This two-part paper presents methods to predict

characterize and ensure the stability of data center power systems based on impedance analysis. The work was motivated by recent power system resonance incidents in new data centers. Part I presents new input impedance models for single-phase power supply units (PSUs) to enable this application. Existing impedance models of single-phase PSU cannot meet the requirements of this application because they exclude DC voltage control that affects system stability at low frequency

or are in a dq reference frame that cannot handle the complexity of data center power systems. The developed new models include DC bus dynamics and are directly in the phase domain to simplify system stability analysis

avoiding the need for multiple-input-multiple-output (MIMO) system models and the generalized Nyquist criterion that are difficult to apply but necessary with dq-frame models. Both the converter and system level models also include the coupled current response that is characteristic of AC-DC converters and important for system stability at low frequency. The simple form of the models and system stability analysis directly in the phase domain also make it possible to develop new PSU design methods and performance specifications that together will ensure the stability of new data center power systems. The developed models are validated by laboratory measurements and are used in Part II of the work to study data center power system stability.