Research On Optimal Control Method Of Battery SOC In Wind-battery Hybrid Power System

Configuring suitable battery energy storage system(BESS) for wind farm is an effective way to reduce the adverse effect of wind power fluctuations and intermittency. However, the BESS could not be charged or discharged in a regular way when it is used to smooth the wind power fluctuations. So the BESS is prone to be over-charging or over-discharging, which will not only reduce its service life, but also make it have not enough capacity to smooth the wind power fluctuations when the power fluctuates violently. If the control of battery state-of-charge(SOC) is added to the control strategy of BESS, the wind power fluctuations can be smoothed in long term without the over-charging or over-discharging of the BESS. So it is necessary to have research on the control method of battery SOC in wind-battery hybrid power system.This thesis proposed an optimal control method of battery SOC in wind-battery hybrid power system which is based on the vanadium redox flow battery(VRB). The main research works are carried out as follows:1. The structure of the wind-battery hybrid power system and the characteristics of wind power fluctuations were analyzed, then the principle of BESS smoothing the wind power fluctuations was explained. The equivalent model of BESS based on VRB was studied, then its charge-discharge characteristics were analyzed.2. The control structure of the optimal control method of battery SOC was designed, which includes optimal SOC calculation module and SOC real-time control module. The optimal SOC calculation module calculates the optimal range of SOC, which should ensure that the BESS has enough charge-discharge ability to smooth the power fluctuations for a period of time to come. The SOC real-time control module generates proper charge-discharge strategy of BESS, and the strategy should regulate the battery SOC to the optimal range without affecting the current smoothing effectiveness of BESS.3. The method for calculating the optimal range of SOC based on the objective optimization was studied, which calculates the optimal range of SOC by establishing an objective optimization model. The objective functions of the model are minimizing the over charge-discharge time of BESS and maximizing the accessibility of the optimal range of SOC. And the requirements of the grid for the wind power fluctuations are considered as the constraints of the model. Then the Particle Swarm Optimization was used to solve the optimization model.4. Based on the fuzzy self-adjusting of the filter time constant, the battery SOC real-time control strategy was studied. Using the method based on the low-pass filter to smooth the power fluctuations, the control strategy adjusts the filter time constant according to the real-time feedback of battery SOC, and then the reference power of BESS will also be adjusted, so the purpose of controlling SOC was achieved.5. The proposed optimal control method of battery SOC was simulated by using MATLAB and PSCAD, and the effectiveness of the control method in three different power fluctuations situations was compared and analyzed. The results show that the over-charging and over-discharging of BESS was effectively avoided without affecting the smoothness of the wind power, and the BESS can smooth the wind power fluctuations in long term.