FRAX series sweep frequency response analysers

SFRA and DFR are entirely different tests. SFRA looks at any kind of mechanical changes inside the transformer, whereas DFR is used to determine the moisture present in the cellulose (solid insulation) of oil-filled power transformers. The two tests have very different applications.

You derive the most significant diagnostic detail in an SFRA analysis by comparing current tests to previous tests or a transformer’s initial ‘fingerprint.’ Therefore, we recommend performing SFRA on new transformers when commissioning, or if the unit is already in service, ASAP, when the transformer is in known good condition. SFRA has also become very popular when transporting a transformer to its site. A ‘sweep’ can be made at the factory before the transformer’s shipment and once again at the site when received to check that no damage occurred during transportation. For the validity of a direct comparison between results, you must emulate all aspects of the original test, including testing with the test or transportation bushings installed and with the transformer filled with oil (or not).

Yes, each test will provide different information about your transformer. Power factor and DFR tests look at the insulation of the transformer. Turns ratio and winding resistance tests reveal the condition of the windings. SFRA provides notable information about the mechanical integrity of the transformer and can help you determine if a transformer has sustained any mechanical damage. Each electrical test you perform gives you a bit more insight, and together they form a good picture of your transformer’s health. Sometimes a ‘second opinion’ from two or more tests on the same component can help you confirm a suspected problem.

For SFRA: No particular order must be followed for open circuit and short circuit SFRA tests. However, to increase efficiency, you may want to run the tests in an order that will help you minimise lead changes, e.g., after running an HV open test of H1-H3, you can quickly short the low side windings and run the HV shorted test on H1-H3. From here, the leads can be switched to H2-H1 to perform the HV shorted test, then remove the shorts to perform the HV open test on H2-H1, finishing by measuring the H3-H2 open and shorted tests. This test order may save time rather than switching the HV connections between all six tests, particularly in the case of higher voltage transformers where you cannot easily reach the terminals. It is much easier to apply and remove shorts on the transformer’s low side than change the high side connections multiple times.For complete electrical tests: Excitation current and SFRA tests should be completed first and winding resistance tests last. This recommendation is to prevent residual magnetism from the winding resistance test from affecting the results of the other tests. However, you do not have to worry about the test sequence if your winding resistance test set can demagnetise the transformer efficiently after the test. One can argue that there is a benefit in bringing a transformer’s core to a consistent state of magnetisation (via an effective demag feature of a test instrument) at the start of an ensuing test sequence that includes excitation current and SFRA tests.

Yes, SFRA is a very sensitive test that can pick up small physical or mechanical changes inside the transformer. Therefore, having any extra connections to the transformer, e.g., the bus, can significantly change the response (particularly in the higher frequencies). Fully isolating the transformer will help secure the most repeatable results and the best analysis.

In an ideal world, when commissioning the transformer, you would test at all taps, i.e., each position of the DETC with the OLTC set to its nominal position and all tap positions of the OLTC with the DETC set to its in-service position. With each tap position requiring 15 sweeps for a two-winding transformer, this quickly becomes impractical with time and resource constraints. The general recommendation is to place the DETC in its in-service position and perform the standard 15 sweeps with the OLTC in the fully raised position; this assures that all winding turns on the OLTC will be in the measurement. Additionally, the sweeps are often repeated with the OLTC in the nominal position. When placing the OLTC in the nominal position, you should approach from the ‘raised one’ tap. By making SFRA measurements in both the fully raised and nominal OLTC positions, you will have a general picture of the transformer with the OLTC fully engaged and not engaged. Note: when performing SFRA measurements, always note the tap positions at which the test was made for future reference and analysis. 

Proper connections and grounding are paramount to a successful SFRA test. Ensure sufficient clamp pressure connecting to the transformer terminals and use the shortest braided ground principles. If there is any paint or corrosion at the connection points, clean it off or verify that the clamps have penetrated through it. Additionally, you can perform a ground loop check to ensure that the grounding lead connections and the transformer ground are at a common point. You can perform a ground loop check by pressing the “GLD” button on the FRAX 101 and 150 instruments or manually check with an ohmmeter if your SFRA unit does not have this feature. Poor grounding and connection issues will typically manifest in the highest frequencies (approximately 500 kHz and above). It is advisable to check connections and perform the sweep again if the sweeps vary significantly in this frequency range compared to previous measurements.

Yes. Open circuit test sweeps will change in the lower frequencies if the core is magnetised. The sweep generally will shift up and to the right. The effects of magnetisation on SFRA results are why it is important to perform SFRA tests before a DC winding resistance test if planned. If this is not possible, you should demagnetise the transformer before performing an SFRA test.

Proper test preparation and setup are essential with any electrical test. SFRA tests, in particular, are acutely sensitive to small mechanical changes within the transformer, meaning that any change in the setup can affect the response. Therefore, you should pay meticulous attention to your connections, test practices, and preparations to ensure repeatable results. Always make solid connections in the same location, follow the shortest braided ground principle, and ensure the transformer is in the same conditions when tested, including tap changer positions, bushings, and oil level. You should note anything that has changed on the transformer since the previous test. Always note the tap positions of the transformer when performing the test and, if applicable, the position from which the tap was transitioned into.

Yes, after a transformer is rebuilt, it is essentially a new transformer, so its previous measurements will differ from its current measurements. At this point, the transformer needs to be re-commissioned, and a new SFRA fingerprint needs to be taken.

Yes. SFRA test results, when performed correctly and under similar conditions (correct grounding, same tap position, and similar connections), are comparable. Factors that can affect test results include residual magnetism and poor grounding practices. Megger’s FRAX software can uniquely import previous results from any other manufacturer’s test set and compare results. The FRAX 101 and FRAX 150 also can adjust the output voltage to match legacy products from other manufacturers that didn’t use a 10 V p-p input signal.

When you select “File” under the main FRAX software menu, there is an “Import” and an “Export” option. Several alternative formats are available for selection (CIGRE, csv, txt, Doble).

Open circuit SFRA sweeps are voltage-dependent in the lower frequencies due to the magnetising impedance of the transformer. Therefore, the sweep will vary as the voltage changes. Once the sweep moves into the mid-range frequencies, where the windings fully influence the sweep, the curves will line up independent of voltage. In those cases where you desire to compare results to older sweeps at different voltages, we recommend running the test at the previous voltage and then performing the test again with the default 10 V peak voltage that the FRAX uses.

On the FRAX101 and 150, the output voltage level may be adjusted from the standard/default of 10 V up to 12 V and down to 0.1 V by changing a command line in a file in the FRAX directory. The file name is “connectioncommands.txt,” and its default location is C:\Program Files\Megger\FRAX. To adjust the output voltage, open the file in Notepad and add the command “gen:gainx=k” to the file. K is a factor for setting the voltage and is defaulted to k = 1 for 10 Vpeak. For example, to set the output voltage to 2.828 Vpeak (2 V RMS), change the value to k = 0.2828. The value must be between 1.2 and 0.01. Save the changes and exit. You must disconnect and reconnect to the FRAX to activate the new setting.

IEEE C57.149 states that a “Large temperature difference, typically much more than 10 ºC, between two measurements will slightly influence the response at higher frequencies”.For practical purposes, the effect of temperature on SFRA measurements is very small and can be ignored as long as there is not a considerable temperature variation between the two comparison traces.

You would have to run a total of 30 different tests.

• 12 open circuit tests, one on each winding (4 windings x 3 phases =12 tests)
• 18 short circuit tests:
  • 9 tests (From the high side with three secondaries shorted one at a time)
  • 6 tests (From the X side with other two secondaries shorted one at a time)
  • 3 tests (From the Y side with the last secondary shorted)

In such cases, IEEE C57.152 recommends performing all electrical tests, including power factor and SFRA. A power factor test may reveal a change in insulation condition and capacitance, while an SFRA trace will help diagnose any issues or failures associated with transformer windings.

There is no industry guideline for using SFRA based on a transformer’s VA ratings. In theory, you can perform SFRA on any size transformer (or windings, such as motors). If subsequent tests are performed under similar conditions, the results can be compared and analysed. Other electrical tests like transformer turns ratio (TTR), excitation current, and DC insulation tests will also give valuable information on smaller transformers.

Yes. SFRA looks at the response of the complex RLC network inside a transformer. You can perform baseline or reference measurements on dry-type transformers and compare results over the years. For dry-type transformers, you need to be aware of the effect that ground capacitances can have on the traces. Additionally, the response on the low side may have slight deviations because of low signal levels. A very good ground plane will produce more repeatable measurements. 

Traditional open and short circuit tests are typically performed in factories to determine the transformer’s no load and copper losses. The manufacturer commonly uses sources equal to the transformer’s ‘rated value’ when performing these tests. By determining the no load and copper losses, you can resolve the different components in an equivalent circuit of a transformer. Although they share similar names and connections, SFRA open circuit and short circuit tests are entirely different. The SFRA open circuit test looks at the electrical response of the core and winding, and the SFRA short circuit test isolates the transformer’s winding response. These tests are operated at a low voltage of 10 V p-p but help you narrow down the areas where a problem might be.

Per IEEE C57.149, testing with oil is the most common and preferred method for frequency response analysis. Safety should be considered when testing a transformer without oil so that excessive voltages are not applied. The presence of oil changes the frequency response. Results with and without oil will cause variations in the SFRA traces. Below is an excerpt from the IEEE guidelines:“For new equipment, this may require the performance of two FRA tests after receipt of the equipment at the final destination; 1) one test with the transformer in its shipping configuration, 2) and one test with the transformer assembled and oil-filled as required for insulation resistance testing, to be used as baseline data for future testing. If no shipping damage is suspected, the test in the as-shipped configuration may not be necessary as a receipt test”.Often, the manufacturer fills and drains the transformer before shipment. You should know the conditions under which the manufacturer performed an SFRA test before shipment from the factory. IEEE further states that:“If the equipment is to arrive drained of oil, the shipping configuration should specify that it will be tested pre and post movement without oil. If the equipment is to be shipped after being drained of oil, it should be tested pre-movement without oil. Testing the unit prior to shipment in this case without oil and prior to a first fill, may not be adequate and could lead to false failures due to residual oil being held in the windings, or additional oil draining from the winding during weeks of shipment. If the equipment is to be shipped with oil, it should be fully filled for both pre and post movement tests. If the equipment is to be shipped partially filled, it should be tested with the same level of oil, or preferentially after oil has been added. Ensuring oil is at the same level before and after transportation for partially filled transformers can be difficult and sometimes leads to incorrect assessments.”