In back-to-back converter systems used with Doubly-Fed Induction Generators (DFIG), managing harmonics generated by the rotor-side and grid-side converters is a notable challenge, as the harmonic spectra produced by each converter differ significantly due to their distinct operating characteristics. The rotor-side converter benefits from the generator’s inherent inductance for harmonic mitigation, while the grid-side converter requires carefully designed LCL filters to meet power quality standards.

Due to these differing filtering characteristics, it is feasible to adopt separate switching frequencies for each converter. Specifically, while the switching frequency of the grid-side converter is typically fixed to match the design of the grid filter, the machine-side converter allows for more flexibility in frequency adjustment.

To enhance system efficiency, the switching frequency of the generator-side converter is reduced from 3000 Hz to 2000 Hz, thereby lowering switching losses. However, this reduction leads to an increase in harmonic distortion—particularly in frequency ranges where filtering is less effective.

To mitigate the resulting harmonic content, a sinusoidal Periodic Carrier Frequency (PCF) modulation technique is introduced. This method imposes a controlled “wobble” on the carrier frequency over time, causing harmonic energy to cluster naturally near the spectral edges where the switching frequency reaches its minimum and maximum. While this technique helps in distributing harmonics across the spectrum, it may inadvertently increase low-frequency harmonic energy, where filtering is inherently less effective.

To overcome this limitation, the proposed method leverages resonant interactions between sideband harmonics resulting from the wobble effect and those intrinsic to Space Vector Modulation (SVM). By selecting switching frequencies that are integer multiples of the fundamental frequency—specifically three and six times—the harmonic spectrum can be shaped more advantageously. The key innovation lies in the resonant synchronization of the wobbling frequency at six times the fundamental (6f₀) with precise phase alignment to the base waveform. This configuration effectively redirects harmonic energy away from the critical low-frequency region and into higher frequency bands, which are more amenable to attenuation by the generator’s natural inductance.

The study is based on original simulation models and spectral analysis of a 5.7 MW DFIG system. Multiple PCF parameters—including modulation depth and phase angle—were tested to identify optimal conditions for harmonic redistribution. Initial simulations at a 2000 Hz switching frequency (without PCF) exhibited severe harmonic violations, with magnitudes reaching 240% of grid code limits. Conventional PCF modulation proved insufficient, as harmonic energy remained concentrated in low-frequency bands with inadequate filtering.

In contrast, the proposed resonant PCF-SVM approach successfully redistributed harmonic energy into higher frequencies, resulting in full compliance with grid standards. The results demonstrate that this technique not only maintains current waveforms within harmonic limits at lower switching frequencies but also improves overall system efficiency. Furthermore, its straightforward implementation makes it highly practical for enhancing converter performance in large-scale wind energy applications.