An Analysis of Grid-Forming Inverters in Blackstart Applications During Transformer Energization Sequence

The increasing penetration of inverter-based resources (IBRs) necessitates a reassessment of current grid practices. Blackstart is one specialized area that has drawn renewed attention due to the capability of grid-forming inverters (GFMIs) to emulate synchronous generators (SG). The ability of IBRs to carry out a blackstart represents a novel concept necessitating deeper investigations, particularly given the constraints of IBRs such as over-current limitations, which affect the energization of non-linear network devices. This paper presents a blackstart case study that uses a GFMI, focusing on the energization of a large unloaded plant transformer. Consequently, a unique inrush is demonstrated, known as a sympathetic inrush between the plant and the inverter medium-voltage (MV) transformer. Subsequently, an alternative method for the execution of soft energization (SE) is introduced using a power plant controller (PPC), eliminating inverter-level control modifications. Furthermore, the impact of employing the SE method on inrush currents is evaluated using the proposed method, and a qualitative analysis is lastly presented comparing SE to the hard switching (HS) blackstart method. Collectively, this paper aims to demonstrate the GFMI's ability, using both the SE and HS methods, to energize transformers which play an important role during a blackstart.