Nonlinear control strategy for battery charge and discharge in microgrid

The introduction of microgrid further improves the utilization of new energy on the basis of ensuring the reliable power supply of local load, but the development of microgrid is limited due to the fluctuation and intermittence of microgrid output power. In order to solve the problems of complex control strategy of microgrid and difficult coordination of micropower source and energy storage side power, considering the change of wind-solar output power and load, a nonlinear control strategy of storage battery charging and discharging with integral controller and online correction parameters was proposed. Based on the model of wind, light and storage battery, the disturbance observation method was used to realize the maximum power tracking of distributed power supply, and the correctness of the model was verified under different temperature, light intensity, and wind speed. According to the power constraint conditions of the wind-solar storage system and the load power demand as a benchmark, the output of each micro power source and the state of charge of the storage battery were comprehensively considered to obtain the output law of the energy storage system. Based on the output law of energy storage system, the input/output mathematical models of DC/DC converter in boost circuit and buck circuit were studied; the DC side bus voltage and the storage battery side inductance current were selected as the state vector, and the duty cycle of each switch tube was the control vector to construct the state space matrix of the battery charging and discharging circuit; according to the charging and discharging characteristics of the battery, the macro variable of the system was constructed by the integral term of voltage deviation and online correction parameters. The duty cycle mathematical model of each switch tube was obtained by combining the state space matrix of the DC/DC converter. The working mode of the DC/DC converter was adjusted to control the charging and discharging of the storage battery, coordinate the power transmission between the bus side and the energy storage side, meet the power balance of the microgrid system and restrain the power fluctuation. Through the simulation experiments of the constructed microgrid model, the dynamic response of the DC bus voltage of the proposed storage battery charge-discharge nonlinear control strategy under various conditions of the microgrid system was studied. The simulation results showed: 1) When the weather conditions changed normally, the nonlinear control strategy could control the battery to reduce the output power within 0.05 s, stabilizing the DC bus voltage at 640 V. The traditional proportional-integral control strategy needed about 0.3 s to achieve the above control process. 2) When the load changed normally, the nonlinear control strategy made the DC/DC converter convert between the boost circuit and the buck circuit within 0.05 s, maintaining the DC bus voltage stable at 640 V. The traditional proportional-integral control strategy needed about 0.2 s to achieve the above control process. 3) When the light intensity suddenly dropped to 0 W/m2, the DC bus voltage under the traditional control strategy recovered from 620 V to 640 V in 0.3 s; the DC bus voltage under the nonlinear control strategy recovered from 625 V to 640 V in 0.05 s. 4) When the large load was put into operation, the DC bus voltage recovered from 626.5 V to 640 V in 0.3 s under the traditional control strategy; the DC bus voltage recovered from 629 V to 640 V in 0.05 s under the nonlinear control strategy. In all case, the dynamic recovery time of DC bus voltage fluctuation was shortened by 0.2 s on average, the transient impact was reduced by 0.6% on average, the proposed control strategy could effectively regulate the power balance between micro power source and energy storage system, ensuring the stability of power supply of microgrid system. The research results should provide a reference for the control research of the microgrid energy storage system.