The growing demand for power and increased environmental concerns gave an impetus to the growth of clean and renewable energy sources like wind, solar etc. There is a remarkable increase in the penetration of wind energy systems in the last decade and this trend is bound to increase at a much faster pace in future. This ever increasing penetration of wind power generating systems pose multi-fold challenges related to operational and stability aspects of the grid. Present day wind energy systems mostly comprise of variable speed wind generators. A large fraction of present day variable speed wind turbine generators use doubly fed induction machine (DFIM). This thesis deals with modeling and grid coordination aspects of variable speed wind gener- ators. In particular, the short coming of the existing steady state equivalent circuit of a DFIM is identified and subsequently, an accurate equivalent circuit of a DFIM is proposed. Relevant mathematical basis for the proposed model is presented. The proposed steady state equivalent circuit of a doubly fed induction machine is further validated using dynamic simulations of a standalone machine. Based on the proposed equivalent circuit, two approaches for computing the initial values of state variables of a DFIM is proposed. The first approach is a linear formulation where the losses due to resistance of the stator and rotor windings are neglected. The second approach is a non-linear formulation which takes the losses into consideration. Further, analysis is carried out on grid connected doubly fed induction generators (DFIG). A framework to incorporate DFIG based variable speed wind farms in the steady state power flow analysis is proposed. The proposed framework takes into consideration important aspects such as voltage dependent reactive power limits and mode of reactive power control of associated converters. Some of the challenges in a grid connected DFIG especially during su- persynchronous mode of operation are identified. The advantages of a non-Maximum Power Point Tracking (MPPT) mode of operation under certain operating conditions is highlighted. Finally, aspects pertaining to coordination of grid connected variable speed wind generators are studied. A trust region framework to determine the reference values to the control loops of converters in a variable speed wind generator is proposed. The proposed framework identifies the reference values considering other reactive power controllers in the grid. Moreover, the proposed framework ensures that the steady state voltage stability margin is maximized. On the computational front, trust region algorithms ensure global convergence. The mathematical models and initialization algorithms proposed in this thesis are tested on standalone systems under various control scenarios. The algorithms proposed to incorporate a grid connected DFIG in steady state analysis tools have been tested on a sample 6-bus system and a practical 418-bus equivalent system of Indian southern grid.

Recent decades have witnessed a significant increment in power contribution from wind generators. This increment in penetration requires power engineers to tackle multi-fold challenges concerning operational and stability aspects. There exists a significant attention among the researchers in analyzing the impact of wind generation on various system aspects. This thesis focuses on steady state voltage stability aspects with penetration of Variable speed wind generators. Traditionally, ancillary services are supplied by large conventional generators. However, with the huge penetration of wind generators as a result of the growing interest in satisfying energy requirements, and considering the benefits that they can bring along to the electrical system and to the environment, it appears reasonable to assume that ancillary services could also be provided by wind generators in an economical and efficient way. Certain types of wind generators can support reactive power for the Power Market. Fixed and Semi-Variable speed wind turbine generators were predominantly employed during the early installations of wind generators. These units require reactive power support from the grid and are usually equipped with capacitor banks to provide the necessary reactive power. Further, STATic synchronous COMpensator (STATCOM) and Static Var Compensator (SVC) with various configurations were proposed to enhance the system operations under normal and disturbed conditions. On the other hand, Variable speed wind turbine generators provide flexibility in control and hence are becoming increasingly popular. Popular among this class of wind turbine generating units are Doubly Fed Induction Generator(DFIG) and induction/synchronous Generator with Front End Converter(GFEC). Contrary to Fixed and Semi-Variable speed wind machines, Variable speed wind turbine generators are capable of providing reactive power to the grid. The converter and control schemes associated with these machines permits controlling the active and reactive power output to desired level. It is possible to control the reactive power output of these machines independently of the active power. Researchers in the past have investigated the impact of reactive power output of variable speed wind machines on system stability. In literature, approaches are proposed to utilize the flexibility in reactive power generation of DFIG to reduce system losses, improve reliability in static and dynamic system operation. Approaches in literature investigate the impact on voltage stability of system by considering the flexibility of reactive power output of wind machine in isolation. However, significant improvement in static voltage stability, voltage profile, system power losses etc. can be attained if the Reactive Power output of Variable Speed wind machines can be properly coordinated with other reactive power controllers. The prime objective of the thesis is to propose algorithms to coordinate the reactive power output of Variable Speed Wind Generators with other reactive power controllers for Enhancement in voltage stability margin, system losses and voltage profiles.