7. Review of previous works

A journal article on the feasibility of a simple small wind turbine with variable speed regulation [19] was looked at. The objective of the research was to suggest and evaluate a Very Small Wind Turbine (VSWT) with a Squirrel-Cage Induction Generator (SCIG) that can contend favorably with VSWTs connected to the grid with respect to cost and relative ease. To assess the dynamic response of the system other different wind speeds, two tests were made. The results were gotten from real life scenarios not computer simulations and showed the drop in performance with regards to its nominal value was about 75%, when working at 50% of the nominal power which is reasonable enough. In summary, the VSWT and its intended control algorithm is feasible for controlling direct-shaft grid-connected VSWTs.

A paper on control of active and reactive power of a wind energy conversion system with variable speed [20] details a design of wind energy conversion system with different speed employing a three-phase SCIG driven by a HAWT. A static VAR compensator is suggested and linked with the SCIG terminals so as to run the system parameters.

### Modeling and Simulation of a 10 kW Wind Energy in the Coastal Area of Southern Nigeria… DOI: http://dx.doi.org/10.5772/intechopen.85064

Pitch angle control was utilized in controlling the mechanical power and the system was simulated using SIMULINK software. From the simulation results, the response of the suggested system offers quick recovery faced with different dynamic system disturbances with the controller boosting power thereby improving system efficiency.

A Research by the Czech Technical University studied the need for gearless wind turbines is looked into due to positives such as reliability, and reduction in downtime due to less moving parts. The design simulation was carried out using MATLAB SIMULINK. Results showed the wind turbine had the ability to sustain an electric-power scheme. The system allowed for the independent control of both reactive and active power, suggesting that the gearless design is suitable for turbines with variable speed [21].

In a paper on the transient response of Doubly Fed Induction Generator using an accurate model [22], the transient execution of various models of DFIG considering saturation impact was looked at and a few parameters that influence rotor overcurrent due to voltage sag was simulated. The findings from the paper include the importance of consideration of saturation effect on transients but less so for steady state analysis. Also, the rotor speed of saturated model reaching steady state value quicker than unsaturated model.

A detailed model of fixed speed wind turbine (FSWT) stability studies with stator transient was addressed in a PhD work by [23]. The addition of the stator current transient permit a precise speed divergence forecast. A model for stability of power system analysis like Doubly-Fed Induction Generator (DFIG) wind turbine was also suggested in the same work including the stator flux transient. By doing so, the analysis of Fault Ride-Through (FRT) is done. However, such representation gives rise to difficulties when looking into the implementation of the positive sequence fundamental frequency simulation tools, as a result of small time-step prerequisite and inconsistency with normal power system parts.

A model of DFIG wind turbine was introduced in [24], the stator transient was not considered at normal operation. However, the use of a current controller still demands high simulation resolution.

A basic model of a DFIG wind turbine, compatible with the natural frequency representation was projected by [25]. Both stator and rotor flux dynamics were neglected in the model. This model is comparable to a steady state representation, while the controller of the rotor current is assumed to be instantaneous. Therefore, iteration process which is not favorable in the implementation model is required to solve algebraic loops between the grid model and the generator model.

With the introduction of time lags representing current control delays, algebraic loops can be avoided [26]. Nevertheless, it is assumed that the maximum power tracking (MPT) in this model is directly proportional to the arriving wind speed, although in common practice, the generator speed or the generator output power drives the MPT.

Miller et al. [27] presented another DFIG simplified model. According to this model, the generator is simply modeled as a current source that is controlled; hence the rotor parameters are omitted. This proposed simplified model did not take into account the limiters of rotor current and the FRT schemes are not clearly modeled.

Demonstrations of detailed FSWT models for power system are presented in [28]. In this paper, the generators are modeled thoroughly. They need very small time-step therefore complicating the execution in a standardized fundamental frequency simulator. Ref. [29] proposed a simplified model of an FSWT model.

In a power system network comprising different generation unit, there is bound to be frequency stabilization and control issues. In the work of [31] the frequency responses of the grid power system network and other variables of the grid connected wind during the period of grid dynamics show improved performance as shown in the simulation results.

where ρ is the density of air, A is effective area of disk, v is wind velocity, and P

The power accessible from the wind is directly proportional to the cube of the speed of the wind. Meaning if the speed of the wind is doubled then the output power from the turbine is given eight times. Therefore, wind turbine designs have to take this into account by ensuring designs can support higher wind loads than those from which they can generate electricity, in order to prevent them from damage. Wind turbines approach maximum efficiency at wind speeds between 12 and 15 m/s. Over this wind speed, the power yield of the rotor must be controlled to diminish main thrusts on the rotor blades and in addition the load on the general

As wind energy is free, wind-to-rotor efficiency, losses in the generator and power electronics are the major factors that affect the final cost of wind power generation. To keep parts from corroding, extracted power is fixed above rated operating speed as theoretical power increments at the cube of wind speed, which reduces the efficiency. Turbine efficiency can diminish somewhat after some time because of wear. Examination of 3128 wind turbines 10 years or older in Denmark demonstrated that half of them did not diminish in efficiency, while the other observed a decrease of 1.2% per year [18]. Vertical turbine efficiency is lower than their horizontal counterparts.

A journal article on the feasibility of a simple small wind turbine with variable speed regulation [19] was looked at. The objective of the research was to suggest and evaluate a Very Small Wind Turbine (VSWT) with a Squirrel-Cage Induction Generator (SCIG) that can contend favorably with VSWTs connected to the grid with respect to cost and relative ease. To assess the dynamic response of the system other different wind speeds, two tests were made. The results were gotten from real life scenarios not computer simulations and showed the drop in performance with regards to its nominal value was about 75%, when working at 50% of the nominal power which is reasonable enough. In summary, the VSWT and its intended control

A paper on control of active and reactive power of a wind energy conversion system with variable speed [20] details a design of wind energy conversion system with different speed employing a three-phase SCIG driven by a HAWT. A static VAR compensator is suggested and linked with the SCIG terminals so as to run the system parameters.

algorithm is feasible for controlling direct-shaft grid-connected VSWTs.

is power.

50

Figure 2.

wind turbine system [17].

The air flow through area A [36].

Wind Solar Hybrid Renewable Energy System

7. Review of previous works
