Reducing stator current harmonics for a doubly-fed induction generator connected to a distorted grid
Pierce, Seth J.
Julian, Alexander L.
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The Department of Defense (DoD) is increasing its demand for reliable renewable energy sources. The doubly-fed induction generator (DFIG) is widely used to extract electrical energy from wind and is a useful means for the DoD to achieve its renewable energy goals. The DFIG is susceptible to electric grid voltage harmonics, which is a potential obstacle for implementing stable wind-energy systems. Two existing rotor voltage controllers are modeled in this thesis for eventual implementation in a laboratory DFIG wind energy system. The first controller uses multiple proportional-integral (PI) controllers to filter out the fifth and seventh stator current harmonics. Each PI controller operates in a reference frame that rotates in synchronicity with the harmonic that is being filtered. The second controller operates in the synchronous reference frame and simultaneously filters both the fifth and seventh stator current harmonics using a double integrator called a proportional-resonant controller (PR). The PI controller is shown to be more effective at eliminating the stator current than the PR controller but has a slower reaction time. The PR controller requires fewer computations but has more stability concerns. Both controllers reduce torque oscillations resulting from the grid distortion by approximately the same amount.
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