Investigation of Voltage Dip Problems during Faults on a Grid-Tied Doubly Fed Induction Generator in a Wind Energy System

A voltage dip is a sudden drop of voltages - generally between 10 and 90 % of the rated RMS value during a period lasting from half a cycle to a few seconds on the phases of the power lines. It is one of the most important power quality problems affecting the stability of the Doubly Fed Induction Generator (DFIG) in Wind Energy Conversion System (WECS) and hence needs to be analyzed for a given machine for its performance analysis under grid disturbances. When a voltage dip problem happens in the given power as of faults, the magnitude of the rotor and stator currents of DFIG get increased, and hence disturbs steady state operation of the system. Therefore, in this paper, the worst voltage dip of 90 % is tested for grid faults on a 5 kW DFIG to validate its performance under this phenomenon. Based on the results of the simulation - along with its experimental validation - the machine is found to be robust for faults staying a shorter period without disconnecting it from the grid. On the other hand, operating a machine for a longer period while keeping it connected to a grid during a heavy dip, may result in its degradation as the rated limits are violated. To rectify and mitigate the aforesaid problems, a control system - with and without a crowbar - is developed for symmetrical faults to tackle such a problematic situation, and to discuss its fulfillment of the current grid code requirements. Finally, a complete model of the DFIG coupled with the grid is developed, modeled, analyzed and simulated using MATLAB/Simulink user-defined function toolbox block.