16 / 2022-02-23 21:26:50

An Improved Analytical Method for Radiator Thermal Modelling of Natural Cooled Transformers

CFD simulation,transformer radiator,ONAN,experiment

Transformer thermal modelling is important for predicating the hot-spot temperature (HST) within the transformer both at the design stage and during the operation. To a liquid-immersed transformer in the natural cooling modes (ONAN/KNAN), the HST is primarily dependent on the total liquid flow rate, which is determined by the liquid temperature distribution in the active part and the radiator. To the transformer radiator, there have been three common methods to estimate the liquid temperature distribution, i.e. a computational fluid dynamics (CFD) model, an analytical calculation and a thermal hydraulic network model (THNM). The radiator CFD models in air natural (AN) cooling mode can also be categorized into two groups by its modelling methodology, i.e. full CFD model including air simulation and reduced CFD model excluding air simulation. The analytical calculation, THNM model and reduced CFD model have less processing time than the full CFD model, however the accuracy of them is highly dependent on the air convection heat transfer coefficient. Historically, the convection heat transfer coefficient used in transformer radiator thermal models were referred to the empirical equations from the handbook.

In this paper, the air convection heat transfer coefficient equations used in different methods are calibrated by the full CFD model parametric sweep of a transformer radiator. Different equations for the reduced CFD model and analytical calculation have been compared. The new air convection heat transfer coefficient equation for the analytical calculation was adopted and then verified by a set of experiments. It was found that the difference of total heat dissipation between the analytical calculation and experiments is within 6.2%, the difference of bottom liquid temperature between those is less than 1 K. Moreover, the validity of the air convection heat transfer coefficient equation used in the analytical calculation under different ambient temperature and in different insulating liquids are studied with additional CFD simulations.