Effect of strong DC Electric Fields on the flow properties of insulating liquids at different temperatures

The ever-increasing interest in High Voltage Direct Current (HVDC) power transmission systems, (e.g. those associated with the transport of electrical energy from rapidly expanding offshore wind farms, the bulk transmission of electrical power over long distances, etc.), is driving a great deal of research towards the development of more reliable and robust converter transformers, which, unlike their traditional power transformers counterparts, are subjected to both AC and DC stresses [1]. Consequently, classical insulation systems based on pressboard and insulating oils are becoming a critical field of research to prevent stress-driven failures. Within this context, knowledge of the influence of operational parameters, such as operating temperature and local shear rate, is of paramount importance for a thorough understanding of the phenomenological behavior of insulating and cooling oils employed in high voltage transformers, especially under strong electric fields. To this purpose the present study presents a preliminary investigation on the correlation between the flow regime of three different insulating oils and strong DC electric fields under various operating temperatures. In a previous study, the authors of the present research have already investigated the temperature dependence of dynamic viscosity of insulating oils employed in power transformers [2]. As a natural extension of previously obtained results, the current study involves a more detailed investigation on both commercially available natural ester and mineral oils. Results provide a sound basis for the rational design of high power/high performance transformers for both AC and DC applications.