iL_ref PI + R iL Vref PI - Modulator ~r_ref + R ~r MPPT SA iL Vdc dc-dc Boost Converter PMSG Rectifier ~r (a) PI ~r iq_ref + R iq + R id_ref = 0 id PI PI Vq_ref Va_ref dq Vb_ref Vd_ref ab is ~r dq ab(c) SA Modulator MPPT ~r_ref + R SB SC Three-Phase ac-dc Converter Position and Speed Estimator va va vb ia ib ia Vdc vb ib PMSG (b) FIG 10 The block schemes of the control algorithms for (a) dc-dc boost converters and (b) three-phase ac-dc converters. MPPT Algorithms Based on Knowledge of Turbine Parameters Based on Iterative Search Data Required : Knowledge of Pm(~) Characteristic : Power and Rotational Speed Measurement : Knowldge of mopt : Wind Speed and Generator Rotational Speed Measurement With Outer Generator Rotational Speed Control Loop: : Power and Rotational Speed Measurement With Power Control With Rotational Speed Control FIG 11 The classification of MPPT algorithms. 22 IEEE PowEr ElEctronIcs MagazInE z September 2015 With Duty Cycle as Direct Reference: : Power Measurement To extract the maximum energy from the SWT, the generator reference rotational speed has to be controlled by the MPPT algorithm, independently on an ac-dc converter topology [12]. There are many different MPPT algorithms, but all can be categorized into two groups. Algorithms in the first group require prior knowledge of the turbine parameters to calculate the operating point. The second group relies on iterative search of the optimum with the use of reference speed increments. Figure 11 presents a more detailed division of these algorithms, which differ in implementation and type of required sensors. However, the principle of operation remains the same.