Influence of single-phase excitation and magnetizing reactance on transformer leakage reactance measurement

Low voltage and single-phase excitation are often used to measure leakage reactance or short-circuit impedance of transformers. Winding distortion may be detected by comparing field results with factory nameplate values measured at 25-100% of the rated current using three-phase excitation. The results of single- and three-phase excitation will not always correlate. If the voltage applied in the field is not properly selected then the data may be influenced by the magnetizing reactance. The resulting difference between the nameplate value and the field data might then lead to erroneous analysis of test results and could be interpreted as an indication of winding distortion. The intent of this paper is to analyze the physics of the process and gain insight on test circuit selection and analysis of the results. Methods for avoiding erroneous test results are suggested.