Dielectric dissipation factor and resistivity of liquid refrigerants and dependency on temperature

In order to increase the power density of electrical equipment the direct cooling of live parts by using refrigerants is a promising solution. R134a and R227ea are two refrigerants which are widely used due to their favorable low vapor pressure characteristics. In this study they are analyzed concerning their resistivity and their dielectric dissipation factor. The physical quantities are measured by using a plate-plate arrangement according to IEC 60093. In two separate experiments the refrigerants are then stressed at different temperatures ranging from 20 °C up to 90 °C by a DC-voltage of ± 500 V and a low AC-voltage of max. 5 V, respectively. The experiments show that R134a tends to exhibit a lower resistivity compared to R227ea. The dielectric dissipation factor of R134a is significantly higher than in case of R227ea. In R134a the high dielectric dissipation factor leads to a marked energy dissipation in the liquid refrigerant even at low electric voltage stress. There is evidence that low electric field strengths are sufficient to cause decomposition of R134a.