FRAX series sweep frequency response analysers
Detect potential mechanical and electrical problems in power transformers - FRAX099, FRAX101, FRAX150
Detect potential mechanical and electrical problems in power transformers - FRAX099, FRAX101, FRAX150
Key Benefits
- High dynamic range and accuracy
- Easy-to-use support software with advanced analysis tools
- Smallest SFRA instrument in the industry
- Hardware designed to ensure repeatable connections
Description
The FRAX99, FRAX101, and FRAX150 sweep frequency response analysers (SFRA) are the smallest and most rugged instruments of their type and powerful tools for revealing potential electrical and mechanical problems in power transformers, many of which are difficult or impossible to detect using other methods.
Meeting all international standards for SFRA measurements, these innovative instruments offer a larger dynamic range and better accuracy than any other comparable test sets currently available. In addition, for the test connections to the transformer, they use a special cabling technology that ensures the repeatability of results.
The FRAX series operates by applying a sweep frequency test signal to the transformer and monitoring its response. The result is a unique fingerprint that reveals a wide range of faults when compared with a reference fingerprint for the same transformer. These include winding deformations and displacements, shorted and open windings, loose and broken clamping structures, core connection problems, core movement, and hoop buckling.
Megger’s FRAX series incorporates powerful analysis and support software. In addition to offering the traditional magnitude versus frequency/phase display, this software allows you to present data in an impedance or admittance versus frequency view, a powerful analytical tool for many transformer types.
The test frequency range covered by the FRAX is 0.1 Hz to 25 MHz, and you can set the range employed for individual tests to match the application’s needs. By default, the number of test points used for each frequency sweep is 1046, but you can extend this up to a maximum of 32 000. The typical measurement time is 64 seconds, but a fast mode is available that can deliver results in just 37 seconds.
Small and easy to carry, with an operating temperature range of -20 ºC to +50 ºC, the FRAX sweep frequency response analysers are ideal for use in the field. They are supplied with ground cable, four 3 m braid sets, two C clamps, 9 m or 18 m connection cables, a user guide, and Windows software.
There are three models in the FRAX series:
FRAX099: optional battery, connects to an external laptop for control and data analysis with standard USB cable
FRAX101: optional battery, supports both Bluetooth and standard USB connection for control by and exchanging data with an external laptop, includes built-in ground loop detector
FRAX150: mains-operated with an integrated PC that has a high-resolution colour screen featuring a powerful backlight that makes it easy to read even in direct sunlight and includes a built-in ground loop detector
Frequently Asked Questions
Are there any accessories that can help with training?
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.
What is SFRA testing, and why is it useful?
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.
Since the best analysis method of SFRA results is a comparison to previous measurements, how repeatable is the FRAX?
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.
If SFRA is a comparative test, is it still useful if I don’t have any factory or previous measurements?
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.
How long does the battery last?
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.
Is variable frequency testing of a power transformer the same thing as an SFRA test?
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).
Is SFRA testing similar to DFR testing?
No, it is quite different. Dielectric Frequency Response (DFR) testing, sometimes known as frequency domain spectroscopy (FDS), is a series of tan delta or power factor tests carried out over a range of frequencies. The frequency range is much lower than that used for SFRA – typically, DFR is performed from 1 mHz (millihertz!) to 1 kHz. In contrast, SFRA is usually performed from 20 Hz to 2 MHz. The DFR results are typically presented as capacitance and dissipation factor/power factor curves. When used in conjunction with insulation modelling, DFR testing provides invaluable information about the condition of the transformer’s insulation system, particularly the moisture content of cellulose insulation and the oil conductivity. In comparison, SFRA tests assess the mechanical integrity of the transformer. Megger offers DFR testing with instruments in the IDAX range and narrowband DFR (NBDFR) testing capabilities in the Delta and TRAX test sets.
What is FRSL testing, and why is it used?
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
Is there a published standard for SFRA?
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.
Should I incorporate SFRA into my transformer maintenance program?
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
How long does it take to complete a FRAX test on a transformer?
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.
Can you import other manufacturers’ data into the FRAX software?
Yes. The FRAX software can import and export data in multiple formats to compare to other measurement data from different instruments.
What lead length should I purchase?
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.
