The transformer fault other tests can’t see

A power transformer can pass its power factor test, its turns ratio test and its excitation current test — and still be carrying hidden mechanical damage that takes it out of service months later. Winding deformation from a through-fault. A loose clamping structure after transport. Core movement that only shows up once the unit is re-energised and the vibration starts again. None of these are electrical insulation problems, so the tests built to find insulation problems won’t find them.

This is the gap sweep frequency response analysis (SFRA) was built to close, and it’s why SFRA now sits alongside power factor, turns ratio and winding resistance testing as a standard part of transformer commissioning and condition assessment programs across Australian utilities, substations and industrial sites. If you’re specifying, procuring or running an SFRA program, the Megger FRAX150 and FRAX200 are the instruments most field teams reach for — and for good reason.

How SFRA actually works

The FRAX injects a low-voltage swept sine wave into one end of a transformer winding and measures the response at the other end across a frequency range from 0.1 Hz up to 25–30 MHz (model dependent). Because the transformer behaves as a complex network of resistance, inductance and capacitance, the shape of that response is effectively a fingerprint — unique to the physical geometry of the core, windings, clamping structures and connections.

Compare a fresh fingerprint against a reference trace (taken when the transformer was new, commissioned, or known to be healthy) and deviations jump out: resonance peaks that have shifted, additional resonances that weren’t there before, lost resonance, or an overall change in magnitude. Each of these patterns points toward a specific class of fault — winding deformation and displacement, shorted or open turns, broken clamping structures, core connection problems, core movement, or hoop buckling — long before any of it shows up as an electrical failure.

It’s a comparative test by nature, which is exactly why data discipline matters. A FRAX trace on its own tells you less than a FRAX trace measured against a documented baseline, a sister transformer, or a phase-to-phase comparison. Teams that start collecting fingerprints at commissioning — and standardise how they do it — get far more diagnostic value out of every subsequent sweep.

Built for the field, not just the lab

Where the FRAX series earns its reputation is repeatability. SFRA is acutely sensitive to connection quality — a marginal ground, a different clamp position, or a longer lead run between two test sessions can introduce curve variation that has nothing to do with the transformer itself. Megger’s answer is colour-coded, wide C-clamp connectors with adjustable braided grounds and shielded cabling built to the shortest-braided-ground principle specified in IEC 60076-18 Method 1, so the same connection technique produces the same result regardless of which technician is holding the clamp.

Both instruments also automate the two steps that most commonly compromise SFRA data in the field: grounding and magnetisation. An automatic ground loop detector verifies every connection before a sweep is allowed to run, catching poor grounds before they corrupt a trace rather than after. A built-in demagnetisation circuit clears residual magnetism from the core before testing starts, eliminating the low-frequency shift that magnetised iron would otherwise introduce — and the reason Megger, along with IEEE C57.152, recommends running SFRA before any DC winding resistance test on the same outage.

At the low end, the FRAX series is genuinely field-sized: from 1.8 kg with batteries and roughly 25 x 17 x 5 cm, it travels in a case with the instrument, cables and accessories together. A full sweep with ground check completes in under a minute on the FRAX200, and a two-winding transformer with accessible bushing terminals is typically fully tested — connections included — in around 45 minutes.

FRAX150 vs FRAX200 — which one fits your program

FRAX150 is the mains-operated option with a built-in PC and 12″ high-resolution touchscreen, bright enough to read in direct sunlight, plus the same ground loop detector and cabling technology that defines the range. It’s a strong fit for teams who want a self-contained, no-laptop workflow at the transformer.

FRAX200 is the current-generation performance test set, available as the fully-featured FRAX200 ADV or the streamlined FRAX200 BAS. Both add automatic demagnetisation, USB-C data transfer and charging, and Bluetooth Low Energy connectivity to a laptop or tablet running the FRAX software. The FRAX200 ADV additionally supports the optional FSX200 switchbox, which uses patent-pending active clamps to connect all three phases of a transformer through a single set-up — the leads at the bushing end disconnect electrically once a phase isn’t being measured, removing the influence of unused cabling from the result and cutting out repeated ladder climbs on tall units.

Both models meet IEC 60076-18, IEEE C57.149-2012, CIGRE Technical Brochure 342 and DL/T 911-2004, and both ship with the ground cable, braid sets, C-clamps, connection cables (9 m or 18 m, extendable to 30 m on the FRAX200), a Field Test Box for self-verification, user guide and the Windows FRAX analysis software — included with no licence control and no per-seat annual fee.

Where FRAX earns its place in an Australian testing program

• Substation and utility asset management — building an SFRA fingerprint library across a transformer fleet as part of a condition-based maintenance program, rather than relying purely on time-based replacement schedules.
• Commissioning new transformers — capturing a factory-condition baseline before energisation, and again after transport, to confirm nothing shifted in transit.
• Post-fault and post-fault-current investigation — after a through-fault, short-circuit event or suspected mechanical disturbance, SFRA is one of the fastest ways to confirm (or rule out) internal mechanical damage without opening the tank.
• Renewables and industrial step-up transformers — wind, solar and battery storage projects increasingly specify SFRA at commissioning precisely because these transformers are transported to remote sites and benefit from a documented pre/post-transport comparison.
• Calibration labs and test houses offering SFRA as a service to utility and industrial clients across the eastern seaboard and beyond.

Analysis software that does more than plot a curve

The FRAX software (Windows, included, unlimited seats) presents data as magnitude-versus-frequency, phase-versus-frequency, or impedance/admittance-versus-frequency — useful across different transformer designs. Automated analysis compares two curves algorithmically for amplitude and frequency-shift deviation and flags the result as severe, obvious or light, with pass/fail criteria available for shorted-winding measurements. It also imports and exports data from other manufacturers’ formats (CIGRE, Doble, IEC 60076-18 XML, CSV, TXT), so a fleet with a mixed testing history isn’t locked out of comparative analysis.

What to have in place before you order

SFRA is not a fault detector in isolation — it’s most powerful when you’re comparing today’s trace to a known-good reference. If you’re speccing a FRAX for a new testing program, factor in the time to establish baseline fingerprints across your fleet early, and standardise your connection procedure (tap position, isolation, grounding) so future comparisons are valid. For teams testing low-capacitance objects like bushings or current transformers with a dielectric frequency response (DFR) requirement alongside SFRA, Megger’s IDAX series covers that complementary insulation diagnostic — the two techniques answer different questions about the same asset.

Get your FRAX sorted

Rapid-Tech supplies the Megger FRAX150 and FRAX200 with local support, and can talk through which configuration — cable length, ADV vs BAS, with or without the FSX200 switchbox — matches your fleet and your access conditions. Request a quote and we’ll get your instrument, cables and Field Test Box ready to go.

Technical Specifications

Note: FRAX101 and FRAX99 are shown on Megger’s website as discontinued and are not covered in this content package.

What is sweep frequency response analysis (SFRA) testing? SFRA testing checks the mechanical integrity of a transformer’s core, windings and clamping structures. A low-voltage signal is injected into one end of a winding and the response is measured at the other end across a range of frequencies, producing a unique “fingerprint.” Comparing this fingerprint to a reference trace — from commissioning, a previous test, or a sister transformer — reveals mechanical changes such as winding deformation, core movement or broken clamping structures that other electrical tests typically miss.

What’s the difference between the Megger FRAX150 and FRAX200? The FRAX150 is a mains-powered instrument with a built-in PC and 12″ touchscreen for self-contained field testing. The FRAX200 is Megger’s current-generation platform, available as the ADV or BAS variant, with USB-C, Bluetooth Low Energy, automatic demagnetisation and battery operation, controlled via laptop or tablet. Only the FRAX200 ADV supports the optional FSX200 switchbox for simultaneous three-phase connection.

How long does an SFRA test take on a transformer? Each individual sweep takes a little over 40 seconds on the FRAX series. For a standard two-winding transformer with accessible bushing terminals, a full set of SFRA tests — including making and remaking connections — typically takes around 45 minutes once the unit is isolated. Higher-voltage transformers requiring a manlift for bushing access generally take roughly double that time.

Is SFRA the same as DFR (dielectric frequency response) testing? No. SFRA and DFR both sweep across frequency, but they measure different things. SFRA assesses the mechanical integrity of a transformer’s core and windings by measuring signal transfer through the winding, typically from 20 Hz to 2 MHz. DFR (available on Megger’s IDAX series) measures capacitance and dissipation/power factor to assess insulation condition and moisture content, typically from around 1 mHz to 1 kHz. They answer different diagnostic questions and are often used together as part of a full transformer assessment.

What faults can SFRA detect that other transformer tests miss? SFRA is specifically sensitive to mechanical rather than purely electrical changes, including winding deformation and displacement, shorted or open turns, broken or loose clamping structures, core connection problems, core movement and hoop buckling. Power factor, turns ratio and winding resistance tests can all return normal results on a transformer that has sustained this kind of mechanical damage, which is why SFRA is recommended as a complementary test rather than a replacement for them.

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