Abstract
Medium-frequency (MF) transformer has gained much popularity in power conversion systems. Temperature control is a paramount concern, as the unexpected high temperature declines the safety and life expectancy of transformer. The scrutiny of losses and thermal-fluid behavior are thereby critical for the design of MF transformers. This paper proposes a coupled, semi-numerical model for electromagnetic and thermal-fluid analysis of MF oil natural air natural (ONAN) transformer. An analytical model that is based on spatial distribution of flux density and AC factor is exploited to calculate the system losses, while the thermal-hydraulic behavior is modelled numerically leveraging the computational fluid dynamics (CFD) method. A close-loop iterative framework is formulated by coupling the analytical model-based electromagnetic analysis and CFD-based thermal-fluid analysis to address the temperature dependence. Experiments are performed on two transformer prototypes with different conductor types and physical geometries for validation purpose. Results suggest that the proposed model can accurately model the AC effects, losses, and the temperature rises at different system components. The proposed model is computationally more efficient than the full numerical method but it reserves accurate thermal-hydraulic characterization, thus it is promising for engineering utilization.
Highlights
Power transformer is a critical component in electricity distribution systems
Instead of using the complicated numerical method, this section describes an analytical method for the determination of core loss and winding loss, which are respectively based on the spatial distribution of flux density and AC factor
The proposed model scrutinizes the spatial distribution of flux density contributes to improving the modeling accuracy of core loss
Summary
Power transformer is a critical component in electricity distribution systems. Nowadays, the medium-frequency (MF) transformer has become an important part of many power conversion systems. It should be noted that the accurate description of fluid circulation, which determines the heat dissipation condition to be essential to the thermal analysis of oil-immersed power transformers In this regard, the thermal-hydraulic network model has been widely used to characterize the circulation of oil and the temperature rise [20,21,22]. Computational fluid dynamics (CFD) model was carried out and it was shown that the 3D model, albeit computationally intensive, was needed to simulate the non-uniform flow and temperature distribution accurately [29] To this end, a 3D finite element modelling (FEM)-based thermal model was used in [30] to determine the loss and temperature rise of a high-frequency transformer. A quasi-3D coupled numerical method that combines the 3D core loss and velocity simulation with the 2D fluid-thermal analysis was proposed in [34] to analyze the temperature rise of transformer.
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