The effect of temperature and field strength on the resistivity of transformer oil
Safety operation of total ultra-HVDC transmission system could be directly affected by the operation reliability of converter transformer which is one of the most important equipments in the system. The transformer oil is the main liquid dielectric medium in the converter transformer. Thus, it is significant to study the dielectric properties of the transformer oil. In this paper, the dc resistivity of transformer oil under different conditions has been systematically investigated, and then the temperature and field strength were the test parameter variables. The variation range of temperature was 25~80°C, and under each temperature point the field strength changed ranging from 0.5V/mm to 10kV/mm. Meanwhile, 15 min is selected as the duration of testing resistivity. In this paper, the authors obtained the systematical data points and analysis curves of resistivity under different temperature and field strength, and obtained a empirical equation which describe the relationship between resistivity, temperature and field strength. It enhanced the understanding of the resistivity for transformer oil and provided the theoretical and test basis to expand the engineering application of this transformer oil.
Effect of temperature on the dielectric properties of hydrofluoroethers and fluorinated ketone
The AC and DC dielectric properties of hydrofluoroethers (HFE) [C3F7 OCH3 ] and fluorinated ketone (FK) [C2F5 C(O)CF(CF3)2] have been characterised by dielectric spectroscopy and DC conductivity at different temperatures. Results show that DC conductivity and imaginary permittivity of both fluids are positively correlated with increasing temperature. However, the real permittivity decreases with increasing temperature. The breakdown voltages of HFE and FK at 295 K are∼10 kV mm−1 . Reducing temperature is an effective method to increase the breakdown voltage of the FK coolant, but the breakdown voltage of HFE is less temperature dependent. Finally, as expected repeated breakdown had no significant effect on the AC dielectric strength of both liquids.
An electrical resistance test for insulating oils
This paper describes a d.c. resistance test which has been developed in connection with the maintenance of transformer and switch oils on the Central Electricity Board's system. Details of the testing technique are given, and the results of resistance measurements made on representative samples of oil in various stages of deterioration are discussed.It is shown that the d.c. resistance is related to the power factor sufficiently closely to enable either parameter to be used equally well as an indication of the characteristics of the oil. Data are given on the variation of resistance with temperature, and it is found that, for used oils of relatively low resistance, the logarithm of the resistance is proportional to the reciprocal of the absolute temperature.It is demonstrated that d.c. resistance measurements can be used as a criterion of oil condition if the following limits of resistivity at 20°C are adopted:—“Good”—Greater than 20 × 106 megohm-cm.“Fair”—From 2 to 20 × 106 megohm-cm.“Bad”—Less than 2 × 106 megohm-cm.It is concluded that the resistance test gives more information about the condition of used oil than any other single test, and it is suggested that consideration should be given to its adoption as a standard test.