Field measurements of transformer single-phase exciting current as a diagnostic tool, and influence of load tap changers

The single-phase exciting-current test can be used to detect undesirable conditions in single- and three-phase transformers. Normally the test results are analyzed by comparing currents between all three phases in a given transformer and between similar single-phase units. Certain problems, however, can be detected on the basis of current change when the measurements are performed on different load tap changer positions. Accordingly, understanding how the load tap changer affects the current magnitude of individual phases is essential for developing proper analysis. This paper identifies twelve patterns of test result behavior, and an attempt is made to show different load tap changer designs as they affect the measurement. The test results and test circuit diagrams are given. The derived patterns will be implemented in a DOS-based expert system in combination with a data acquisition system which is currently used in the field by more than 80 utilities