An improved control strategy for charging solar batteries in off-grid photovoltaic systems

Abstract In off-grid photovoltaic (PV) systems, a battery charge controller is required for energy storage. However, due to unstable weather conditions as well as the frequent variations in load demand, the PV power flow delivered to the load could be fluctuated while the battery charging efficiency will be reduced. To overcome these issues, this paper presents an improved power balance control strategy based on two proportional and integral (PI) compensators only, which can adequately balance the PV power flow delivered to the DC load and the battery, so that the PV power is effectively utilized and the battery is properly charged. First, a modeling of the entire system based on the linear PV array model is performed to simplify the design of the PI compensators. In addition, four operating modes are adapted to deal with the aforementioned issues regarding any variation in weather conditions and load demand. Namely, Maximum Power Point Tracking (MPPT)-mode, Non-MPPT mode, Night-mode, and Off-mode. Afterwards, a digital anti-windup control strategy associated with PI compensators is also developed to ensure a smooth switching from an operating mode to another. Moreover, an improved Incremental Conductance IC-MPPT algorithm is adopted to extract the maximum power from the PV array. Finally, MATLAB/SIMULINK simulations are carried out and the results obtained demonstrate the good performance of the proposed control strategy under different atmospheric conditions, both in power balance control and MPPT control, notably in terms of efficiency (99.79%), steady-state power oscillations (0.03 W), and convergence time (0.03 s).