This paper is Part I of a two part series. The two papers will report part of the research project RP-1138 sponsored by EPRI and BPA on HVDC-AC System Interaction from AC Harmonics. The two papers deal with the harmonic impedance measurement of Bonneville Power Administration's (BPA) system as seen from the Celilo HVDC substation, the development of analytical techniques to calculate the harinonic impedance of the system, and comparison of the calculated and measured results.