Sweep Frequency Response Analysis of Transformer Windings

What it is

Sweep frequency response analysis, usually shortened to SFRA, is an electrical test that checks the mechanical integrity of a power transformer's core and windings without opening the tank. A low-voltage signal is injected at one terminal and its frequency is swept across a wide range, while the response is measured at another terminal. The instrument plots the ratio of response to input against frequency, producing a trace that works as a fingerprint of the transformer's internal geometry. The windings, the core and the leads together form a network of inductances, capacitances and resistances, and any physical change to that network alters the trace. Several traces are taken on each unit, covering each winding in turn, open-circuit and short-circuit connections, and the different phases. The test is performed mainly on power transformers, and also on reactors and other wound equipment. It is valued because it can reveal damage that other routine electrical tests tend to miss, such as a winding that has shifted but has not yet failed electrically.

Why it is done

A transformer can be shaken out of shape without any sign on the outside. Short-circuit currents from nearby faults produce very large mechanical forces in the windings, and rough transport, relocation or a seismic event can loosen clamping or move a winding or the core. Deformation of this kind is often invisible to routine electrical tests such as turns ratio and winding resistance, yet it leaves the unit weaker, because a displaced winding has less margin against the next fault and may be left with reduced clearances that favor a later dielectric failure. Frequency response analysis gives a way to ask the question directly. It is therefore used to establish a baseline when a transformer is new or commissioned, after transport or relocation, after a through-fault or other severe event, and when other evidence suggests that something inside has changed. A clean comparison can avoid an unnecessary internal inspection, and a clear difference can justify one before the unit is returned to service.

The principle

Every transformer is, electrically, a ladder network of coupled inductances and capacitances: turns of each winding, the spaces between windings, the core and the tank all contribute. When a low-voltage signal is swept in frequency, the way that network responds depends on how those elements are arranged, so the measured trace is a function of geometry. If a winding is displaced, compressed or bulged, or if the core is deformed or grounded in an unintended place, the capacitances and inductances shift, and the trace shifts with them. Different parts of the frequency range are sensitive to different features. The lowest region is dominated by the core and its magnetization, and it responds to core deformation, shorted turns and residual magnetism. The middle region reflects the interaction between windings and tends to reveal movement or deformation of the windings relative to one another. The highest region is governed by the winding ends, the leads and the connection of the measuring cables, and it responds to lead position and to problems in the test setup itself. Because the trace is a comparison tool, its meaning comes from a reference and not from any single curve taken alone.

How it is performed

The transformer is de-energized, isolated and grounded under the owner's safety procedures, and the external connections, such as bus, cables and surge arresters, are removed from the bushings so that only the transformer itself is measured. The instrument is connected to the terminals with the shielded measuring cables supplied for the purpose. The shield grounds are connected with short, solid connections to the bushing flange or to the tank at the base of each bushing, because loose or long grounds distort the high end of the trace and prevent a fair comparison. The position of the tap changer is selected and noted, and the same position is used whenever a comparison is planned. Tests are arranged so that each winding is measured in the open-circuit configuration, and the short-circuit and inter-winding configurations are added as the procedure requires. A repeated sweep is useful to confirm that a trace can be reproduced before the leads are moved. Tests that magnetize the core, such as direct-current winding resistance measurements, are scheduled after the frequency response sweep so that residual magnetism does not alter the low end. Everything about the setup, including lead positions and the identity of the instrument, is written down so that the next test can be performed the same way.

What is recorded

The record includes the traces themselves for every connection that was measured, together with the information needed to repeat the test: the unit and its winding connection, the tap changer position, the configuration of each sweep, the instrument and its settings, the cables and ground connections used, and the date. The condition of the transformer at the time is noted too, such as whether it was filled with oil, recently transported, or just removed from service, along with the reason for the test. These details are important because a trace can differ for reasons that have nothing to do with the windings, including a different tap position, a different lead arrangement or residual magnetism, and the reviewer needs the information to tell a real change from a setup difference.

How results are judged

Interpretation is a comparison, and this page deliberately gives no pass or fail limit. The preferred reference is an earlier trace from the same unit, typically recorded at the factory or at commissioning, taken in the same configuration. When no baseline exists, the trace can be compared with a sister unit of the same design, or between the phases of the same transformer, where similar windings are expected to give similar traces. In each case the reviewer looks at how closely the traces overlay and where they separate. Differences in the low region point toward the core, differences in the middle region toward winding movement or deformation, and differences in the high region toward leads, connections or the test setup. Small shifts can have harmless explanations, and a larger or growing separation calls for more investigation. The comparison methods and the judgment of differences are set out in published guidance on frequency response analysis. ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023 include the test among the electrical tests for liquid-filled transformers, at acceptance and in maintenance; refer to your own licensed copies for how each treats the test and its results.

Variations

The most informative case is a test against the transformer's own earlier trace, which is why a baseline taken at the factory or at commissioning is worth the effort. Without one, a sister unit or the other phases serve as a weaker reference, and the conclusions are more cautious. The test is used differently by purpose: after transport or relocation it checks that the unit arrived intact, after a through-fault it asks whether the windings moved, and as a periodic measure it tracks slow change. Construction changes the traces. Three-phase units with delta-connected windings, autotransformers and units with tertiary windings give traces that look different from those of a simple two-winding unit, and the tap changer position can alter the result. Reactors and other wound equipment are measured in the same spirit, with their own reference traces. In every case the configuration should match the reference exactly.