Sampling frequency design to optimizing MPP-tracking performance for open-loop-operated converters

Fixed-step perturbative maximum-power-point (MPP) tracking algorithms, such as perturb & observe and incremental conductance technique, are the most popular techniques in single and double-stage grid-connected photovoltaic PV systems due to their relatively good performance with a simple implementation. However, in order to optimize the performance of such algorithms, the design parameters - sampling frequency and perturbation step size - need to be designed in respect to interfaced power electronic converter. Recent studies have provided state-of-the-art MPP-tracking design rules for single and two-stage grid-connected PV systems. In perturbation frequency design, the basic guideline is to ensure that the interval between the perturbations is chosen to be long enough so that oscillatory behavior of PV power transient is attenuated to its steady-state value. Unfortunately, the perturbation frequency design in recent studies is treated at the MPP, which does not represent the worst-case from the power settling time point of view. Due to the natural behavior of the perturbative MPP-tracking algorithm, the operation point moves from the MPP into constant-current and constant-voltage regions with significantly different PV power settling time. In this paper, deterministic analysis and experimental results reveal that MPP-tracking design needs to be performed in constant-current region, where the settling time of the PV power transient is the longest. Thus, the design of the perturbation frequency is very deterministic and entirely governed by the design of the converter.