You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: Grid Ancillary Services From Doubly Fed Induction Generator-Based Wind Energy Conversion System: A Review 
Chapter Title:  A. COMMON ELECTRICAL GENERATORS USED IN WIND TURBINES 
1. Abstract of Cited Paper (BIB001):  Adjustable speed induction generators, especially the doubly fed induction generators (DFIG), are becoming increasingly popular due to their various advantages over fixed speed generator systems. A DFIG in a wind turbine has the ability to generate maximum power with varying rotational speed, ability to control active and reactive power by the integration of electronic power converters such as the back-to-back converter, low rotor power rating resulting in low cost converter components, and so on. This study presents an extensive literature survey over past 25 years on the different aspects of DFIG. 
2. Abstract of Cited Paper (BIB002):  Preface. List of Symbols. Acronyms and Abbreviations. 1. Introduction. 1.1 Introduction. 1.2 Overview of Wind Energy Conversion Systems. 1.3 Wind Turbine Technology. 1.4 Wind Energy Conversion System Configurations. 1.5 Grid Code. 1.6 Summary. 2. Fundamentals of Wind Energy Conversion System Control. 2.1 Introduction. 2.2 Wind Turbine Components. 2.3 Wind Turbine Aerodynamics. 2.4 Maximum Power Point Tracking (MPPT) Control. 2.5 Summary. 3. Wind Generators and Modeling. 3.1 Introduction. 3.2 Reference Frame Transformation. 3.3 Induction Generator Models. 3.4 Synchronous Generators. 3.5 Summary. 4. Power Converters in Wind Energy Conversion Systems. 4.1 Introduction. 4.2 AC Voltage Controllers (Soft Starters). 4.3 Interleaved Boost Converters. 4.4 Two-Level Voltage Source Converters. 4.5 Three-Level Neutral Point Clamped Converters. 4.6 PWM Current Source Converters. 4.7 Control of Grid-Connected Inverter. 4.8 Summary. 5. Wind Energy System Configurations. 5.1 Introduction. 5.2 Fixed Speed WECS. 5.3 Variable Speed Induction Generator WECS. 5.4 Variable-speed Synchronous Generator WECS. 5.5 Summary. 6. Fixed-Speed Induction Generator WECS. 6.1 Introduction. 6.2 Configuration of Fixed-Speed Wind Energy Systems. 6.3 Operation Principle. 6.4 Grid Connection with Soft Starter. 6.5 Reactive Power Compensation. 6.6 Summary. 7. Variable-Speed Wind Energy Systems with Squirrel Cage Induction Generators. 7.1 Introduction. 7.2 Direct Field Oriented Control. 7.3 Indirect Field Oriented Control. 7.4 Direct Torque Control. 7.5 Control of Current Source Converter Interfaced WECS. 7.6 Summary. 8. Doubly-Fed Induction Generator Based WECS. 8.1 Introduction. 8.2 Super- and Sub-synchronous Operation of DFIG. 8.3 Unity Power Factor Operation of DFIG. 8.4 Leading and Lagging Power Factor Operation. 8.5 A Steady-State Performance of DFIG WECS. 8.6 DFIG WECS Start-up and Experiments. 8.7 Summary. 9. Variable-Speed Wind Energy Systems with Synchronous Generators. 9.1 Introduction. 9.2 System Configuration. 9.3 Control of Synchronous Generators. 9.4 SG Wind Energy System with Back-to-back VSC. 9.5 DC/DC Boost Converter Interfaced SG Wind Energy Systems. 9.6 Reactive Power Control of SG WECS. 9.7 Current Source Converter Based SG Wind Energy Systems. 9.8 Summary. Appendix A. Per Unit System. Appendix B. Generator Parameters. Appendix C. Problems and Answers Manual.

Literature Review:
Wind turbines can be divided into four main categories. The first category is fixed speed wind turbines. This type uses an asynchronous squirrel-cage induction generator (IG) directly connected to the grid via a transformer. Also, this type of wind turbines cannot be automatically regulated. The second category is partial speed wind turbine with variable rotor resistance. This type of wind turbine uses a wound rotor induction generator which is directly connected to the grid. The slip and output power of this type of wind turbines can be regulated by a series controlled resistance. The third category is variable speed wind turbine with partial-scale power converter. This type of wind turbines is also known as the doubly-fed induction generator (DFIG). It uses a variable speed controlled wind turbine. The rotor phase windings are connected to a back-to-back converter via slip rings whereas the stator phase windings are connected directly to the grid. The speed range of the double-fed induction generator is between −30% and +20% of the synchronous speed. This is due to power converters ability to control the rotor speed by controlling the rotor's frequency. The last category is variable speed wind turbine with full-scale power converter. This type of wind turbines uses a full-scale power converters and a permanent magnet synchronous generator (PMSG). The stator windings are connected to the grid through full scale converters BIB003 . The first two categories of the wind turbines are very simple and maintenance free. However, they have many drawbacks. First, they do not have reactive power compensation. Second their mechanical construction must support high mechanical stress. Third, they require a stiff grid. Finally, the most important disadvantage is that they do not support any speed control. DFIG based wind turbines, third category, have many advantages. Firstly, their construction is simple and cheaper compared to the fourth type. Secondly, by controlling the rotor terminal voltages, they can decouple the control of the active and reactive powers and control power factor as a result. Thirdly, the DFIG wind turbines power converters rating is 30% of the DFIG rated power. This reduces the switching losses, converters cost, filter volume, and harmonic injection into the grid BIB002 . However, double fed induction generators wind turbines have disadvantages such as the need of protection schemes, periodic maintenance of slip-rings and gearbox, complex power control schemes, and limited fault ride through. PMSG based wind turbines, forth category, have many advantages. First, they do not have mechanical stress problems because they can avoid gearbox. Second, they can achieve full speed regulation, and hence maximum power point tracking (MPPT) is possible. Third, they do not need slip-rings; thus, they are more stable than the doubly-fed induction generators turbines. Finally, their control schemes are easy to implement and simple. Despite the advantages fourth category of wind turbines, they also have disadvantages. Firstly, they are heavy and large. Secondly, their power converters have high cost, generate high harmonics, and have high power losses. Thirdly, they use permanent magnet which may demagnetize at high temperature and increase the wind turbine total cost. used in wind turbines. Further information related to different wind generators and their comparisons is available in BIB001 .