Percent Impedance/Leakage Reactance Test
This is normally an acceptance test to see that nameplate percent impedance agrees with the measured percent impedance when the transformer arrives onsite. Normally, a 3% difference is considered acceptable. However, after the initial benchmark test, the percent impedance should not vary more than 2% from benchmark. As the transformer ages or suffers events such as through faults, nearby lightning strikes, and other surges, this test is used in the field to detect winding deformation. Winding deformation can lead to immediate transformer failure after a severe through fault, or a small deformation can lead to a failure years later.
Percent impedance/leakage reactance testing is performed by short circuiting the low-voltage winding and applying a test voltage to the high-voltage winding. Reluctance is resistance to lines of magnetic flux. Reluctance to the magnetic flux is very high in spaces between the high- and low-voltage windings and spaces between the windings and core. Reluctance is very low through the magnetic core so that the vast majority of total reluctance is in the spaces. When winding movement (distortion) occurs, these spaces change. Therefore, the reluctance changes, resulting in a change in the measured leakage reactance. Changes in leakage reactance and in capacitance tests (explained in section 9.3.2) serve as an excellent indicator of winding movement and structural problems (displaced wedging, etc.). This test does not replace excitation current tests or capacitance tests but complement them, and they are used together. The excitation current test relies on reluctance of the core while the leakage reactance test relies on reluctance of the spaces. See Doble’s Leakage Reactance Instrument Users Guide, and IEEE®, Guide for Diagnostic Field Testing of Electric Power Apparatus-Part 1: Oil-Filled Power Transformers, Regulators, and Reactors (IEEE 62-1995™ ).