Integrated Harmonic Mitigation and Fault Current Limiting for Grid-Forming Inverters

In microgrids with high penetration of power electronics-interfaced sources, harmonic distortion from nonlinear loads degrades the power quality at the terminals of virtual synchronous generators (VSGs). Conventional impedance-reshaping methods mitigate harmonics by reducing output impedance but often exacerbate fault currents during voltage sags. This letter proposes an integrated control strategy for harmonic mitigation and fault current limiting based on Virtual Forced Excitation Control (VFEC). The scheme extracts harmonic components from the point of common coupling (PCC) voltage and directly superimposes them onto the amplitude command of the internal electromotive force (EMF) of the VSG. Under normal operation, this mechanism reduces the equivalent output impedance at harmonic frequencies, thereby attenuating nonlinear harmonic content and improving PCC voltage quality. During grid faults, the same architecture leverages inherent amplitude limiting to prevent overmodulation without control mode transitions or auxiliary circuits, effectively restricting fault currents to levels comparable to conventional VSGs and significantly below those of impedance-reshaping approaches. The proposed strategy ensures full compatibility with standard VSG control frameworks. Experimental results validate its effectiveness in both grid-connected and islanded microgrids.