Ratio, polarity and magnetics
Leakage reactance testing short-circuits one winding and applies a low alternating voltage to another to measure the reactance of the magnetic flux that links only one winding. Because it depends on winding geometry, a change from nameplate or earlier results can point to winding movement or deformation.
Leakage reactance is the part of a transformer's impedance caused by magnetic flux that links one winding but not the other, flux that leaks through the space between and around the windings. The test measures it by shorting the terminals of one winding, applying a reduced alternating voltage to another winding, and measuring the resulting current and voltage, from which impedance and reactance are derived. It is also known as a short-circuit impedance measurement, and the result is commonly expressed per phase or as a percentage of the transformer base. The test is used on liquid-filled power and distribution transformers, on step voltage regulators, and on the series winding of load tap-changer units. It is listed in the standards as an optional test in several sequences, applied when the owner wants an additional check on the mechanical integrity of the windings, or when a baseline is being built for a unit that may later be subject to faults or relocation.
The leakage reactance is fixed by the dimensions and spacing of the windings. When a winding is displaced, bulged, compressed or telescoped by short-circuit forces, by rough handling during transport, or by a seismic event, the geometry changes, and so does the leakage flux and the measured reactance. This makes the test a sensitive indicator of mechanical damage that does not show up in simple ratio or insulation tests. Detecting winding movement early allows the owner to remove a transformer from service for inspection before an electrical failure occurs, rather than after. The test is also valuable for confirming that a replacement or repaired unit matches the design, that the windings of a multi-phase unit are consistent with each other, and that a regulator has not been damaged by a through-fault. Finding a deviation at commissioning establishes the baseline against which later changes are read.
In an ideal transformer, all the flux produced by one winding links the other. In a real one, some flux leaks into the space between the windings and into the insulation, oil and surrounding structure. This leakage flux behaves like an inductance in series with the winding, which is the leakage reactance. When one winding is short-circuited, the voltage needed to drive current through the transformer is limited by this series impedance, mostly reactive, together with a small resistive part from the winding resistance. A low applied voltage therefore produces a measurable current, and the ratio of the two gives the impedance. Because the reactance is set mainly by the geometry of the windings, such as the radial gap and the height, it stays constant for a healthy unit and shifts if the windings move. Because the core is not driven toward saturation at this low voltage, the measurement reflects the windings alone, and it is relatively insensitive to the core condition.
The transformer is de-energized, isolated and grounded under the owner's safety procedures, with external connections removed from the bushings. The tap changer is set to a defined position that is noted, since the active turns and therefore the reactance depend on it. For each phase, one winding is shorted with a conductor of adequate size and good contact, because resistance in the short affects the result. A low-voltage alternating source is connected to the other winding, and voltage, current and, where the instrument provides it, the power are measured. Three-phase units can be tested phase by phase or with a three-phase source according to the unit's connection and the instrument used. Purpose-built test sets calculate impedance and reactance automatically and report them in ohms or as a percentage. The measurement is repeated on each phase and on the tap positions that the procedure requires, including those of the series winding of a regulator. Tests that magnetize the core are scheduled after this one so that residual magnetism does not influence the reading. Instrument calibration, winding temperature and the connections used are recorded.
The record states the unit and the winding pair tested, the phase, the tap position, and the source and the measurement arrangement. It holds the measured impedance or reactance for each phase, in the units reported by the instrument or converted to the same basis as the nameplate, along with the quantity of source voltage and current applied. The instrument and its calibration status, the date, the winding temperature and the ambient conditions are noted. The as-found state of the unit is described, such as new from the factory, recently transported, or returned from a fault. The record preserves the configuration so that the same test can be repeated identically later.
Results are judged by comparison and not by an absolute limit, and this page gives none. The measured value is compared with the impedance on the nameplate or with the factory test report, and a close agreement is expected for a unit with intact windings. The values for the three phases are compared with each other, since a phase that differs from the others suggests local winding deformation. Comparison with earlier results on the same unit, taken in the same configuration, is the most sensitive form of evaluation, as a gradual or sudden shift points to movement. A deviation is interpreted together with other evidence, such as frequency response analysis, dissolved gas analysis and excitation current, since each addresses a different aspect of the same unit. The comparison method and the treatment of regulators are covered in IEEE C57.15-2017, and ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 list leakage reactance as an optional test; refer to your licensed copies for the values and criteria.
At acceptance, leakage reactance is sometimes performed to confirm the unit against factory data and to set a baseline, and it is more often applied to larger or more critical units. In maintenance, it is used after a through-fault, relocation or any event that could have moved the windings, and when other tests such as frequency response analysis give a questionable result. Step voltage regulators are tested across the series winding for each tap position of interest, and units with load tap-changers are tested with the changer on defined positions. Three-phase transformers with delta or zigzag windings require the test arrangement to match the connection. Small distribution transformers are quickly done with a portable set, whereas large power transformers need a higher-capacity source and care with the connections. The most informative case is always a comparison with the same unit's own earlier result.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
It reflects the geometry of the windings, because the leakage flux depends on the gaps and dimensions between and around them. A change from the nameplate or from an earlier measurement can indicate that a winding has moved, bulged or been compressed, often after a through-fault or rough transport.
Short-circuit impedance includes the reactive leakage part and a smaller resistive part from the winding resistance. For power transformers the reactive part dominates, so the two are often used interchangeably, and the measurement is the same: short one winding, apply a low voltage to another and measure.
The number of active turns changes with tap position, and so do the geometry and the reactance. A comparison is only fair between results taken on the same tap, so the position is noted and the same one is used whenever a later test is compared with an earlier one.
The published acceptance and maintenance specifications list it as an optional test for several equipment types. Owners typically apply it to larger or more critical units, or when winding movement is suspected. Check the applicable standard and the owner's specification for the requirement in a given project.
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