• State-of-the-art, high voltage DFR instrument tailored for field testing of power transformers, bushings, current transformers, voltage transformers, cables and more.
  • The IDAX322 instrument and its accessories are designed for the most demanding field conditions
  • 2 kV peak and 50 mA capacity – ideal for low capacitance objects such as bushings and instrument transformers
  • Best hardware and specifications for the most accurate results
  • Easy to use software with integrated assessment guidance aligned with standards and informed by 25+ years of field experience with DFR technology.

What does DFR mean?

DFR stands for Dielectric Frequency Response. The test is also known as FDS (Frequency Domain Spectroscopy). DFR is a measurement technique in which capacitance and losses (expressed as dissipation factor/tan delta or power factor) are measured over multiple frequencies to assess the insulation condition in test objects, such as power transformers, bushings and instrument transformers. DFR technology is an established test procedure in laboratories that, in an innovative effort by Megger, has been adapted for field use through the IDAX range of instruments. In transformers, bushings and instrument transformers, issues are not often visible at conditions at which it is easy to perform diagnostic tests (typically, at ambient temperature and line frequency). Rather, problems are generally best exposed at higher temperatures or closer to the operational limits of the objects. Unfortunately, temperature is not easily or efficiently controlled in a field test environment. The power of a DFR test is that tan delta or power factor is the basis for its measurements. Tan delta or power factor is primarily a function of insulation system geometry, aging byproducts, moisture, liquid insulation conductivity, frequency, and temperature. Capitalising on knowledge about these relationships, an assessment of aging byproducts, moisture and conductivity can be made in the frequency domain, via DFR, rather than in the much more difficult to control temperature domain. Therefore, DFR makes it easy to find problems in the insulation under conveniently achieved conditions in the field.

Technical Documents
Technical guide
Case study: Identifying aging and installation issues in an HV bushing
Technical guide
The Megger IDAX322: Achieving DFR success for all applications
IDAX Software
47.2 MB | 13/04/21
What information does a DFR test using the IDAX provide?

DFR test curves are provided for all measurements. Depending on the asset, additional, discrete test results are reported. For example, a transformer report includes the moisture content of the transformer’s solid insulation, the conductivity of its liquid insulation, and the transformer overall insulation’s 50/60 Hz tan delta or power factor value. When testing a bushing, the percent dissipation factor or power factor value is reported at three different frequencies. 

How much time does a DFR test take?

The time required to complete a DFR test depends upon the asset being tested and, in the case of a transformer, its condition. The better the transformer’s insulation health, the longer the test requires. However, a DFR test using the IDAX will generally be less than 20 minutes. For a bushing, a DFR test takes less than 5 minutes.

What makes the IDAX so fast?

Any time that you can perform an entire measurement in only FDS (frequency domain spectroscopy) mode, you have a better, more reliable measurement. A true frequency domain method that speeds up measurement time is accomplished by applying several frequencies simultaneously, and extracting measurement data from the applied frequencies simultaneously. Because the voltage level of each frequency needs to be reduced, the measurements are a bit more sensitive to AC interference, but DC current interference will not affect the measurements. This multi-frequency approach is an advance over the older approach of combining FDS with PDC, which is more sensitive to AC interference and also quite sensitive to DC interference.

The time savings of the multi-frequency method are significant. For example, using three frequencies simultaneously reduces measurement time by about 40%.

Why is having 2 meters in a DFR test set (a feature that is unique to the Megger IDAX range of instruments) important?

The benefit of having two available metering systems in one instrument is the unique advantage of testing two capacitances simultaneously. For example, the IDAX can test two HV bushings at the same time. It can also measure both of a three-winding transformer’s interwinding insulation systems, e.g., CLH and CLT, at once.

No other instrument is capable of simultaneous measurement in the frequency domain. In some dual-channel instruments, both channels share a single ammeter. With these instruments, either half as many measurements are made (i.e., less accuracy), or no timesaving is realized as compared to performing two separate tests.

For which applications do I need a high voltage (HV) DFR instrument like the IDAX 322?

The most commonly used voltage level is 140 V RMS and this is enough for measurement of the CHL insulation of a transformer in most conditions. However, in situations where there is a high level of interference or when measuring CH, CL (of a transformer), reactors, bushings and current transformers (CTs), a 140V RMS signal does not provide a high enough measurement signal-to-noise ratio to get meaningful results. Using a higher test voltage, like the IDAX 322’s 1,400 V RMS/2,000 V peak, in these instances will improve the measurement accuracy and is recommended.

When should DFR be used instead of taking an oil sample or having a constant DGA on the transformer?

Moisture assessment of a transformer’s solid insulation by DFR is more accurate than taking an oil sample for a moisture content test. The latter often results in over estimations of water-in-paper content. However, an oil sample can be taken while the transformer is still energized. A DFR test is performed when a transformer is deenergized. An on-line moisture monitor installed in the transformer provides ‘anytime’ trending of moisture but requires an outage to install and is, thereafter, tracking only one transformer. Therefore, this approach is relatively expensive. As an asset owner, the course of ensuing action that you plan to take under various scenarios should inform the method you choose to assess moisture in your transformers. If one wants an accurate assessment of the moisture content in their transformer’s solid insulation so that they can determine whether or not to process the unit, DFR is an excellent choice.

What is the difference between DRF & SFRA?

DFR and SFRA, Sweep Frequency Response Analysis, are two very different tests that are often confused because they both entail making measurements at many different frequencies. As noted above, DFR assess how the losses in insulation behave as frequency changes. SFRA, on the other hand, assesses how effectively a voltage signal is propagated through a winding at many different frequencies. An SFRA test provides a mechanical assessment of a transformer.