Dielectric frequency response measurements and dissipation factor temperature dependence

The condition of the insulation is an essential aspect for the operational reliability of electrical power transformers, generators, cables and other high voltage equipment. Transformers with high moisture content can not without risk sustain higher loads. Bushings and cables with high moisture content at high temperature can explode due to “thermal runaway”. Typically, dissipation factor (DF) or power factor (PF) test at power frequency 50/60 Hz is carried out in the field following well known procedures. DF measured values are then normalized to 20oC for comparisons with guidelines and trending. However, the temperature correction factor for the normalization of the field measurement is questioned because cellulose with different moisture contents as well as oil with different conductivity will have different correction factors. It has been proved that good insulation has less temperature dependent response than the bad insulation. DFR modeling, accurately taking the temperature effect into account, can be used to model the temperature dependence of DF over a wide temperature range based on measurements over a frequency range. Naturally, from such modeling, also temperature correction factors for correcting a measurement values obtained at one temperature, e.g. 32C, to a reference temperature, e.g. 20C, can be calculated. This paper will provide a background of DFR and its modeling specifically applied to model the effect of temperature and calculating temperature correction factors based on actual DFR response. The theoretical basis is backed up with several case studies of measurement on samples as well as on real objects, e.g. bushing and transformers at multiple temperatures.