Modeling a Grid-Forming Inverter Dynamics Under Ground Fault Scenarios Using Experimental Data From Commercially Available Equipment

In order to address the recent inclement weather-related energy events, electricity production is experiencing an important transition from conventional fossil fuel based resources to the use of Distributed Energy Resources (DER), providing clean and renewable energy. These DERs make use of power electronic based devices that perform the energy conversion process required to interface with the utility grids. For the particular cases where DC/AC conversion is required, grid-forming inverters (GFMI) are gaining popularity over their grid-following (GFLI) counterpart. This is due to the fact that GFMI do not require a dedicated Phase Locked Loop (PLL) to synchronize with the grid. The absence of a PLL allows GFMI to operate in stand-alone (off-grid) mode when needed. Nowadays, inverter manufacturers are already offering several products with grid-forming capabilities. However, modeling the dynamics of commercially available GFMI under heavy loads or faults scenarios has become a critical task not only for stability studies, but also for coordination and protection schemes in power grids (or microgrids) that are experiencing a steady growth in their levels of DERs. Based upon experimental low-impedance fault results performed on a commercially available GFMI, this paper presents a modeling effort to replicate the dynamics of such inverters under these abnormal scenarios. The proposed modeling approach relies on modifying previously developed GFMI models, by adding the proper dynamics, to match the current and voltage transient behavior under low-impedance fault scenarios. For the first inverter tested, a modified CERTS GFMI model provides matching transient dynamics under faults scenarios with respect to the experimental results from the commercially available inverter.