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Diagnostic Test: 1-500 Hz Narrow Band Dielectric Frequency Response

Diagnostic Test: 1-500 Hz Narrow Band Dielectric Frequency Response

Diagnostic Test: 1-500 Hz Narrow Band Dielectric Frequency Response

For approximately 25 years, dielectric frequency response (DFR) has been investigated and proven to provide an accurate method of determining both the moisture condition of paper + oil power transformers and bushings. Together with general insulation testing, DFR provides enhanced insulation condition of the transformer under test.

DFR testing method is now accepted internationally – as evidenced by its inclusion in the CIGRE TB445 as an accepted method for moisture determination. IEEE is presently in the process of completing a new guide for use of DFR in transformer moisture assessment for both factory and field testing, which should be completed and available by 2019. Instruments like the Megger IDAX300, which was the first commercially available unit to perform this test, is a valuable tool in this determination of moisture content.

IDAX300 has included the ability to perform PF tests at frequencies from 1 to 500Hz which can be defined as a narrow frequency band dielectric frequency response test. We are now able to provide a shorter duration DFR test which gives us additional information into the overall insulation condition of the transformer or bushing.

This paper illustrated a narrow band DFR test performed on a transformer located in Thailand – EGAT (power utility) and the result was recorded. It is proved that narrow band DFR test can provide a base line for any future testing comparison. We can use both a curve shape and/or a trend change for looking for issues. We no longer need to only rely on the line frequency PF(DF) to detect future issues.

Author(s): Vince Oppedisano and Mohammad Tariq
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