Excitation Current Test
Perform the excitation test before any dc tests. Excitation current tests should never be conducted after a dc test has been performed on the transformer. Results will be incorrect because of residual magnetism of the core left from the dc tests.
The purpose of this test is to detect short-circuited turns, poor electrical connections, core de-laminations, core lamination shorts, tap changer problems, and other possible core and winding problems. On three-phase transformers, results are also compared between phases.
This test measures current needed to magnetize the core and generate the magnetic field in the windings. Doble software only gives two indications on this test: “G” for good and “Q” for questionable. On a three-phase, wye/delta or delta/wye transformer test, the excitation current pattern will be two phases higher than the remaining phase.
Compare the two higher currents only. If the excitation current is less than 50 milliamps (mA), the difference between the two higher currents should be less than 10%. If the excitation current is more than 50 mA, the difference should be less than 5%. In general, if there is an nternal problem, these differences will be greater. When this happens, other tests should also show abnormalities, and an internal inspection should be considered. The results, as with all others, should be compared with factory and prior field tests.