## 2 Challenges and Opportunities of Large Wind Turbines  
Considering the growing nature of large wind turbines, many technological challenges are to be resolved. Wind power intermittency is a well-known problem. To address this issue, it is important to exercise well how to predict the wind speed, and only then can this intermittency problem be handled precisely. The smoothing of the variable wind power is the next step in which the energy storage system and other advanced control approaches may play vital roles [3; 4]. The intermittency of wind power may also lead to voltage and frequency instabilities. Many FACTS devices and energy storage systems can help in augmenting these stabilities. A modern approach to address the frequency dip problem is to use the synthetic inertial [5]. Grid Interfacing, fault ride through and power electronic converters' reliability are other important issues to be addressed [6] in order to get a reliable grid operation. The wind energy sector is blessed by the contribution from many engineering and science disciplines in the last few decades, mainly from mechanical, electrical, electronic, computer and aerospace. The individual and joint efforts from scientists within different disciplines are prime driversfor the wind industry to reach maturity. In this special issue, the present and future development schemes of wind turbine generator systems are depicted based on the contribution from many renowned scientists and engineers in different disciplines. To make this special issue collection useful for students and researchers, a wide variety of research outcomes are merged together, putting focus on the variability, stability and scalability issues of wind energy conversion systems. To address the challenges mentioned above, 10 different articles are included in this special issue collection, selected from many submissions, and a brief description for each article is given below. The first paper [7], authored by [PERSON], [PERSON], [PERSON] and [PERSON], presents a pure technical review on Wind Farm Automatic Generation Control (AGC), focusing on frequency regulation. In this article, the contribution of wind farms in the supplementary/load frequency control of AGC is overviewed first. Then the authors proposed a fractional order proportional-integral-differential (FOPID) controller to regulate the speed of the turbine rotor in the participating AGC. The second paper [8] provides another review on Fault Current Limiting Devices to augment the fault ride through the capability of large wind turbines using doubly fed induction generators, authored by [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON], giving a focus mainly on fault current limiters and series dynamic braking resistors. This is indeed a timely reporting. There are three papers addressing wind power variability and intermittency issues. In the first paper [9], authored by [PERSON], [PERSON] and [PERSON] computed the potential uncertainties of wind power in constructing prediction intervals (PIs) and prediction models using a wavelet neural network (WNN). In order to optimize the forecasting model, the authors have proposed a multi-objective artificial bee colony (MOABC) algorithm combining multi-objective evolutionary knowledge, called EKMOABC. It appears that in this way, a better short-time wind power forecasting is very much possible. In the second paper [10], the novel output power smoothing control strategy for a wind farm based on the allocation of wind turbines is proposed by the Authors [PERSON], [PERSON], [PERSON] and [PERSON]. The wind turbines in the wind farm are divided into control wind turbines (CWT) and power wind turbines (PWT), separately. The rotor inertia based power smoothing method is adopted in that study for the sake of better performance. This is another timely reporting in addressing wind power variability problem. In the third paper [11], authored by [PERSON] and [PERSON], the wind power intermittency problem is addressed by using Flywheel Energy Storage System (FESS).