Large-Signal Stability Improvement of Parallel Grid-Forming Inverter-Driven Black Start: Preprint

With the rapid increase of inverter-based resources in modern grids, advanced grid-forming (GFM) inverter capabilities are urgently needed to realize the power-electronics dominant grids at scale, such as system restoration and operation under faults. One such capability is inverter-driven black start using GFM inverters. This paper analyzes the ability of two recently proposed advanced GFM controls to help GFM inverters sustain system-wide off-nominal conditions and remain synchronized until they can overcome the momentary overloading as more GFMs join the process without generator sequence coordination or communication and finally stabilize the grid. This work shows that even if pre-synchronization methods are used to obtain a smooth grid connection of GFM inverters, the grid can still become unstable due to system overloading and inverter controller saturation during a black start. Through an extensive set of 1200 full-order electromagnetic transient (EMT) simulations, we evaluate the black start performance of the various GFM inverter controls. The results show that the GFM current limiter and primary control have a significant impact on the stability of the system during dynamic operating conditions and thus the success of an inverter-driven system restoration. GFM control enhancements that improve the stable operation and grid synchronization under nonideal conditions can relax the pre-coordination and communication requirements necessary for black start.