Performance Analysis of Doubly-fed Induction Generator in Wind Energy Conversion System

Abstract

The statistical data conveys that Doubly-fed Induction Generator (DFIG) based wind turbine with variable speed and variable pitch control is the most common wind turbine in the growing wind market. This machine is usually used on the grid connected wind energy conversion system to satisfy grid code requirements such as grid stability, fault ride through (FRT), power quality improvement, grid synchronization and power control etc. Though the requirements are not fulfilled directly by the machine, the control strategy is used in both the stator as well as rotor side along with power electronic converters to fulfill the requirements stated above. To satisfy the grid code requirements of wind turbine, usually grid side converter is playing a major role. So in order to improve the operation capacity of wind turbine under critical situation, the intensive study of both machine side converter control and grid side converter control is necessary. The system uses two back-to-back converters in the rotor circuit. The basic concept behind the total system (DFIG with back-to-back converters) is that the machine-side converter controls the active and reactive power by controlling the d-q components of rotor current (i.e. idr and iqr), while the grid-side converter controls the dc-link voltage and ensures the operation at unity power factor by making the reactive power drawn by the system from the utility grid to zero. By means of a bidirectional converter in the rotor circuit the DFIG is able to work as a generator in both sub-synchronous and super-synchronous modes. In this thesis the performance of the DFIG system is analyzed under grid voltage fluctuations. The voltage fluctuations are made by lowering and raising the voltage values in the utility grid intentionally for the purpose of simulation keeping in view of different grid disturbances. The effect of change in wind speed and change in supply frequency are also taken into consideration for the performance analysis of DFIG. The analysis is also done by changing the demand of reactive power of machine. The theoretical results are obtained from the MATLAB/ SIMULINK environment. The experimental validation is made using Real Time Distributed Simulation package (RT-LAB).