## 1 Introduction  
Wind power penetration in power systems has been rapidly growing in recent years. Regarding environmental pollution of conventional generating units and economic benefits of fuel savings, there has been a great concern toward renewable energy sources [1]. Thus, with improving techniques, reducing costs and low environmental contact, wind energy seems to play an important role in the future of world energy supply [2]. In earlier projects, wind turbine generators (WTGs) were not committed to grid operation. By improving wind power generation, most of the installation projects were oriented towards large-scale wind farms (WFs) interconnected to the power system [3]. Reduction of conventional generating units in the power system results in reduction of system inertia and hence the overall system frequency will be affected. Frequency stability studies related to utilization of WTGs alongside the conventional generating units have grown in recent years. It is of special interest to consider the impact of wind power on power system operation and stability [2]. Currently, most WFs are operating in the mode of maximum power point tracking (MPPT). This operating condition is beneficial from the economic point of view, but there exists a great disadvantage with this mode of operation. In the off-peak hours in an isolated power system, with high percentage of wind power penetration, some conventional generating units may be forced to be offline, thus frequency oscillations may affect the system frequency stability [4]. In order to involve WTGs in the system frequency control, their operating point must be receded from maximum power point into suboptimal points. Hence, the dispatching wind power generation technology is gradually becoming an option in WFs' operation [5]. The presence of WF controller is necessary to allocate the best values of Pcmds to individual WTGs in a WF. Input command to the WF control unit is a signal from automatic generation control (AGC) unit. AGC unit sends its commands to individual WFs' control units according to their participation factor in supplying load demand of the power system. The next approach is dedication of proper Pcmds to individual WTGs in a WF to reach the best frequency stability conditions. In [6; 7], allocating Pcmds to individual WTGs in a WF is proposed according to weighting coefficients from a lookup table. Of course, in this procedure, the differences in weighting factors, changing by available wind velocity of each WTG, cause frequency oscillations in power system. In this paper, in order to smoothen sudden changes in weighting factors, fuzzy logic controller (FLC) has been applied. In order to investigate frequency stability in a power system with high percentage of wind power penetration, an exact modeling of WTGs alongside the conventional generating units must first be performed. Conventional units' modeling has been proposed in [8; 9] which is suitable for frequency stability studies. WTG models can be classified into two categories: (A) Simplified model; (B) Detailed model. Various simplified models are employed for modeling. Of course, in these models, some dynamics in WTG behavior may be neglected. In [10; 11], the dynamics related to converter switching and voltage regulation of DC-link in doubly fed induction generators (DFIGs) is not considered. In [12; 13], the proposed model is simplified by neglecting the DFIG rotor transients. Detailed model of DFIG-based WF helps proper investigation of the interactions between WTGs and conventional generating units, but most of the extracted models, like the model in [14], are electromagnetic models, which are suitable for power converter control strategies and cannot be employed for frequency stability studies. In [15], a detailed model of DFIG wind turbine is proposed, but the modeling of its power controller, rotor-side and grid-side controllers has made it too complex for power system studies.