy Storage System(BESS),and then study the capacity configuration and control strategy of the battery storage system under the operating mode is necessary.In this paper,the battery energy storage is used to smooth the output power fluctuations of the wind farm,and the battery energy storage is divided into two group to carried out the charge or discharge task separately.Considering that the total charging energy of the charging battery group is not balanced with the total discharging energy of the discharge battery group,this paper study on capacity optimization configuration and control strategy of energy storage system under this operation mode.The main research contents are as follows:1)On the basis of the existing literature,the relationship among DOD and battery life,total energy transfer is analyzed to reveal the importance of reasonable choice of charge-discharge depth for the economical operation of battery energy storage system.The fluctuation characteristics of the historical output power of actual wind farm is researched,and an evaluation index of fluctuation in specific frequency bands is proposed to provide a basis for evaluating the smoothing effect of energy storage system on wind power.2)In order to prolong the service life of battery and improve the energy utilization rate,considering the factors such as the charge and discharge efficiency of battery,State of Charge(SOC)constraint,and the rated capacity limit,the charging and discharging operation model of Dual Battery Energy Storage System(DBESS)is established,which the charge and discharge task is executed separately under a given depth of discharge.Discrete Fourier Transform(DFT)processing is performed on the original wind output power,and the high frequency band is used as the compensation frequency band.The minimum balance power of battery under the requirements of wind power grid standard is preliminarily determined.Considering the imbalance of charge and discharge energy in the calculation period,the required balance power is revise.With the lowest Life Cycle Cost(LCC)as the optimization goal,the genetic algorithm is used to optimize the rated capacity and DOD of battery.A simulation example is set up on MATLAB and the economy of DBESS is verified from battery daily equivalent cycle loss and the results of capacity optimal configuration,which LCC is the minimum.3)Considering the extreme operating conditions and the fluctuation of wind power during the long-term operation of DBESS due to unbalanced charge and discharge energy and fluctuation of wind power,the charging-discharging saturation index and the charging-discharging stability index of DBESS are proposed in this paper.Taking the two indicators as input,fuzzy control is designed to adaptively change the first-order low-pass filter time constant,so as to change the value of battery power in real time and optimize SOC of the two battery groups.And on the MATLAB simulation platform,the effectiveness of the proposed control strategy is verified from the optimization control effect of SOC and power fluctuation smoothing effect.4)Considering inertial lag in changing the filter time constant for actual projects,a variable-domain step of Model Predictive Control(MPC)with low-pass filtering method is proposed to optimize the two groups of battery SOC because of the predictive control.The model predictive control is used to optimize the charging and discharging stability index of DBESS and power fluctuation rate.The optimization results are used in the conventional first order low pass filtering method to adjust the grid power value to obtain the reasonable charge-discharge power of battery.By changing the length of controlling time domain,the influence of uncertainty of wind power prediction on the control effect is reduced.Combining the actual wind power data and prediction data,it is verified through simulation that the proposed control strategy is effective in alleviating the extreme operating conditions of DBESS described above.