The probable source of certain spurious frequencies found in the output of a variable speed generating system using slip recovery

As part of US-Department-of-Energy-sponsored research on wind energy, a Mod-0 wind turbine was used to drive a variable-speed, wound-rotor, induction generator. Energy resulting from the slip frequency voltage in the generator rotor was rectified to DC, inverted back to utility frequency AC, and injected into the power line. Spurious changing frequencies displayed in the generator output by a spectrum analyzer are caused by ripple on the DC link. No resonances of any of these moving frequencies were seen in spite of the presence of a bank of power-factor-correcting capacitors. The data collected doing the Mod-0 tests have yielded some interesting spectrograms of generator current showing the expected odd harmonics of a line-commutated inverter. Also present, however, are other components of changing frequency. This work discusses the relation of some of these harmonics to the changing rotor speed