Safe Reconnection Time for Large-Scale Data Center Loads: An Analytical Framework for Transient Stability Assessment

Abstract

The rapid growth of large, power-electronics-rich data center (DC) loads is creating new operational challenges for bulk power systems. A key risk arises when a DC uninterruptible power supply (UPS) disconnects the facility during voltage/frequency disturbances and then reconnects it while the bulk grid is still dynamically settling to a new equilibrium point. Poorly timed reconnection can amplify electromechanical oscillations, deepen frequency deviations, and lead to repeated connect-disconnect \emph{flapping}. In this paper, we develop an analytical framework to characterize the \emph{safe reconnection time} for large DC loads after a disturbance-induced disconnection that avoids flapping. Using a model in the spirit of the classical single-machine infinite-bus system, we capture (i) swing dynamics during the disconnection interval and (ii) voltage-angle coupling at the load bus, which determines the electrical power step at reconnection under constant-power load assumptions. Using energy function method, we characterize the critical safe reconnection time such that for any reconnection time after the critical safe reconnection time, the post-reconnection trajectory is guaranteed to remain within operational limits (frequency/angle/voltage) and converge to the post-reconnection equilibrium, thereby preventing flapping. Time-domain simulations validate the effectiveness of the proposed analytical approach. The results provide a simple, physics-informed criterion that can be used to bound reconnection windows for large DC facilities and inform UPS reconnection logic.