OptiQU: Coordinated Multi-Level Voltage and Reactive Power Control for Enhanced Voltage Quality and Secure Grid Operation. - Irene Hammermeister; Eric Tönges; Nils Bornhorst; Johannes Heid; Gabriela Fritzler; Timo Rehwald; Andrea Schoen; Ronald Halbauer; Jan Meschede; Michael Kramer; Julia Holl; Sina Straußberger; et al. (CoRR, 2026)