Accelerated Transformer Energization Sequence for Inverter Based Resources in Black-Start Procedures with Active Flux Trajectory Manipulation in the Stationary Reference Frame
Jiyu Lee, Shenghui Cui
TL;DR
The paper addresses transformer flux offset and inrush during black-start of grid-forming converters by exploiting inverter voltage programmability to actively prevent offset rather than merely limiting inrush. It introduces two soft-magnetization strategies—the ultra-fast constant-vector approach and an Archimedean spiral voltage trajectory—alongside a demagnetization sequence to handle residual flux, and analyzes the flux dynamics in the stationary frame using Clarke transformation with $\lambda_0 = \hat{V}/\omega_0$ and $T_D = 1/\omega_0$. The approaches yield offset-free magnetization, with the Archimedean spiral further mitigating LC-filter surge, and the demagnetization step ensures reliable startup under residual-flux conditions. Experimental validation on a 5 kVA GFM with an LC filter demonstrates start within a few milliseconds and safe operation of the power electronics, highlighting the practical impact for fast, semiconductor-safe black-start in next-generation inverter-based grids.
Abstract
This paper proposes advanced soft-magnetization techniques to enable ultra-fast and reliable black-start of grid-forming (GFM) converters. Conventional hard-magnetization with well-established three-phase voltages during transformer energization induces severe inrush currents due to flux offset, which can damage power semiconductor devices. To overcome this drawback, an ultra-fast soft-magnetization method is firstly introduced, leveraging the voltage programmability of the inverter to actively reshape the initial voltage profile and thereby eliminate flux offset of the transformer core. By suppressing the formation of flux offset itself, the proposed approach prevents magnetic saturation and achieves nominal terminal voltage within a few milliseconds while effectively suppressing inrush current. However, this method can still trigger surge currents to power semiconductor devices in the presence of an LC filter due to abrupt voltage magnitude and phase transitions. To address this issue, an enhanced Archimedean spiral soft-magnetization method is developed, where both voltage magnitude and phase evolve smoothly to simultaneously suppress inrush and surge currents. Furthermore, residual flux in the transformer core is considered, and a demagnetization sequence using the inverter is validated to ensure reliable start-up. Experimental results confirm that the proposed methods achieve rapid black-start performance within one fundamental cycle while ensuring safe and stable operation of GFM converters.