## 1 Introduction  
Wind energy possesses an enormous potential for simultaneously addressing the worldwide rapidly growing energy demand, contributing to sustainable development and alleviating global warming concerns. Over the past few decades, this technology has undergone a very fast development. At present, the variable speed wind turbines (VSWTs) have gained a lot of attention among their other counterparts for achieving the maximum efficiency and delivering better quality of power [1; 2]. Generally, due to the inconsistency of the wind speed, the wind energy conversion system (WECS) exhibits variation in its electric power output. This makes the inherent characteristics of the WECS highly nonlinear; thus, the maximum power extraction becomes a challenging task. For maximizing the efficiency of a WECS, the application of a maximum power point tracking (MPPT) control strategy is essential [3]. Different versions of alternating current (AC) generators have been employed in WECS, such as the squirrel-cage induction generator (SCIG), doubly-fed induction generator (DFIG) and permanent magnet synchronous generator (PMSG). Among the stated generators, the PMSG is preferred for a variable speed WECS due to its higher power density, higher efficiency, lower maintenance cost, lack of excitation and a wide operating speed range (0-100%) [4; 5; 6]. Different MPPT strategies have been formulated for maximizing the power extraction from the WECS. Most of the MPPT control strategies, formulated in the domain of the WECS, have focused on the development of nonlinear techniques. Among these strategies, one of the most promising nonlinear control strategies is the sliding mode control (SMC). The SMC possesses several attractive attributes, including simple design, fast dynamic response, insensitivity to parameter variations and disturbance rejection [7]. Apparently, the SMC strategy seems to be an ideal solution for MPPT due to its positive attributes; however, the chattering phenomenon as well as the asymptotic convergence because of a linear sliding surface in the first-order SMC invites a high level of criticism. The chattering causes the system response to oscillate around the desired reference (sliding surface), thus leading to higher mechanical wear and tear, reduced tracking performance and overheating of the power circuits [8; 9]. The SMC control signal basically consists of two parts: an equivalent (or a continuous control) signal and a discontinuous (or switching control) signal. The continuous control part controls the system states once they are on the sliding surface whereas the discontinuous control part deals with the uncertainties (disturbances) but also gives rise to the chattering. The greater the uncertainties in the system, the larger the amplitude of the discontinuous control signal, and hence the chattering phenomenon is more severe. A normal strategy for the chattering minimization is the insertion of a boundary layer near the sliding surface. In this strategy, the continuous control signal is replaced by the discontinuous one once the system response is in the vicinity of the sliding surface. Through this strategy, however, the chattering is minimized, but it also gives rise to a finite steady-state error. Hence, a trade-off is required between the tracking accuracy and chattering. Minimizing the amplitude of the discontinuous control signal can be employed as an alternate strategy for the chattering reduction [10]. However, this strategy degrades the robustness as well as the transient response of the controller. In [11], the authors have proposed a nonlinear backstepping integral sliding mode control (BISMC)-based MPPT strategy for a standalone PMSG-WECS. The system Lie derivatives have been computed using a multilayer feed-forward artificial neural network (ANN). This technique exhibited a good performance in alleviating chattering. The authors in [12] have proposed a discrete-time integral SMC strategy, consisting of a new discrete-time reaching law, to accomplish the MPPT of a PMSG-WECS.