IDAX322

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. 

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.

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%.

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.

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.

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.

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.