Modeling and Analysis of Data Center Power System Stability by Impedance Methods

This paper presents a study of recently experienced power system instability and resonance problems in large data centers using impedance modeling and analysis. The approach developed to analyze the problem consists of 1) modeling each power supply unit (PSU) by its input impedance and the power distribution network by an equivalent source impedance; 2) converting the 3-phase system impedance model into separate positive-, negative-, and zero-sequence impedance models by ignoring asymmetries in the network impedance; 3) determining system stability in each sequence based on the Nyquist criterion; and 4) applying matrix perturbation theory and eigenvalue analysis to account for network asymmetries. Inclusion of the zero-sequence model is necessary because of the use of a three-phase, four-wire distribution system and single-phase power supplies operating between phase and neutral. The analyses presented in the paper identifies zero sequence as the weakest link in which instability occurs first. The simplified system model and analysis also lead to new specs and design guidelines for data center power systems to assure stability. Laboratory and field measurements are presented to demonstrate the nature of this new problem and to validate the proposed method.