Control of DC bus voltage in battery energy storage hybrid power system using interval type-2 fuzzy logic controller

In this thesis, in an off-grid hybrid power system consisting of a PV panel and a battery, the battery charging/discharging current is controlled using an interval type-2 fuzzy logic based proportional + integral (AT2-BM PI) controller based cascade control method. The model of the hybrid power system is created in Matlab/Simulink. The model consists of PV panel, lithium-ion battery, step-up type DC-DC converter, bidirectional DC-DC converter, load and control algorithms. The PV panel is connected to a step-up type DC-DC converter and the battery is connected to a bidirectional DC-DC converter in parallel to the common DC bus. The PV panel is controlled by an adaptive neural fuzzy inference system based maximum power tracking (ANFIS-MPPT) algorithm while the bidirectional DC-DC converter is controlled by AT2-BM PI controller based cascade control method. The cascade control method consists of two loops: inner and outer control loop. In the outer control loop, the DC bus voltage is controlled and the reference battery current is obtained from the output of this control loop. In the inner control loop, the battery charge/discharge current is controlled using the reference battery current obtained from the outer control loop. The AT2-BM PI controllers used to control the load voltage and battery current have two inputs (inputs are error and change in error) and one output (change in output duty ratio). Five Gaussian type upper and lower membership functions are used for inputs and output. The effectiveness of the proposed method is investigated for three different operating conditions. The first operating condition is considered as reference variation for DC bus voltage, the second operating condition is load variation and the third operating condition is irradiance variation. Some simulation results from an off-grid hybrid power system are presented for all operating conditions. Also, the simulation results for the variation of the DC bus voltage are compared with the results of the conventional PI controller. The results show that the proposed control method improves the settling time of the DC bus voltage and the overshoot in the DC bus voltage, and provides power balance quickly. Keywords: Hybrid power system, PV panel, Battery, Interval type-2 fuzzy logic PI controller.