## 3 MG Overview and Experimental Test Bench
### Lppt Control Strategies  
The use of power control strategies to maximize the efficiency of the WT system is still relevant. MPPT control is a well-known mechanism developed to keep the power output of the WT system at the MPP [11] (green point in Figure 5). However, some power limitations can be associated with this tracking method and then the WT is supposed to provide less power [29]. Thus, the LPPT mode is chosen and activated (pink line in Figure 5). The suggested control power strategies presented in this  
Figure 4: (**a**) Electrical power \(p_{WT}\) based on the bus voltage \(v_{WT}\); (**b**) electrical power \(p_{WT}\) based on the bus current \(i_{WT}\); (**c**) electrical power \(p_{WT}\) based on mechanical speed \(\Omega\).  
study focus on LPPT characteristics and evolution. The principle of LPPT control for the WT system is explained with the help of the \(p_{WT}-v_{WT}\) curve shown in Figure 5.  
When the SSWT converter operates in LPPT mode, the \(P_{\text{lim}}\) is under maximal value, which means that the user or loads ask for less power. Therefore, to function correctly in LPPT mode, the absolute value of the difference between \(p_{WT}\) and \(P_{\text{lim}}\) should be calculated as \(p_{diff}=\left|p_{WT}-P_{\text{lim}}\right|\). This \(p_{diff}\) must be low in order to reach the operating point. For this reason, algorithms used in the LPPT control strategies aim to decrease the value of \(p_{diff}\) as much as possible. Overall, in LPPT mode, two limited power points (LPPT and LPP2) could be achieved by reducing \(p_{diff}\). Consequently, the \(p_{WT}-v_{WT}\) curve is divided into four regions, which are characterized by different conditions (see Figure 6), allowing the transition from one operating point to another according to the needs of the user and the constraints of the used equipment. Thus, it is crucial to make a wise choice based on the advantages and disadvantages of each point. To avoid the generator's over-speeding and minimize the mechanical stress, the proposed power control strategies should seek LPP1, whereas LPP2 ensures less power loss and low currents. In this study, the limits of the test bench equipment restrict the WT system to operate at a specific point. In this context, the characteristics of the SSWT used in this study (Figure 4a,c) show that for high wind speeds, the system can operate only in LPP1. Indeed, the rotor maximum rotation speed estimated at 1250 rpm can be exceeded while trying to reach the second point LPP2, which can damage the WT.  
The selected controller in this present study is P&O, which is very simple and allows for a fast dynamic in tracking the operating power points. It does not require knowledge of SWECS components. It consists of perturbing the operating voltage of the system and analyzing the resulting change of power to decide the direction of the next perturbation. The P&O principle is used and presented in Figure 6. It is different from the flowchart for classical P&O [11] because it takes into account two operating points. Thus, several conditions were analyzed in order to identify the four regions (\(r\)) presented in Figure 5. The value of \(v_{WT\_REF}\) is calculated following the conditions in Figure 6. This flowchart measures at each k instant the variables \(v_{WT}(k)\) and \(i_{WT}(k)\), calculates \(p_{WT}(k)\) and \(p_{diff}(k)\), and then compares them with powers calculated at the \(k-1\) instant: \(p_{WT}(k-1)\) and \(p_{diff}(k-1)\). Three LPPT control strategies based on P&O are presented below.  
Figure 5: Operating cases for (**a**) limited power point 1 (LPPI); (**b**) LPP2.