Harmonic analysis of a Doubly Fed Induction Generator

The aim of this paper is to develop a framework for analysis of harmonics in a Doubly Fed Induction Generator (DFIG) caused by non-sinusoidal conditions in the rotor as well as unbalance in stator. Non-sinusoidal rotor voltages are decomposed into harmonic components and their corresponding sequences are identified. Then induced harmonics in rotor and stator are analyzed and computed, from which the torques produced by these interactions between stator and rotor harmonic components can be found. During unbalanced stator conditions, symmetric component theory is applied to the stator voltage to get positive-, negative and zero- sequence components of stator and rotor currents. The steady-state negative sequence equivalent circuit for a DFIG is derived based on reference frame theory. Harmonic currents in the rotor are computed based on the positive and negative sequence circuits. In both scenarios, the harmonic components of the electromagnetic torque are calculated from the interactions of the harmonic components of the stator and rotor currents. Three case studies are considered, namely, (i) non-sinusoidal rotor injection, (ii) an isolated unbalanced stator load scenario and (iii) grid-connected operation. The analysis is verified with results obtained through numerical simulations in Matlab/Simulink. The second case is verified using experiments. The simulation results and experimental results agree well with the results from analysis.