Fuzzy logic PID controller-based lithium ion battery charge control

In this study, the DC bus voltage of the hybrid power system consisting of a DC voltage source and a lithium-ion battery is controlled using a fuzzy logic proportional + integral controller (FL-PI) based cascade control scheme. The simulation model of the hybrid power system was built in Matlab/Simulink. The model created consists of a DC voltage source, lithium ion battery, bidirectional DC-DC converter, DC voltage control, battery charge/discharge current control and generation of switching pulses. The lithium-ion battery is connected to the common DC line via a bidirectional DC-DC converter. In addition, bidirectional DC-DC converter is controlled with FL-PI based cascade control scheme and this converter enables DC bus voltage to be controlled at the desired value. This control loop has an external control loop for DC line voltage control via current reference generation and an internal current loop for controlling battery current. This scheme has superior performance in terms of transient response, stability of control loop. The dynamic performance of the proposed control method was evaluated by means of simulation studies for two cases. In the first case, DC voltage source is activated at the start of the simulation operation and DC voltage source is deactivated in the 3rd second. In the second case, the DC voltage source was not activated at the beginning of the simulation study and the DC voltage source was activated in the 3rd second. In the case of DC voltage source, the source provides the power demanded by the load and charges the battery, while in the opposite case, the power demanded by the load is supplied from the battery and the battery is discharged. Simulation results showing the load voltage, battery voltage, battery current and battery state of charge (SOC) are given for each case. The simulation results obtained from the conventional PI controller are also given to show the superiority of the proposed method. From the obtained simulation results, it is seen that the FL-PI controller is superior to the conventional PI controller in the transients of both operating conditions.