Insulating oil dielectric tests

In order to improve power transformer reliability, a special focus has been carried out on insulating materials and especially on insulating oils. The majority of high voltage transformers are filled with liquids that work as an electricalinsulation as well as a heat transfer medium. Thus, for a transformer with a smaller size (by means of a reduced insulation gap), the liquids have to ensure the integrity of this gap for the existing or even higher voltage levels, and to enable the cooling to be still effective. The most commonly used liquid in power transformers is mineral oil [1] due to its low price and its good properties. However the performance of mineral oil starts to be limited with respect to the new requirements. For that purpose, numerous activities have been initiated to try to improve the properties of mineral oil or to find other substitute liquids.

Breakdown voltage (BDV) measurements are achieved at room temperature, according to the IEC 60156 specification, i.e., under a quasi-uniform electric field obtained with spherical electrodes having a horizontal axis. The gap was set to 2.5 0.05 mm. The voltage is uniformly increased (2.0 0.2 kV/s) and the oil is stirred during all the tests. A Baur Dieltest (100kV-50 Hz) device is used. Before each test, the samples are treated by means of an 11-16 μm sintered glass filter under vacuum (102 Pa). All samples are processed in the same conditions, i.e. they don’t have specific treatment to create the same relative saturation (percentage of saturation with respect to the oil solubility at a given temperature).