FRAX series sweep frequency response analysers

SFRA testing, which you can conveniently perform with a Megger FRAX test set, is used to check the mechanical integrity of transformer components such as the core, the windings, and the clamping structures. The test entails injecting a low voltage signal into one end of a winding and measuring the voltage output at the other end so that the electrical transfer function of the transformer can be determined. The test is typically repeated over a frequency range from 20 Hz to 2 MHz. The results are compared with a reference curve produced using the same technique when the transformer was new or known to be undamaged. This technique reveals many fault types, including core movements, faulty core grounds, winding deformations, winding displacements, partial winding collapse, hoop buckling, and shorted turns. It’s important to note that SFRA is essentially a comparative test. Without a reference curve for the transformer, the information provided by the test is much more challenging to interpret.

You can connect your SFRA test instrument to an optional accessory called the FRAX demo box FDB 101 (part number AC-90050), which allows you to short turns, shift the transformer core, and make other changes to show how different faults affect SFRA traces. This training tool will enable you to familiarise, or re-familiarise, yourself with the FRAX and software before testing in the field. 

Repeatability is at the forefront of the FRAX design, both in the internal electronics and the leads and connections to the transformer. The FRAX 99 has an internal noise level of less than -120 dB, while the FRAX 101 and 150 are even lower at less than -140 dB. Repeatability is also built into the leads by following best practices of the shortest braided ground principle and secure connections with C-clamps. The FRAX 101 and 150 also have an embedded ‘Ground Loop Detector’ to verify proper connections before running the test. 

Yes. Although a time-based comparison is the best method for evaluating SFRA measurements, you can still compare measurements between sister transformers or make a phase-to-phase comparison for an initial evaluation. Additionally, testing the transformer in a known good condition will allow you to evaluate the transformer later if there is a fault or catastrophic event.  

Both the FRAX 101 and FRAX 99 are available with a battery option. The battery will last up to 8 hours of continuous use and 12 hours of standby. With a fully charged battery and laptop, you can test multiple transformers in one day without power at the site. This powering flexibility is especially advantageous when transporting transformers where you may be measuring at transfer points. The battery takes four hours to charge fully, and the FRAX can also run off AC power while its battery is charging.

The FRAX is available with either 9 m (30 ft) or 18 m (60 ft) cable lengths. The 9 m will cover most transformers 245 kV and below, whereas you will need the 18 m cables for higher voltages. The FRAX 101 can be placed on top of the transformer and connected via Bluetooth to help reduce the cable length needed.

FRSL stands for frequency response of stray losses. It’s a technique for assessing the condition of transformer windings by performing short-circuit tests over a wide range of frequencies. Diagnostics based on FRSL rely on comparing results to earlier measurements, tests carried out on an identical transformer, or between phases. Measurements are made on the high voltage side of the transformer, with the low voltage side short-circuited. FRSL testing uniquely reveals strand-to-strand short circuits in a winding. You can perform FRSL tests with Megger FRAX and TRAX test sets

The IEEE guide for SFRA is IEEE C57.149 Guide for the Application and Interpretation of Frequency Response Analysis for Oil-Immersed Transformers. Other relevant SFRA documents include IEC 60076-18 Ed. 1 – 2012, Std. DL/T911-2004, and Cigré Technical Brochure No. 342, April 2008. 

Yes. IEEE C57.152 Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors recommends SFRA as a diagnostic test. SFRA can often pick up mechanical issues that other electrical tests might miss

The individual sweeps take a little over 40 seconds to complete; the longest part of the test is making connections. Once the transformer is fully isolated, you can complete an SFRA test on a two-winding transformer in about 45 minutes, as long as you can reach the bushing terminals while standing on top of the transformer. For higher voltage transformers requiring a manlift to access the bushing terminals, you will need additional time (approximately double) to make the connections.

Yes. The FRAX software can import and export data in multiple formats to compare to other measurement data from different instruments.  

Yes – and no! SFRA (sweep frequency response analysis) testing is the best-known variable frequency transformer testing technique. Still, it’s not the only one. Several other transformer diagnostic techniques are based on frequency, each of which has unique diagnostic functions and values. Other widely used techniques include DFR (dielectric frequency response), narrowband DFR, and FRSL (frequency response of stray losses).