Coupled electromagnetic-thermal analysis of roadway inductive power transfer pads within a model pavement

High performance, highly efficient DC-DC converters play a key role in improving the penetration of renewable energy sources in the context of smart grids in applications such as solid-state transformers, built-in power drives in electric vehicles and interfacing photovoltaic and wind-power systems. Advanced medium-frequency transformers (MFTs) are fundamental to enhance DC-DC converters and determining its behavior, therefore MFT design procedures have become increasingly important in this context. This paper investigates which type of core material, between nanocrystalline and silicon steel, has the best properties for designing MFTs for distinct applications. Unlike to other proposals, in this work, two 1 kVA-120 V/240 V-1 kHz lab MFT prototypes, with a different type of core material, are developed for the purpose of comparing its physical characteristics, behavior, and performance under real-life conditions. A final section, the experimental results show that the nanocrystalline MFT has greater power density and efficiency. The results of this work introduce nanocrystalline MFTs as an option in a wider range of applications in niches in which other materials are currently used.

https://www.mdpi.com/1996-1073/12/11/2062/pdf

Nanocrystalline and Silicon Steel Medium-Frequency Transformers Applied to DC-DC Converters: Analysis and Experimental Comparison

Medium-frequency transformer (MFT) dc bias is critical for the safe operation of dual-active-bridge (DAB) converters. To analyze the dc bias, expressions of dc bias are derived for SiC MOSFET DAB converters. It shows that the dc bias is a current source in steady state; thus, a dc bias current capacity is defined and used as a design constraint in the optimization of MFT. The dc bias current capacity is related to core materials, geometric parameters, and flux density, which is analyzed with analytical expression. Optimization equations with geometric variables are derived, which can be solved theoretically or numerically for both interleaved winding (IW) and separated winding (SW) MFT without and/or with leakage inductance design. Based on the optimization equations, different MFT designs for a 150-kW SiC MOSFET DAB converter are compared on the aspects of core materials, winding structures, and without/with leakage inductance design. The combination of IW structure MFT with auxiliary phase shift inductors is chosen, and the assembled prototype is tested with the 150-kW DAB converter. The designed dc bias capacity, temperature rise, and efficiency are verified.

Design Optimization of Medium-Frequency Transformer for DAB Converters With DC Bias Capacity

In power electronics, magnetic components are fundamental, and, unfortunately, represent one of the greatest challenges for designers because they are some of the components that lead the opposition to miniaturization and the main source of losses (both electrical and thermal). The use of ferromagnetic materials as substitutes for ferrite, in the core of magnetic components, has been proposed as a solution to this problem, and with them, a new perspective and methodology in the calculation of power losses open the way to new design proposals and challenges to overcome. Achieving a core losses model that combines all the parameters (electric, magnetic, thermal) needed in power electronic applications is a challenge. The main objective of this work is to position the reader in state-of-the-art for core losses models. This last provides, in one source, tools and techniques to develop magnetic solutions towards miniaturization applications. Details about new proposals, materials used, design steps, software tools, and miniaturization examples are provided.

/pdf/power-losses-models-for-magnetic-cores-a-review-1ht1fvyy.pdf

Power Losses Models for Magnetic Cores: A Review

Power transformer windings in most Switch Mode Power Supplies (SMPS) are conventionally wound around a plastic bobbin. Bobbins are support structures for the windings and provide electrical insulation between the winding and core. They are generally made from plastics like Polytetrafluoroethylene (PTFE), characteristic of low thermal conductivity (k~0.25 W/mK). Hence, using a bobbin creates a large thermal resistance between the windings and the core. The present paper investigates the benefit of removing the bobbin on the thermal performance of a 3.3kW power transformer fabricated on a PQ4040 core. The modelling is carried out through coupled 3D multiphysics Finite Element Analysis (FEA). First, the electromagnetic model accurately evaluates the spatial distribution of core losses for given operating conditions (core material, frequency, primary voltage and current, etc.) and the evaluated core losses are coupled to the thermal model. The winding losses are also evaluated. Next, the thermal model predicts the temperature distribution in the power transformer based on the loss inputs from the electromagnetic simulation as well as the material and geometrical properties of the transformer. The thermal model accounts for all modes of heat transfer. Further, feedback is established between the electromagnetic and thermal model to account for changes in temperature dependent electromagnetic properties. The evaluated thermal performance of the bobbin and bobbinless transformers are then compared. The FEA result shows that bobbinless transformers have lower hot spot temperatures as compared to transformers with bobbins. Further, bobbinless transformers have a smaller temperature difference between windings and core (T winding − T core ) as compared to transformers with bobbins, indicating lower thermal resistance between winding and core. Finally, the findings are experimentally validated on a 3.3kW SMPS platform with thermocouple measurements and Infrared (IR) Thermography. Bobbinless transformers are shown to be low cost and thermally superior alternatives to their conventional counterparts.

Improving Thermal Performance of High Frequency Power Transformers using Bobbinless Transformer Design

To find balance among multiple design objectives of a medium/high-frequency (MF/HF) high-power (HP) transformer is best addressed employing an optimization technique. In this paper, MF HP transformer design is formulated as a multi-variable optimization problem, where efficiency, power density and temperature rise are chosen as design objectives. Total loss, core volume and maximum temperature rise are modeled as respective cost functions and amalgamated using weighted-sum approach to derive objective function. It is minimized using Steepest descent method. Being a gradient-based search technique, it preserves correlation among design variables during minimization. Using the proposed methodology, optimal design of a 10 kW, 0.5/5 kV, 1 kHz natural oil-cooled transformer with amorphous core and concentric foil winding, is derived. It is estimated to have an efficiency of 99.55% at a power density of 19.79 and maximum node temperature of 52.92 °C. These merit of figures are validated using FEM and CFD studies. Cost-effectiveness of proposed methodology is discussed with the help of a hardware prototype, built from off-the-shelf amorphous core. Benefits like design flexibilities and plausible cost-effectiveness, are inherent to gradient-based optimization method, which augur well for its applicability for MF HP transformer design.

Multi-variable optimization methodology for medium-frequency high-power transformer design employing steepest descent method

Nowadays, medium frequency transformers are widely employed with power electronic converters, offering obvious benefits for modern power system. Rising the switching frequency leads to the need for accurate estimation of the characteristics of a designed transformer. This paper refines the existing analytical frequency-dependent method for accurately calculating leakage inductance in the medium frequency range based on the magnetic field intensity distribution and the stored magnetic energy method. Unlike the previous expressions, the proposed method employs mean turn length of individual layer which makes the calculation more precise. The advantages and impacts of interleaved winding structure to minimize the leakage inductance are also shown by comparing different winding patterns. Transformer prototypes with specifications as 10kW, 500/5000V, 5kHz have been manufactured and tested. The presented equations for inductance calculation are benchmarked to the experimental results on the developed transformer prototypes as well as the finite element analysis (FEM) simulation results.

Calculation and Experimental Validation on Leakage Inductance of a Medium Frequency Transformer

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.

https://www.mdpi.com/1996-1073/12/2/328/pdf

A Coupled, Semi-Numerical Model for Thermal Analysis of Medium Frequency Transformer

Nowadays, medium frequency transformers have gained growing attention in modern power system. Higher power density, which is realized by increasing the operating frequency, leads to the reduction of the magnetic components size. Along with it, cooling surface is consequently reduced which results in high thermal stress. Careful attention must be paid on the loss mechanisms and thermal analysis of a medium frequency transformer at the design stage. This paper develops an equivalent thermal circuit of an oil-immersed medium frequency transformer to predict its temperature profile. Thermal resistance, capacitance and the losses generated in the core and winding are carefully estimated. Two oil-immersed shell-type transformer prototypes with specifications as 5kW, 500/5000V, 5kHz, interleaved winding construction have been developed and built for verification purpose. Loss and temperature measurements have been performed to verify the presented framework. The accuracy of the proposed thermal model is benchmarked and corroborated through experimental measurements as well as FEM-CFD study with good agreement.

Madasamy Palavesha Thevar

Papers

Multi-variable optimization methodology for medium-frequency high-power transformer design employing steepest descent method

Calculation and Experimental Validation on Leakage Inductance of a Medium Frequency Transformer

A Coupled, Semi-Numerical Model for Thermal Analysis of Medium Frequency Transformer

Experimental Verification on Thermal Modeling of Medium Frequency Transformers