IRACST – Engineering Science and Technology: An International Journal (ESTIJ), ISSN: 2250-3498, Vol.2, No. 4, August 2012 603 Modeling of a Grid compatible Variable Speed Wind Turbine with direct drive Synchronous Generator R.Jeevajyothi EEE Department Kalasalingam University Virudhunagar, India D. Devaraj EEE Department Kalasalingam University Virudhunagar, India Abstract— This paper presents the modeling and simulation of a grid compatible variable speed wind turbine (VSWT) with direct drive synchronous generator (DDSG) and power electronics interface. Models and equations of wind turbine, DDSG with field excitation controller and power electronics controller interface are presented and their implementations are explained. Controllable voltage source converter(VSC) with hysteresis band current control are utilized for capturing the maximum power under variable speed operation and maintaining reactive power generation at a desirable level. The control strategy of this model can be employed to regulate the real power, reactive power, generated voltage and generated speed at different wind speeds in the power system. Simulation results are presented which provides the control performance and dynamic behavior of a VSWT with DDSG. Keywords- VSWT, DDSG, VSC with hysteresis band current control, maximum power capture, reactive power control. I. INTRODUCTION Wind energy is a reliable, natural and renewable electrical power supply. The high installed capacity of today’s wind turbines and decreasing plant costs have shown that wind power can be competitive with conventional, more heavily polluting, fuels in the long term. In terms of wind power generation technology, as a result of numerous technical benefits (higher energy yield, reducing power fluctuations and improving VAR supply) the modern MW-size wind turbines always use variable speed operation which is achieved by power electronic converters. Interconnecting large wind farms to power grids and the relevant influences on the host grids need to be carefully investigated. To increase the maximum power extraction the variable speed generators are employed. These variable speed generators necessitate AC-DC-AC conversion systems [1] . The modeled system includes a fixed- pitch, stall regulated wind turbine, a DDSG and a controllable power electronics system, which consists of an uncontrolled rectifier and VSC with hysteresis band current control. The purpose of modeling is to support the project in its architecting effort. The project purpose is always to realize a system in its context. A good system is a system that fits in its context and that is appropriate for its purpose. The schematic diagram of the modeled VSWT driven synchronous generator is shown in Figure 1. Figure 1. Schematic diagram of modeled VSWT with DDSG II. SUBSYSTEM MODELS The VSWT model consists of the following components. - Wind turbine and control - DDSG - Rectifier and inverter A. Wind Turbine The wind turbine is described by the following equation (1) (2) and (3) where λ  is the tip speed ratio, ω M is the mechanical speed of wind turbine in rad/sec., R is the blade radius in m, V W is the wind speed in m/sec., P M is the mechanical power from wind turbine in kW, ρ is the air density in kg/m 3 , C P is the power coefficient and T M is the mechanical torque from wind turbine in N-m. W M V R ω = λ (1) W P 2 M V C R 2 1 P ρπ = (2) 3 2 5 2 1 λ ω ρπ ω M P M M M C R P T = = (3) The mechanical torque obtained from equation (3) enters as the input torque to function of the tip speed ratio (TSR) λ given by equation (2).