Elimination of System-Induced Torque Pulsations in Doubly-Fed Induction Generators Via Field Reconstruction Method

Effects of system unbalance and system harmonics on the operation of doubly-fed induction generator (DFIG) used in wind energy harvesting are of great concern. This is primarily due to the fact that system unbalance and harmonics can generate unwanted torque undulations that can potentially undermine the mechanical integrity of the tower, and reduce the lifetime of the moving components that are attached to the generator shaft. This paper focuses on the development of a solution for the above problem by judicious selection of the rotor currents to actively eliminate/mitigate these undesirable vibrations. The enabling technology for optimal calculation of the rotor currents is based on the field reconstruction method (FRM). FRM is an analytical tool for the approximation of the magnetic field distribution in the middle of the air gap. Once the FRM formulation is setup, it is capable to predict the tangential/normal components of the magnetic forces. In this paper, the FRM is applied to compute the rotor phase currents in lieu of the availability of the real-time stator currents such that the resultant field will generate a smooth torque.