Transformer Testing: Dielectric Frequency Response(DFR)
What is DFR?
Dielectric Frequency Response(DFR) analysis, is mainly used to assess the moisture content of the cellulose insulation. Moisture in oil-paper insulated power transformers is due to ingress via leaky seals or poor breathing as well as normal paper aging mechanisms. Moisture in transformers can lower the dielectric strength of the oil which has a direct effect on the insulation properties. Moisture also prematurely ages the cellulose insulation leaving it with less mechanical strength. Therefore moisture content correlates directly with the longevity of the transformer.
How does it work?
DFR involves measuring the insulation response at different applied frequencies. Since the molecular composition of H2O has a non linear response to applied frequency, the generated curve, slope, peaks and troughs can be used as a comparative tool in assessment.
Measurement setup
The bulk of cellulose insulation in the active part of a power transformer is located between the primary and secondary winding. To measure this insulation, the test instrument injection leads are connected to the high-voltage winding and the input to the low-voltage winding. Unwanted capacitive and resistive currents are captured by the guard connections and negated. The test instrument applies a predetermined frequency sweep and the power factor/dissipation factor of this insulation is then measured over the frequency range.
Analysis
The PF/DF at low frequencies are often correlated with moisture in the solid insulation, while the position of the slope in the mid range frequencies, represent the conductivity of the liquid insulation. The resultant curve must be compared to model curves and /or similar vintage equipment for analysis purposes.
Wideband Vs Narrow Band DFR
Advanced DFR test sets can preform a wide frequency sweep range from µHz to a few kHz. This is referred to as “wide band DFR”. Standard 10KV PF/DF test sets are now available with frequency sweep capabilities from low Hz to a few hundred Hz. This is referred to as “narrow band DFR. This reduced narrow band frequency range cannot provide the comparative moisture levels of wide band DFR, however the shape and slope of the obtained narrow band curve, can still be used for overall condition assessment and insulation trending purposes.
Available test instruments
Widely adopted wideband DFR test sets are Megger IDAX300/350 and Omicron Dirana.
Narrow band DFR sets are Omicron Testrano/CPC TD12/15 and Megger Delta 4000 series.
These instruments vary in measurement methodology as well as power, voltage and frequency capabilities. Care must be taken in the selection methodology to meet the required application. If in doubt locate a qualified engineering and testing firm for support.
Further reading
IEC 60422, Cigre TB 254, Cigre TB 414, Cigre TB 445, Cigre TB 775, IEEE C57.152-2013, IEEE C57.161-2018