Experimental Validation of Virtual Inductance for Synchronization of Grid-Forming Converters

Grid-forming converters provide a voltage and frequency to the grid and can thus be used to enable the temporary islanded operation of distribution grid structures, e.g. during contingencies. When operating several grid-forming converters in parallel, they maintain synchronization by a supplementary controller, as e.g. a conventional droop controller. However, the droop control is designed for grids with a high line reactance to resistance ratio, which is not valid at the low voltage level. As a consequence, grid-forming converters might not be able to synchronize or maintain synchronization to the grid. This paper presents test results from a laboratory setup with a real low voltage microgrid validating the inability of two grid-forming converters to synchronize. To overcome this issue, a virtual output inductance, which effectively increases the line inductance, is implemented in the converter controllers and validated in the test setup. The measurement results prove that the additional inductance diminishes the synchronizing interactions and enables the successful synchronization of the converters. Furthermore, it is shown that the tuning of the inner converter control loops can significantly impact the effectiveness of the virtual inductance during transients. Copyright (c) 2024 The Authors.