Thermal-fluid characterization of alternative liquids of power transformers: a numerical approach Ramón Lecuna, Fernando Delgado, Alfredo Ortiz, Pablo B. Castro, Inmaculada Fernandez and Carlos J. Renedo Mining and Energy Engineering School, Cantabria University Bulevar Ronda Rufino Peón, s/n, 39316, Torrelavega, (Spain) Phone/Fax number:+34 9422013 76/85; e-mail: ramon.lecuna@unican.es ABSTRACT The transformers lifespan depends importantly on its refrigeration. Mineral oils perform this work in the majority of the power transformers. However, this type of coolant has two main drawbacks: low biodegradability and low ignition point. Several alternative liquids are being developed in order to overcome these drawbacks. This paper compares their thermal-fluid behavior with a mineral oil by means of several parameters, such as temperature, flow rate, fluids velocity, convective heat transfer coefficient (h) and the cooling criterion (P). These are calculated using the numerical results of the simulation of a 3D-model of a Low Voltage Winding that belongs to a power transformer with ONAN cooling. The software COMSOL Multiphysics has allowed the simulation of the geometry using a physical model in which buoyancies and viscous forces are the only considered establishing the natural convection. As a result of the comparison, it is clear that the mineral oil is the best coolant liquid. Among the alternative liquids, silicone oil would be the second best coolant fluid, followed by the synthetic and natural esters, respectively. On the other hand, it seems to be clear that the 3D simulations can be used to compare properly the cooling capacities of the liquids. Index Terms —Dielectric liquids, fluid-dynamics, thermal analysis, power transformers, numerical analysis 1. INTRODUCTION 1 MINERAL oil is the most common option as a cooling 2 and dielectric liquid in the majority of the power transformers 3 worldwide. However, in cases where fire risk is an important 4 concern, this type of liquid is not so recommendable. Fire 5 resistant oils (with higher flash and fire points than those of the 6 mineral oils) should be used. Environmental reasons are also 7 supporting the development of new transformer oils with 8 improved biodegradability, so that in the event of a failure or 9 leakage the impact would be lower. Thus, the growing 10 demands for improved fire safety and environmental 11 sustainability have encouraged the research and development 12 of alternative fluids. 13 The main research lines of these liquids are focused in 14 silicone oils, natural and synthetic esters. The characterization 15 of silicone oils and synthetic esters has been studied by a few 16 authors [1-5]. However, the majority of the studies has been 17 focused in the physicochemical characterization of some 18 commercial natural esters, [6-8], or based on some specific 19 crop (coconut, palm, rapeseed…) [9-10]. Finally, some authors 20 have compared the main properties of these new fluids with 21 mineral oil in order to evaluate their suitability [11]. 22 On the other hand, there has been a lot of research about 23 cooling improvement in power transformers. The reason is 24 simple, high temperatures degrade the dielectric materials, oil 25 and paper, shortening their lifespan. In order to ensure a long 26 life for these machines, there are two types of approximations 27 for the calculation of their temperature and velocity 28 distributions: lumped parameter models and Computational 29 Finite Element-based Tools (CFET). The first method provides 30 fast and approximate results based on several simplifications 31 and empirical data. By contrast, the second one is more 32 accurate since it is based on the solution of the differential 33 equations governing processes. 34 Several papers have been published in last decade using CFET. 35 Nonetheless, we have to mention that the main goal of 36 practically all these papers is the determination of the velocity 37 and temperature profiles of a mineral oil inside a 2D section of 38 one winding. For instance, Mufuta and Van den Buck 39 described the flow pattern and its influence in the cooling of 40 the windings of a disc-type transformer by means of 41 dimensionless parameters (Nu, Re, Gr) applied on a 2D model 42 [12]. Six years later, El Wakil et al. studied the heat transfer 43 and fluid flow in two windings wound around a core of a step- 44 down 3-phase layer-type power transformer by means of the 45 analysis of six different 2D-models [13]. In 2007, Rahimpur et 46 Manuscript received on X Month 2005, in final form XX Month 2005.