From Liability to Asset: A Three-Mode Grid-Forming Control Framework for Centralized Data Center UPS Systems

AI workloads are turning large data centers into highly dynamic power-electronic loads; fault-time behavior and workload pulsing can stress weak-grid points of interconnection. This paper proposes a centralized medium-voltage (MV) uninterruptible power supply (UPS) control architecture implemented as three operating modes: Mode 1 regulates a DC stiff bus and shapes normal-operation grid draw, Mode 2 enforces current-limited fault-mode P--Q priority with UPS battery energy storage system (UPS-BESS) buffering and a rate-limited post-fault "soft return," and Mode 3 optionally provides droop-based fast frequency response via grid-draw modulation. Fundamental-frequency averaged dq simulations (50 MW block, short-circuit ratio (SCR) = 1.5, 0.5 p.u. three-phase dip for 150~ms) show zero unserved information-technology (IT) energy (0.00000 MWh vs.0.00208 MWh for a momentary-cessation benchmark), a 0.57 p.u. peak inverter current (vs. 1.02 p.u. for a synchronous-reference-frame phase-locked loop (SRF-PLL) low-voltage ride-through (LVRT) baseline), a nonzero mean fault-window grid draw of 0.20~p.u. (vs.approx 0 for momentary cessation), and an improved settled point-of-common-coupling (PCC) voltage minimum of 0.79 p.u. after one cycle (vs. 0.66 p.u.). A forced-oscillation case study applies a 1 Hz pulsed load (+/- 0.25 p.u.) and shows that the normal-operation shaping filters the oscillation seen by the grid while the UPS-BESS buffers the pulsing component.