A NB DFR test allows for the determination of an insulation system’s unique, or individual temperature correction (ITC). This is significant as testing has revealed that not only does every transformer exhibit unique sensitivity to temperature and require individual temperature compensation, but, over its life, the temperature dependency of a transformer can change. Generally, as insulation deteriorates, an increase in temperature causes power factor/tan delta (PF/TD) to increase dramatically. Also of interest is that a transformer’s individual insulation components (CH, CHL, and CL) may each exhibit differing temperature dependencies.
The ITC method is based on the fact that the shape of the dielectric response (PF/TD versus frequency) for a large group of solid dielectric materials does not change drastically with temperature . Further, as temperature changes, the response shifts with respect to frequency while remaining intact. A PF/DF value measured at 60 Hz and 20 °C will occur at a different frequency if the temperature changes. Therefore, if testing at a non-20 °C insulation temperature, one can locate the 20 °C equivalent 60 Hz power factor somewhere along the measured response if the frequency at which it occurs at that temperature is known. This frequency is determined through the application of the Arrhenius equation.