PLL-free VM-DPC stabilizes data center UPS on weak grids

Data centers are increasingly exposed to weak-grid conditions, driven by converter-dominated power networks and the surge of dynamic AI workloads. In centralized uninterruptible power supply (UPS) architectures, the front-end rectifier continuously processes incoming facility power, making its control strategy critical. Conventional phase-locked loop (PLL)-based proportional-integral (PI) rectifiers struggle under these conditions: strong interactions between converter control dynamics and grid impedance can trigger instability, especially when the short-circuit ratio (SCR) drops to 2 or below. This paper from researchers including Jin Dong and Yilu Liu investigates these failure modes using a detailed switching-level MATLAB/Simulink model, validated in real time on an OPAL-RT platform.

The proposed solution is a voltage-modulated direct power control (VM-DPC) strategy with adaptive reactive-power support. Unlike traditional PLL-based methods, VM-DPC directly regulates active and reactive power without grid synchronization, dynamically supporting the point of common coupling (PCC) voltage during rapid IT load swings. Results show that conventional PI control becomes unstable under SCR ≤ 2, causing DC-link oscillations and degraded downstream power delivery. The VM-DPC approach restores stable operation, improves system damping, and maintains reliable power transfer to highly dynamic IT loads. This means data centers can operate reliably on weaker, renewable-heavy grids without costly hardware upgrades—critical for AI-driven facilities that need steady, high-quality power even as grid conditions fluctuate.