AbstractThis paper presents the results of experimental study on the performance of a Unified Power Flow Controller (UPFC), one of the FACTS (Flexible AC Transmission Systems) controllers. A laboratory‐scale UPFC was manufactured and installed on a laboratory electric power system to investigate its multifunctional capabilities as a power flow controller. The UPFC consists of two 4.5‐kVA, 200‐V back‐to‐back voltage‐sourced converters, labeled “Converter 1” and “Converter 2,” operated from a common DC link provided by a DC storage capacitor of 380 V. It can provide independent control of both the real and reactive power flow in the line. Tests were performed to examine the capabilities of the UPFC, under one‐machine connected to an infinite‐bus system. Steady‐state responses under various kinds of operating conditions were measured and analyzed. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 161(2): 8– 15, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20538