FRAX series sweep frequency response analysers
High dynamic range and accuracy
Allows even the most subtle electromechanical changes within the transformer to be detected
Easy-to-use support software with advanced analysis tools
Easily select and deselect multiple sweeps and sets of sweeps to compare phase-to-phase or previous to current measurements. Advanced analysis and custom formulas allow for sound decision making with regard to further diagnostic analysis and transformer disposition
Smallest SFRA instrument in the industry
FRAX instruments weigh from just 1.8 kg with batteries, with dimensions 25 cm x 17 cm x 5 cm, depending on the model. Easy to transport in a convenient case that houses cables and the test instrument
Hardware designed to ensure repeatable connections
Colour-coordinated connection points and wide C-clamp connectors with adjustable braided grounds ensure consistent connections no matter who uses the equipment, virtually eliminating changes in curves due to connection issues
Fulfils international standards for SFRA
Sweep frequency response analysis (SFRA) measurements for IEC 60076-18, IEEE C57.149, and more
About the product
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
Further reading and webinars
Related products
Troubleshooting
Unplug the USB cable from the FRAX and your PC, check for any foreign objects in the cables or connection ports, and then plug them back in. Start up the FRAX software. Connect to the instrument by selecting “Connect” under the “File” menu, clicking the “Connect” button on the right side of the window, or using the F7 key. If connections are set up correctly, the window name will change from “FRAX (Disconnected)” to “FRAX (Connected).” If the connection doesn’t work, you will get an error message suggesting what to do. Selecting the recommended port number with a green symbol next to it will typically rectify connection issues.
The FRAX 101 has a built-in Class 1 Bluetooth antenna and comes with a Class 1 USB Bluetooth adapter for your computer. We recommend always using this adapter because most PCs only come with Class 2 Bluetooth, which is limited in range and unsuitable for substation environments. To install the Bluetooth adapter, install the software that comes with it before inserting it into your PC. If you insert the adapter before installing the software, you may need to uninstall and reinstall the Bluetooth software and/or driver. When connecting to the FRAX for the first time, you will need to power on the FRAX 101 and add a new Bluetooth device under the windows menu. It should appear as Megger FRAX 101, and the pairing code is “0000.” After pairing is complete, you can connect to the FRAX 101 in the FRAX software.
We recommend using the Bluetooth adapter that comes with the FRAX 101 because the built-in Bluetooth in a PC is limited in range and unsuitable for substation environments. In noisy substations, it is also beneficial to establish a connection to the FRAX when the instrument is close to your PC. Then you can move it to the top of the transformer or farther away as needed. It is easier to maintain a connection once established than connecting for the first time at longer distances. Additionally, if the USB Bluetooth antenna is inserted into a different USB port on your PC, this may switch the COM port that the Bluetooth uses to connect. Verify the COM port before connecting.
A low output voltage usually occurs when there is a short between the signal generator clamp and the measurement clamp. Check all connections and connection points to ensure no unwanted grounds or shorts are present.
You should check test leads for continuity and integrity before use. The best means for checking lead integrity and correct equipment operation is to perform the FRA self-check using a standard test object. This check is particularly beneficial for checking FRA test equipment since there is generally no intuitive way of knowing if the test equipment is giving correct results when making field measurements. The FTB 101 included with the FRAX test set is provided for field verification. In addition to the self-check using the FTB 101, you can perform a short circuit self-test (large C-clamps connected to each other and small ground clamps connected to each other) and an open circuit self-test (clamps isolated and not connected to anything) to help identify any issues. The graph below shows a typical response for short circuit, FTB 101, and open circuit self-tests.
Note: when performing a short circuit self-test, the FRAX software will open a pop-up window that states low output voltage. Just click “OK” and proceed with the test.
Interpreting test results
Understanding how each SFRA response should look before performing these measurements is useful. This way, it is possible to recognise when a measured response differs from what it should be. In these cases, there is a possibility that the cause is a test preparation mistake, e.g., poor grounding or improper test connections. If recognised while still in the field, you can repeat the test after double-checking the test connections/preparation. You should perform a quick test set verification if there is any question on the validity of the measurements (see the instrument verification test in the troubleshooting section). You should also perform a ground loop test with the FRAX by pressing the “GLD” button on the FRAX instrument before beginning each test on the transformer to verify that you have made good ground connections.
SFRA output signal settings typically range from 20 Hz to 2 MHz, inclusive, to inspect the integrity of the complete transformer. You may perform four main types of SFRA tests:
- Open Circuit Self Admittance: the signal is applied on one end of a winding, and the response is measured on its other end. All other connections are left floating (if there is a DELTA stabilising winding, these should remain shorted but not grounded). Six tests are performed on a two-winding transformer, three on the high side and three on the low side. The open circuit test looks at both the winding and core characteristics of the transformer as well as taps and connections.
- Short Circuit Self Admittance: the signal is applied on one end of a winding, and the response is measured on its other end. Three tests are performed, one on each high-side winding, while the three low-side windings are shorted together. This test focuses on the windings. By short-circuiting the low-side windings, you are shorting out the core effects on the test. Evaluating the short circuit tests and the open circuit tests allows you to determine if the change in the curve is due to faults in the core or the windings.
- Capacitive Inter-winding: the signal is applied to one terminal on the high-side winding, and the response is measured on the corresponding terminal of the low-side winding. Three of these tests are performed, one for each phase/winding. This test focuses on the capacitance between the windings and helps detect radial deformations.
- Inductive Inter-winding: this is similar to the capacitive inter-winding with the exception that the opposite ends of each winding that the signal is applied to, and measured on, are grounded. This test focuses on the inductance of both windings.
Since the transformer can be modelled as a complex RLC circuit, every transfer will have a unique response. Still, there will be some commonalities based on transformer design. There is no set frequency range that corresponds to the components of the transformer, but there are some general ranges. The following frequency response ranges for an open circuit test on a transformer are the most common:
- The response in the lowest frequencies, approximately 20 Hz to 10 kHz, is dominated by the transformer's core. However, the windings will have some influence on this section of the response.
- As you move into the midrange frequencies of 2 kHz to 500 kHz, the windings will influence the response most.
- At the highest frequencies starting from a few hundred thousand Hz up to 1 to 10 MHz and beyond, the taps and connections of the transformer will make up most of the response. However, as the frequency sweeps past 1 MHz for transformers greater than 72.5 kV and 2 MHz for transformers 72.5 kV and below, the setup and connections of the instrument will have the most significant influence on the response. These are general guidelines, and the influence of the components can and will vary outside of these frequencies.
SFRA test results are evaluated using comparative analysis. Reference SFRA test results may be in the form of any or all of the following (listed in order of most valuable).
This is the most reliable approach to interpreting SFRA test results. Deviations between SFRA curves are easy to detect and often indicate a problem. For this reason, it is desirable to obtain benchmark SFRA test results on a transformer when it is in known good condition, such as during its commissioning, to have a reliable future reference with which to compare. The test conditions, e.g., the position of the tap changer(s), the type of SFRA test, and any special preparation, should be the same for the reference and repeated measurements for correct interpretation.
Care is required with this approach, as minor deviations between traces do not necessarily indicate a problem. This approach also requires knowledge about the transformer under test.
When evaluating or comparing the results of transformers based on the same design, you want to make sure they are as similar as possible. Comparing transformers with the exact specifications but manufactured by different companies or even by the same company in different years can reveal significantly different traces. Note, too, that just because the same manufacturer builds two transformers with the same ratings that are even just one serial number apart (e.g., single-phase generator transformers or multiple three-phase transformers delivered in the same order) does not guarantee the construction of the units is the same. That being said, the latter is the ideal transformer group for comparative purposes when using this approach.
If the traces are very similar, you can be reasonably confident that the transformers are in good condition. If the traces vary a little, then the differences in readings may reflect a dissimilarity in construction versus an actual issue with the transformer(s).
This approach is the most challenging, as minor deviations between traces may be completely normal. For example, comparing the SFRA responses of a three-phase transformer's two outer phases. The middle (centre) phase response typically differs from the outer phase responses, particularly in the core region of the open circuit test responses. As the sweep moves up in frequency and the winding starts to dominate the response, each phase's trace will mimic each other and, in some cases, look identical. With this being said, there may not be symmetry between the outer-wound phases.
Despite its challenges, the phase comparison approach is an exceptionally insightful diagnostic for short-circuit SFRA tests. For these tests, all three short-circuit responses should be nearly identical. A magnified view of the linear part of the inductive roll-off should reveal no more than a 0.1 dB difference between the three traces, and the roll-off should be close to -20 dB/ decade. Poor connections (i.e., increased resistance) will affect the short-circuit SFRA responses at the lowest frequencies (e.g., 20 Hz). Short of that, you may need to check the transformer with DC winding resistance tests in these cases.
The figure below shows a typical response of a high voltage (HV) open circuit, a low voltage (LV) open circuit, and a HV short circuit test, respectively, on a two-winding DELTA-WYE transformer:
When comparing new traces to a reference trace, any of the following can indicate a potential mechanical change:
- Resonance (i.e., peaks and valleys) shifts
- Additional resonance
- Loss of resonance
- Overall magnitude difference
For a more detailed explanation of SFRA and results interpretation along with examples, contact us for a complementary copy of our comprehensive SFRA Transformer Life Management Bulletin.