Ratio, polarity and magnetics
The excitation current test applies a low alternating voltage to one winding of a transformer, with the other windings open, and measures the current that magnetizes the core. The size and pattern of that current across the phases reveal shorted turns, core damage and tap changer problems.
The excitation current test, sometimes called the no-load current test or the exciting current test, measures the small current a transformer draws to magnetize its core when one winding is energized and the other windings are left open. In the field it is done at a reduced voltage supplied by a portable test set, usually on the high-voltage winding of a power or distribution transformer, one phase at a time. The test is applied to liquid-filled transformers, large air-cooled dry-type transformers and step voltage regulators, including the load tap-changer positions, so the reading is taken on each tap of interest and, on some units, with the neutral accessible. It is called for as an electrical test at acceptance and in maintenance, and in the specifications it is sometimes listed as optional for tap positions and the neutral. The result is a current, often recorded together with the applied voltage and loss, and the value for each phase is compared with the others and with earlier or factory data.
The excitation current is a sensitive indicator of the condition of the magnetic circuit and of turn-to-turn insulation. Shorted turns in a winding, damage to core laminations, a core joint that has moved, a circulating current in the core, residual magnetism, and poor contact in a tap changer or its leads all change the current the transformer draws. These problems may be invisible to insulation resistance and ratio tests, yet they raise losses, heat the unit, and can grow into a failure. Because the test is quick and needs only a low-voltage source, it is a useful screen after transport, after a through-fault, after a tap changer repair, and whenever dissolved gas analysis or other evidence suggests an internal problem. It also builds a baseline for the unit, so that future tests can be compared with it, and it helps distinguish a winding fault, which shows in the excitation current, from a purely insulation problem.
A transformer core needs a magnetizing current to produce the flux that links the windings. The size of that current depends on the core material, its geometry, and the number of turns of the energized winding. In a healthy three-phase transformer with a core of three legs, the magnetic path is longer for the outer phases than for the center phase, so the current is higher in the two outer phases and lower in the center, producing a recognizable pattern. When a few turns are shorted, the shorted loop behaves like a loaded secondary and the primary draws more current to compensate. When core laminations are damaged or a joint is open, the reluctance increases and the current changes. At the low test voltage the core is not saturated, so the measurement reflects the linear region of the core and the structure of the windings. Residual magnetism from earlier direct-current tests or from a switching event can alter the result, which is why the excitation test is performed before core-magnetizing tests such as winding resistance, or after the unit has been demagnetized.
The transformer is de-energized, isolated and grounded under the owner's safety procedures, and the external connections are removed from the bushings. The tap changer is moved to a defined position, which is noted, and the test is repeated on the positions the procedure requires. The test set is connected across one phase of the winding to be energized, with the other windings of the transformer left open and the delta or neutral connections arranged as the procedure directs, since the connection of the unit determines how the test must be set up. A low alternating voltage is applied at a level that is safe for the unit and the instrument, and the current, and where available the loss and the power factor, are recorded for that phase. The step is repeated for each phase. Because the equipment under test can be capacitive and inductive, the instrument handles the source power and gives stable readings. A test set that reports both current and loss is useful, as the two together separate a core problem from a winding problem. Because residual magnetism affects the result, direct-current tests are scheduled afterward. Temperature, tap position, applied voltage and instrument calibration are recorded.
The record states the unit, the winding energized, the winding connections and the phase, along with the tap position. It holds the applied test voltage and the excitation current for each phase and each tap measured, and the loss where the instrument reports it. The instrument and its calibration status, the date, the winding or oil temperature and the ambient conditions are noted, together with the as-found state of the unit, such as new, transported, or returned from a fault. Information about whether the core had been magnetized by earlier direct-current tests is included, since it explains an unexpected difference. The same arrangement is described so that later tests can follow it exactly.
No single figure is a pass or fail threshold, and this page gives none. Results are judged by comparison. The first comparison is between phases: in a unit with a three-legged core the two outer phases are expected to be similar to each other and the center phase different in a characteristic way, and a departure from that pattern points to a local problem. The second is against factory test data or earlier field results taken at the same voltage and tap, where a rise or a change of pattern suggests shorted turns or core damage. The third is across tap positions, where a position that departs from its neighbors suggests a tap changer or lead problem. Findings are read together with other tests, such as turns ratio, winding resistance, leakage reactance and dissolved gas analysis. ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 include the excitation test for transformers and regulators; consult your licensed copies for the requirements and the criteria.
At acceptance, the test is performed to confirm the unit against factory data and to establish a baseline for all taps of interest. In maintenance, it is applied after a through-fault, after transport or relocation, after tap changer service, and when other evidence suggests an internal problem. Liquid-filled and dry-type units are treated alike in principle, though large dry-type transformers present different temperature and access considerations. Step voltage regulators are tested on each tap position, and the series arrangement and the changer are included in what the current reflects. Three-phase units with delta windings, zigzag windings or accessible neutrals need a specific test arrangement, and where the neutral is brought out the test may be extended to include it. Single-phase units give a single reading per configuration. In every case the comparison with the unit's own earlier result is the most sensitive.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
In a three-legged core, the magnetic path for the two outer phases is longer than for the center, so they need more magnetizing current. The result is a characteristic pattern of two similar outer readings and a different center reading, and a departure from it can indicate a fault.
It can reveal shorted turns, damaged or displaced core laminations, an open or poor core joint, circulating currents in the core and faulty contacts in a tap changer. These may not appear in insulation or ratio tests, so the excitation test is a valuable complement.
Direct current used in winding resistance measurements can leave residual magnetism in the core, which changes the excitation current and makes the reading unrepresentative. Doing the excitation test first, or demagnetizing the core before it, avoids that distortion.
The number of energized turns changes with the tap, so the excitation current changes as well. A fair comparison with earlier results or with factory data needs the same tap and a similar voltage, and differences between taps can themselves reveal a tap changer problem.
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