**1. Introduction**

Over the past two decades, one of the most important aspects of our life is electrical energy [1, 2]. Currently, to supply power for a modern life as well as to avoid environmental issues originating from fossil fuels exploitations, the production of clean energy has become the primary objective of major universal power producing nations [3, 4]. Solar energy, wind power, biomass and geothermal are the most useful renewable energy sources [5–7]. Nowadays, wind power has become a crucial renewable energy source [8–10].

To convert wind energy into electric power, many kinds of generator concepts have been used [11, 12]. Previously, the squirrel cage induction generator was basically used in wind energy conversion system [13]. This technology is well known in fixed speed applications. Recently, the technology moves towards variable speed wind energy conversion systems [14].

Thanks to its advantages such as four quadrant power capabilities, variable speed operation, improved efficiency, decoupled regulation and reduced losses, the Doubly Fed Induction Generator DFIG has been extensively used in wind energy conversion systems WECS [1, 12, 15, 16].

In fact, using this concept, the electronic power converters are designed only at 25 to 30% of the generator capacity [12]. Therefore, from an economic point of view, this technology is more attractive compared to others (PMSG) [9, 11, 17]. Moreover, a study demonstrates that the DFIG topology presents 50% of the wind power market [10, 15].

Instead of DFIG based WECS in grid connected operation mode, a very little consideration has been paid towards the stand-alone strategy where consumers are totally disconnected from the distribution network [12]. However, in the last few decades, the availability of electricity problem in remote areas has created opportunities to exploit renewable energy sources to feed isolated loads [15]. Therefore, the implementation of stand-alone or isolated power systems can handle the rural electrification for small or medium power consumers located far from the distribution grid by providing sustainable and reliable energy supply [10, 15, 18].

Owing to the large extension of the doubly fed induction generator in isolated power systems as a primary power source generator for handling the electrification requirements of numerous isolated consumers worldwide, stand-alone wind power systems based on DFIG have become one of the most promising used technologies [10, 15]. From this perspective, the modeling and the control of WECS based DFIG have attracted extensive research efforts [2, 10, 15, 16].

Under variations of wind speed and power demand, the stator output voltage and frequency are no longer constant [12]. Highly fluctuating and unpredictable wind generation can have consequences in terms of system stability and robustness [19]. In fact, the DFIG based on WECS has strong nonlinearities [20] and the stability of power system is confronted to new challenges [21]. Many kinds of control strategies are studied and developed in literature [3, 5, 8, 10, 15].

In [8], the vector control strategy is used for the purpose of control both the active and reactive powers. In fact, to ensure the optimal operation mode, a strategy based on Adaptive fuzzy gain scheduling of the PI controller is developed.

In [10], the application of the DFIG for an isolated wind power system is examined to supply the remote area using Double-fed Induction Generator. The objective of this study is to supply different loads such as balanced, unbalanced and nonlinear loads. This study investigates the application stator/load side converter for load harmonics mitigation in the studied system. The shunt active power filter function is added to the convention control scheme of the load/stator-front voltage source converter so as to improve load harmonics. A simple technique for rotor side converter is invested to regulate Voltage and Frequency at stator/load terminals.

In [15], authors developed a speed-sensor less control strategy for a stand-alone doubly fed induction generator supplying energy to an isolated load. This technique is based on the root mean square (rms) detection. This developed direct voltage control method is applicable for not only the balanced and unbalanced load but also for standalone and grid connected mode. The control of load side converter is beyond the scope of this paper and only a diode rectifier is used for the purpose.

In [18], using fuzzy PI controller, the authors have described a control strategy for variable speed wind turbine based on DFIG. The main goal of this work is to analyze, apply and compare two kinds of controllers such as classical and Fuzzy PI. *Nonlinear Control Strategies of an Autonomous Double Fed Induction Generator Based Wind… DOI: http://dx.doi.org/10.5772/intechopen.94757*

In [19], a technique of terminal voltage build-up and the control of a stand-alone WECS based on DFIG is described. This technique is based mainly on the pitch control of the wind turbine. The active and reactive output powers are controlled and maintained equal to their reference values under sudden perturbations of wind speed and/or load variations.

In this chapter, an improved structure of a variable speed stand-alone WECS based on DFIG is proposed. In this context, a general model of the wind turbine is displayed. A detailed analysis of the autonomous DFIG for transient stability analysis is performed. The main goal of the present work is to control the stator outputs voltage and frequency as well as to maintain them within permissible operational limits (220 V/50 Hz) under wind speed and load demand variations on the one hand and to ensure the rotor side converter security on the other hand. Accordingly, to achieve these purposes, three types of controllers have been explored, modeled and integrated into the global system: The classical PI controller, a Back-Stepping and a Sliding mode controller. Besides, a management strategy is suggested to guarantee the rotor side power under 30% of the DFIG nominal power. The implementation of the overall system and different controller designs with the management strategy is achieved using Matlab/Simulink environment. In fact, simulation performances analysis of the stand-alone DFIG based WECS using the classical PI controller, back-stepping and sliding mode controllers are exhibited and discussed. A comparison between different controller process performances under sudden variation of load and wind speed disturbances is presented.

The remaining parts of this paper are organized as follows. Section 2 depicts different system component models. Section 3 describes various controller designs. Section 4 highlights the controllers management strategy. Section 5 demonstrates and compares simulation results of used controllers. Finally, Section 6 presents some drawn conclusions.

### **2. System modeling**

The simplified schematic of the studied system is shown in **Figure 1**. It consists of a stand-alone double fed induction generator driven by a variable speed wind turbine through a gearbox. The stator of the machine is directly connected to an isolated three phase resistive inductive load (*RLp*, *LLp*). In fact, in order to make the stator outputs voltage and frequency independent from the load demand changes as well as the rotational speed variations, the rotor of the machine is supplied through a rotor side controller followed by a rotor side converter. The modeling of different components is presented and explained subsequently.

**Figure 1.** *Block diagram of the autonomous WECS based on DFIG.*
