Investigation of Alternative Liquids and Ambient Temperature on Transformer Thermal Behaviour

The influence of alternative liquids and ambient temperature on transformer thermal behaviour becomes an increasingly important topic to the transformer community. Collective research efforts on this topic are being made by a transformer research consortium, which consists of the University of Manchester and industrial sponsors. The influence of different liquid types and a wide range of ambient temperature on transformer thermal behaviour can be combined into the influence of different liquid properties. To capture the essence of the liquid property effect, dimensional analysis is adopted to guide the investigation and to gain simplicity, insight and universally applicable results. Computational fluid dynamics (CFD) models and experimental measurements for liquid flow distribution and temperature distribution in a winding test rig were implemented for both pump-driven oil directed (OD) and buoyancy-driven oil natural (ON) cooling modes.

For OD cooling modes, the liquid flow and temperature distributions are mainly controlled by the Reynolds number (Re) at the winding pass inlet. The higher Re the more uneven the flow and temperature distributions. When Re is high enough, reverse liquid flows occur at the bottom radial cooling ducts in the winding pass and cause localised overheating. Alternative liquids with higher viscosity and lower ambient temperature both result in lower Re, making flow and temperature distributions relatively more uniform at the expense of higher pressure drop.

For ON cooling modes, the liquid flow and temperature distributions are mainly controlled by the Richardson number (Ri) and Re. For a fixed Ri, lower Re caused by viscous liquid or lower ambient temperature results in more uniform flow and temperature distributions. In reality, however, more viscous liquid and lower ambient temperature results in lower total liquid flow rate and therefore higher Ri. The higher Ri the more uneven the flow and temperature distributions. When Ri is high enough, reverse flows occur in the top radial ducts of the winding pass, opposite to OD cooling modes, causing localised overheating.