Owing to the good trade-off between implementation and performance, fixed-step direct maximum power point tracking techniques (e.g., perturb and observe and incremental conductance algorithms) have gained popularity over the years. In order to optimize their performance, perturbation frequency and perturbation step size are usually determined a priori. While the first mentioned design parameter is typically dictated by the worst-case settling time of the combined energy conversion system, the latter must be high enough to both differentiate the system response from that caused by irradiation variation and match the finite resolution of the analog-to-digital converter in case of digital implementation. Well-established design guidelines, however, aim to optimize steady-state algorithm performance while leaving transients nearly untreated. To improve transient behavior while keeping the steady-state operation unaltered, variable step direct maximum power point tracking algorithms based on adaptive perturbation step size were proposed. This paper proposes a concept of utilizing adaptive perturbation frequency rather than variable step size, based on recently revised guidelines for designing fixed-step direct maximum power point tracking techniques. Preliminary results demonstrate the superiority of the proposed method over adaptive perturbation step size operation during transients, without compromising the steady state performance.Energies 2019, 12, 399 2 of 16 between transient and steady-state operation [21]. In order to tackle this drawback, adjustable (or adaptive) step size versions of non-model-based algorithms have been proposed [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38], in which the step size is adjusted, typically in proportion to the derivative of PVG power with respect to control variable. Indeed, such algorithms demonstrate superiority over the fixed step versions, achieved at the expense of more complex implementation. It is interesting to note that the performance similarity of fixed step direct non-model-based algorithms versions have been revealed [39,40], while the equivalence of major variable step size MPPT algorithms has been demonstrated [41].Consider, without loss of generality, a photovoltaic energy conversion operating under a single-loop direct non-model based MPPT. For example, the interfacing power converter (IPC) duty cycle is the perturbed variable (see Figure 1a). A small perturbation ∆d is injected into the system every T p seconds. Following a T ∆ -lasting transient, the corresponding change in generated power is observed either at the PVG or load side (see Figure 1b), and the next perturbation polarity (in fixed step versions) and size (in variable step versions) is determined so that the operation point is driven towards the MPP. It was shown in References [13,21] that the maximum perturbation frequency (reciprocal of T p ) is limited by settling time of the generated power transient induced by the perturbation while the minimum perturbation step size dep...