Investigation into the Mechanisms of Negative Sequence Current Distribution Among Grid-Forming Sources

Distribution-level grids such as microgrids may contain multiple grid-forming (GFM) sources, such as inverter-based resources with GFM inverters and synchronous generators (SGs). As load unbalance is commonly seen in distribution grids, negative-sequence (NS) currents from the unbalanced load need to be provided by GFM sources. With different source combinations, the NS current sharing among GFM sources can be significantly different. An inappropriate NS current distribution may decrease the source's energy usage efficiency or cause malfunction of the sources. Therefore, analyzing the NS current distribution among different sources is critical for system planning, which is lacking in the existing literature. In this paper, a comprehensive analysis of NS current distribution among different GFM sources is conducted, considering different GFM source types. An analysis method is proposed to compare different distribution conditions, and source selection guidelines are proposed. The analyses are first verified through simulation and then experimentally demonstrated on a converter-based hardware testbed.