## 1 Introduction  
Global interest in renewable energies has surged in recent years, and the development of green technologies has encouraged the applications of renewable energies to promote energy feasibility and reduce carbon emissions. In 2005, [PERSON] et al. [1] investigated the effects of blade number and angle for wind turbine systems and proposed a tip speed ratio (TSR) to optimize wind power efficiency. [PERSON] [2] studied the influence of turbine blades with a pre-twisted angle on the turbine efficiency and the wind speed distribution. In 2007, [PERSON] [3] studied the aerodynamic influence of wind speed on the starting torque for wind turbine systems. The majority of such systems transmit power using planetary gear systems based on the analysis conducted in 1972 by [PERSON] and [PERSON] [4]. In that study, the kinematics, forces, and power flow for the planetary systems were widely discussed. Using the finite element method, in 1996, [PERSON] and [PERSON] built a three-dimensional model for a planetary gear system to investigate the system's dynamic responses and tooth load sharing [5]. [PERSON] et al. [6] proposed five kinds for the gear systems characterized by their mesh parameters,including mesh stiffness, damping ratio, frictional coefficients, etc. To stabilize the system's angular velocities and output torques, [PERSON] [7] designed a variable inertia flywheel for a planetary gear system composed of a shaft and spring. [PERSON] [8] proposed another, fluid-controlled variable inertial flywheel to investigate the energy states for planetary gear systems. The influence of variable wind conditions on the wind turbine was studied by [PERSON] and [PERSON] [9], and the corresponding control system was designed and modelled. Among renewable energy sources, the small wind turbine (SWT), used in rural or urban areas, has the advantages of easy accessibility, flexible capacity, and low costs. According to IEC 61400-2:2013 [10], the small wind turbine has a power output of less than 50 kW, and the corresponding rotor swept area is less than 200 m\({}^{2}\). The SWTs used in urban or rural areas were studied in different countries, such as the United Kingdom [11], Kenya [12], and Venezuela [13], etc. It showed that the wind conditions have a strong effect on the effectiveness of SWTs. In the study by [PERSON] et al. [14], it was suggested that the classification of the urban wind conditions should be revised for IEC 61400-2, and the small wind turbines could be designed to be more consistent with wind conditions in the urban environment. In the study by [PERSON] et al. [15], the small wind turbine effectiveness in the urban environment was discussed in relation to the wind fluctuation. Both the vertical axis wind turbine (VAWT) and horizontal axis wind turbine (HATW) were investigated for the trend of rotor inertia versus radius, the response time of rotor inertia to the wind fluctuation, and the corresponding control strategy, etc. In 2019, [PERSON] et al. [16] tested the performance of the small VAWTs with the nominal powers of 1 kW and 3 kW. The maximum power outputs were 0.69 kW and 1.6 kW at the wind speeds of 14.5 m/s and 10.5 m/s for the 1 kW and 3 kW VAWTs, respectively. It also pointed out that the VAWTs enabled the braking mechanism when the wind speed was over the prescribed wind limit. The results showed that the real outputs were far less than the nominal powers for the commercial wind turbines in the market. To accurately estimate the influence of the wind condition in the urban area, [PERSON] et al. [17] developed the tunnel test for VAWTs in the turbulent flows. Using the aeroelastic model, [PERSON] et al. [18] analyzed the horizontal-axis wind turbine of 5 kW. Compared to VAWTs, HAWTs are more sensitive to wind fluctuation. In 2013, [PERSON] et al. [19] studied the maximum peak power tracking (MPPT) algorithm for a HAWT with a permanent magnet synchronous generator (PMSG) of 5 kW.