Accelerated Transformer Energization Sequence for Black-Start by Grid-Forming Inverters With Active Flux Trajectory Shaping in the Stationary Reference Frame

This paper proposes an accelerated, inrush-free transformer energization method for black-start by grid-forming (GFM) inverters that overcomes the inherent low speed of conventional soft-energization (SE) based on voltage ramp-up. In conventional SE, the ramp-up duration is often conservatively selected due to large transformer electromagnetic time constants and unknown post-blackout residual flux. In contrast, the proposed method actively suppresses magnetic-flux bias instead of relying on its passive decay. To this end, an inverter-based demagnetization step is first implemented to eliminate the unknown residual flux. Then, the inverter terminal voltage is synthesized to generate a time-optimal, bias-free magnetic-flux trajectory. To enable systematic trajectory design, the three-phase voltage and magnetic flux are represented as vectors in the stationary reference frame (SRF), providi…