  This paper focuses on the challenging task of bottom-up restoration in a
complete blackout system using Grid-forming (GFM) converters. Challenges arise
due to the limited current capability of power converters, resulting in
distinct dynamic responses and fault current characteristics compared to
synchronous generators. Additionally, GFM control needs to address the presence
of unbalanced conditions commonly found in distribution systems. To address
these challenges, this paper explores the black start capability of GFM
converters with a generalized three-phase GFM droop control. This approach
integrates GFM controls individually for each phase, incorporating
phase-balancing feedback and enabling current limiting for each phase during
unbalanced faults or overloading. The introduction of a phase-balancing gain
provides flexibility to trade-off between voltage and power imbalances. The
study further investigates bottom-up black start operations using GFM
converters, incorporating advanced load relays into breakers for gradual load
energization without central coordination. The effectiveness of bottom-up black
start operations with GFM converters, utilizing the generalized three-phase GFM
droop, is evaluated through electromagnetic transient (EMT) simulations in
MATLAB/Simulink. The results confirm the performance and effectiveness of this
approach in achieving successful black start operations under unbalanced
conditions.
