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 in conditions in 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 by-products, moisture, liquid insulation conductivity, frequency, and temperature. Capitalising on knowledge about these relationships, an assessment of aging by-products, 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.