A Fundamental and Harmonic Frequencies Decoupled Control Scheme to Improve the Output Current Quality of Grid-forming Inverters
Bo Yang, Hao Li, Liang Shan, Longhai Wei, Shuai Lu
Abstract
Grid-forming inverters (GFI) are used to retain the grid inertia and frequency stability with increasing inverter-based sources in the grid. However, as GFI is essentially a fundamental frequency voltage source, grid voltage distortion would induce significant grid current harmonics. In this article, the limitations of the existing types of GFI harmonic current suppression methods are analyzed from a perspective of the equivalent circuit model at harmonic frequencies. Based on this perspective, an intuitive fundamental and harmonic frequencies decoupled control (FHDC) scheme is proposed. At the fundamental frequency, FHDC tracks the voltage command from the GFI power control loop. At harmonic frequencies, the FHDC eliminates the grid-side current harmonics, by canceling out the capacitor current harmonics and eliminating the harmonics otherwise introduced into the inverter-side current command. Compared with existing methods, FHDC is more effective, and also adaptive to wide ranges of grid impedances; meanwhile, it needs no additional sensors and hardware, making it applicable to existing GFI hardware. Finally, in the experiment, the proposed FHDC is not only compared with the conventional GFI, the existing GFI harmonic suppression methods are also implemented in lab test to comparatively verify the superiority of the FHDC.