Winding resistance and excitation current testing address two different aspects of transformer condition that power factor and turns ratio testing do not cover directly. Winding resistance finds problems in the conductor path — the copper or aluminum winding itself and its connections. Excitation current finds problems in the core and in turn-to-turn insulation. Together they round out a complete diagnostic test program.
Winding resistance is measured by injecting a stable DC current through the winding terminals and measuring the resulting voltage drop. The resistance, calculated from Ohm’s law, is compared to the factory nameplate value or the baseline reading from a previous test. The comparison identifies any resistance increase that has developed since the transformer left the factory or since the last test.
The test set must charge the inductive winding to a steady-state DC current before the reading stabilizes. For large power transformers, this charging time can be substantial — on high-voltage windings with significant inductance, waiting for the current to stabilize before recording the reading is critical. A reading taken before the current has settled will appear higher than the true resistance, leading to a false indication of a problem.
High-resistance connections at the winding terminations, lead exits, or tap changer contacts show up directly in the measurement. A winding that tests 15% above nameplate is carrying that resistance increase somewhere in the current path. At full load, that extra resistance creates additional I²R heating that shortens insulation life in the vicinity of the fault and may be detectable thermally on a DGA sample as elevated methane and ethane.
LTC contact problems are a frequent source of elevated winding resistance. The LTC contacts are in series with the winding path on the tap being tested. A worn, pitted, or misaligned LTC contact that is not making full surface contact adds measurable resistance. Winding resistance readings taken at every tap position isolate which tap position is contributing the problem — if resistance is elevated at one specific tap and normal at adjacent taps, the LTC contact at that position is the likely cause.
Shorted turns reduce resistance rather than increasing it — fewer conducting turns in parallel means lower measured resistance. A reading that is noticeably below nameplate is as significant as one above it. Shorted turns in conjunction with a turns ratio deviation that points to the same winding or section are a strong combined indicator of a real winding fault.
Phase balance is checked by comparing the three phase readings against each other. For a balanced three-phase winding, all three phases should read within about 2% of each other. A single phase that is significantly higher than the other two points toward a problem in that phase specifically — either in the winding itself or at the lead connection or LTC for that phase.
Temperature correction is required because conductor resistance changes with temperature. Correct all readings to 20°C or 75°C (whichever the nameplate uses as its reference) before comparing to the factory value. Record winding oil temperature at the time of the test — not ambient air temperature, which can differ significantly from the winding temperature on a transformer that was energized until recently.
The excitation current test applies a low AC voltage to one winding of the transformer with all other windings open-circuited, and measures the current that flows. That current is the no-load magnetizing current required to drive flux through the core. Because the other windings are open, no load current flows — only the magnetizing current. The test is run at a voltage low enough that the core is not saturated, typically a few percent of rated voltage.
Three-phase transformers are tested one phase at a time, with the voltage applied to each of the three phases in sequence. On a core-type transformer, the center phase typically draws less excitation current than the two outer phases because the center phase magnetic path is shorter. This is a normal asymmetry that is present in factory test data and should be consistent between test cycles.
Core lamination faults increase the excitation current. The core is built from thin laminations of electrical steel separated by insulating coatings. These coatings prevent circulating currents (eddy currents) from flowing between laminations. When the insulating coating breaks down — from mechanical damage, corrosion, or a manufacturing defect — eddy currents flow across laminations, increasing core losses and the magnetizing current required to drive the flux. An excitation current that has risen significantly between test cycles, without a corresponding winding change, points to core degradation.
Winding deformation from through-fault forces changes the geometric relationship between the winding and the core, altering the mutual inductance and therefore the excitation current. A transformer that has sustained a close-in fault event and shows changed excitation current readings on the subsequent test has experienced some degree of mechanical distortion, even if the winding resistance and turns ratio are within tolerance. SFRA is the more definitive test for winding deformation, but excitation current provides a corroborating data point.
Turn-to-turn insulation faults that allow current to circulate within a portion of the winding alter the flux distribution in the core, which shows up in the excitation current. This is the same fault mode that turns ratio testing catches, and the two tests corroborate each other — a turns ratio deviation alongside an excitation current anomaly on the same phase is a stronger indicator of a winding fault than either result alone.
Residual magnetism affects the excitation current reading. A transformer that has been de-energized under load retains residual flux in the core, and this residual affects the apparent excitation current in subsequent tests. Demagnetizing the core before the test, or running the test with both positive and negative voltage polarity and averaging, eliminates residual magnetism as a variable. Modern test sets handle this automatically.
A complete transformer diagnostic program on a maintenance or acceptance visit typically runs power factor, TTR, winding resistance, and excitation current as a suite. Each test covers different failure modes, and the results cross-reference each other. A winding resistance elevation at a specific tap position that coincides with a TTR deviation at that same tap position identifies the LTC contact for that tap as the problem with high confidence. An excitation current anomaly on one phase alongside a power factor result that shows a difference between CHL and CH-only measurements suggests core or winding interaction that merits SFRA investigation.
The value of the suite is not just in catching individual problems — it is in the pattern across all four tests. A transformer with clean power factor, clean TTR, clean winding resistance, and clean excitation current is a transformer that has been fully characterized and shows no indication of developing faults. That conclusion is more defensible than one based on any single test result.
We run winding resistance, excitation current, TTR, power factor, and SFRA testing on power transformers across Florida and the Southeast. Send us your equipment list and we’ll respond within one business day.