Current Limiting of Virtual Synchronous Machines on Unbalanced Faults Considering Grid Protection

Lately, grid-forming converter (GFC) control attracts a lot of attention. Looking at decreasing synchronous generation in electrical energy systems worldwide, this key technology may provide similar auxiliary services for stable grid operation as synchronous generators. In this context, further urgent issues need to be addressed in a targeted manner, such as the coordination of GFC and grid protection. Expected steps to ensure safe operation contain, first, a definition and standardization of the behavior of the GFC directly after fault occurrence regardless of the manufacturer, second, an adjustment of the setting of the protection devices and third, development of new functions for the protection devices to achieve the highest possible selectivity. Dealing with the first step, the contribution of the short-circuit current and the harmonic content of the currents and voltages injected into the network during the event of a fault are decisive for the correct operation of numerical selective protection relays. In this context, the degrees of freedom in the inverter control allow several possibilities to deal with the current limitation during short circuits in the grid. As a result, our short circuit investigation favors an expanded GFC current limiting method that ensures the trouble free grid operation of the grid protection devices.