Data Center Power System Stability – Part II: System Modeling and Analysis

This is the second part of a two-part paper on stability study of data center power systems by impedance-based methods. As the basis for this application, Part I [1] developed new impedance models for power supplies that are the most dominant loads in data centers. This second part presents system modeling and analysis methods that can support practical data center power system design to ensure stability. The proposed methods comprise: 1) building distribution network modeling by impedance scaling; 2) system modeling and model reduction based on equivalent source impedance; 3) system stability analysis in the sequence domain to include zero-sequence dynamics; and 4) expansion of system models and analyses to account for network asymmetry and uneven loading. These methods are used to characterize practical resonance problems observed in data centers, explain their root causes, and develop solutions. For systems using Y-connected power supply units (PSUs), the zero sequence is identified as the weakest link and the first to become unstable. The expanded system model and analysis reveal a new, differential-mode instability that is responsible for high frequency resonances. To guarantee system stability, new impedance-based product and system design specifications are developed based on sufficient conditions derived from the Nyquist stability criterion. Laboratory and field measurements are presented to substantiate the proposed methods and conclusions.