Dielectric frequency response (DFR) analysis for moisture measurement in transformer

The dielectric response is a unique characteristic of the particular insulation system. The increased moisture content of the insulation results in a changed dielectric model and, consequently, a changed dielectric response. By measuring the dielectric response of the equipment in a wide frequency range, the moisture content can be assessed and the insulation condition diagnosed.

The test connections and modes are the same as used in a traditional transformer insulation power-factor test with the difference being the test is performed at a low voltage, up to 200 Vp-p, and the test is performed at frequencies from 1 kHz to 1μHz.

In this Figure the response curve for oil-impregnated paper. This curve shows a frequency vs. dissipation factor relationship.

The higher frequencies display the moisture and aging of the cellulose.
Moving from left to right the frequency is reduced and the oil conductivity properties are displayed. In the millihertz range, the insulation geometry comes into play.
As the moisture properties of the cellulose change so does the shape of the curve.

The most common techniques used to measure this response are frequency domain spectroscopy (FDS) and polarization and depolarization current (PDC) methods. Using the frequency domain spectroscopy, the dissipation factor of the insulation system under test is measured by frequency sweep. The FDS allows fast measurements at high frequencies but requires long measurement times at frequencies down to 0.1 millihertz.

After the test is completed the results must be compared to a standard to determine the actual moisture content. IEC 60422 and IEEE 62-1995 have defined moisture classifications.

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