Multi-physics modeling of cooling of power transformers immersed in a ferrofluid

Overview & Synopsis

Ecological considerations lead to nanoparticles with low Curie temperature. The use of vegetable oils seeded with magnetic nanoparticles as an alternative to petroleum oils in power transformers. However, the macroscopic physical properties of these suspensions and the resulting forces are poorly known. To study them, we propose a coupled approach, both numerical and experimental, of this multi-physical problem combining thermal, magnetic, and fluidic effects. The multiphysics problem consists of the Navier-Stokes equations with Boussinesq approximation, the energy conservation equation, and the magnetostatic equations. The simulations are performed first with two finite element codes to verify the impact of the thermomagnetic convection in a solenoid immersed in a ferrofluid. Numerical results have shown a decrease by 2°C of the temperature of the coil. In parallel, the numerical results have been compared to experimental ones and have shown a very good agreement. Then, we have made two tests to improve the thermomagnetic convection in the solenoid model. The first test consists of studying the effect of magnetic nanoparticles with low Curie temperature. The second test investigate the impact of an annular magnet at an optimized location to maximize the leakage flux in the ferrofluid. We have also inserted a ferromagnetic core in the coil system as a first approach for the power transformer to increase the heat transfer inside the tank. We have also performed a 3D validation of the solenoid system as a transition to the 3D modeling. A 2D axisymmetric model of a 40 kVA, 20kV/400V single-phase power transformer is then studied. The impact of the thermomagnetic convection on the heat transfer process is verified. The maximum temperature decrease in the windings is evaluated to 2°C when using ferrofluid. We have performed a 3D study for the same model of the simplified transformer when it is immersed in regular oil. Results have shown a relatively good agreement. Finally, we have modeled a 3 kVA, 400/230V non-axisymmetric single-phase power transformer. The numerical results have shown a significant decrease in the maximum temperature of the windings by 10°C when the magnetic force is present.