## 1 Introduction  
Global warming is one of the most serious environmental problems facing the world today. Most remote areas and isolated communities depend on diesel generators to meet electricity demands [1]. Diesel fuel has several drawbacks, including the fact that it is expensive because transportation to remote areas adds extra cost. Moreover, it emits sulfur oxide and carbon dioxide by engine exhaust that is harmful to the environment [1; 2]. Due to the environmental and economic effects of the diesel generator, interest in alternative, cost-effective, and clean energy sources has grown significantly. Wind turbines are attracting a lot of attention, and represent the fastest growing and most widely utilized form of renewable energy technology in power systems [3]. Wind-diesel hybrid power systems can be installed in isolated communities to improve quality of life. Many small scale wind farms are connected to the distribution network, while many wind farms of 50 MW or more are directly connected to the transmission network [4]. This results in many challenges. Among them is the fact that wind power is not constant and can diverge quickly, since it is proportional to the cube of wind speed. Wind power fluctuations lead to some problems, which can be summarized as follows [4,5]:  
1. Wind power fluctuations may cause the grid frequency to fluctuate. 2. The variation in wind speed causes variations in the active power generated and thus the absorbed reactive power by the induction generator from the grid, which leads to voltage flicker at the buses of the power grid. 3. Poor power quality and instability problems appear in the power system as a result of frequency fluctuation and voltage flicker, especially in the case of sensitive loads for frequency and high-voltage deviations. The effects of these problems appear clearer in isolated power systems as penetration of the wind turbine generator (WTG) increases. Therefore, various power smoothing methods are applied to wind energy conversion systems (WECSs) to overcome these problems. Many pitch angle control techniques are utilized to smooth the output power of WTGs using model predictive controls (MPCs) [6], the genetic algorithm (GA) [7], linear quadratic Gaussian (LQG) [8,9] (which provide robustness in terms of phase and gain margins), and the GA with particle swarm optimization (PSO) [10]. However, the performance of all these linear controllers is limited due to the highly nonlinear characteristics of wind turbines. Fuzzy logic has been applied for pitch angle control [11, 12, 13, 14]. However, these studies differ from each other significantly by type and number of inputs and outputs, number and type of input and output membership functions, type of control rules, inference engines, and defuzzification method. No complete studies of the power system performance from the voltage and frequency stability point of view nor evaluations of the robustness of the proposed controllers under severe operating conditions such as generator outage or transmission line faults have been presented. The \(H_{\infty}\) control method is used to smooth the output power of WTGs [15, 16, 17]. However, the weighting functions in the \(H_{\infty}\) control design cannot be easily selected, and this affects the process of design significantly. Also, the order of the \(H_{\infty}\) controller depends on that of the plant. This causes a complex structure which is not easy to realize in practical applications, especially for large-scale systems. On the other hand, the frequency control approach in power systems has a long history and there is plenty of related literature. Power system frequency can be negatively affected due to wind power fluctuations, especially with high-penetration wind farms. This leads to a frequency deviation from its nominal value. As a result, under/over frequency relays may operate and disconnect some portions of system loads and generation. However, there are only a few articles on the frequency regulation schemes in the presence of wind turbines.