Foil Windings Research Articles

Impact of one and two partial cooling ducts on temperature rise in transformer windings

We apply our quasi-1D model to the temperature rise in oil-filled transformer foil windings with two symmetric partial ducts. We compare the results with a two-dimensional FEM simulation. To compare the results to industrial tests we try to find a relationship between the effective values for heat transfer coefficients of different foil windings. Finally, we compare the effectiveness of adding one vs two partial cooling ducts to the winding.

ТЕПЛОВИЙ РОЗРАХУНОК СУХИХ ТРАНСФОРМАТОРІВ З ЛИТОЮ ІЗОЛЯЦІЄЮ ТА З ФОЛЬГОВИМИ ОБМОТКАМИ

The method of analytical thermal calculation of dry transformers with cylindrical conductors with cast insulation and foil windings is presented. The surface and volume averaged temperatures of the magnetic system core and the wind-ings are determined from the one-dimensional non-linear substitute thermal circuit of the transformer. To clarify the temperature distribution across the cross-section of the anisotropic foil winding, the solution of the boundary value problem in the form of a two-dimensional Poisson equation with heat transfer coefficients calculated from the substitute scheme is used. Calculation example of a dry transformer with a capacity of 2500 kVA is given. Numerical CFD model-ing was performed to verify the calculation results. Bibl. 10, fig. 9, tab. 3.

Homogenization of foil windings with globally supported polynomial shape functions

In conventional finite element simulations, foil windings with thin foils and with a large number of turns require many mesh elements. This renders models quickly computationally infeasible. This paper uses a homogenized foil winding model and approximates the voltage distribution in the foil winding domain by globally supported polynomials. This way, the small-scale structure in the foil winding domain does not have to be resolved by the finite element mesh. The method is validated successfully for a stand-alone foil winding example and for a pot inductor example. Moreover, a transformer equipped with a foil winding at its primary side is simulated using a field-circuit coupled model.

A Comparative Study of Finite Element Method and Hybrid Finite Element Method–Spectral Element Method Approaches Applied to Medium-Frequency Transformers with Foil Windings

This study aims to improve the computational efficiency of the frequency domain analysis of medium-frequency transformers (MFTs) with the presence of large clearance distances and fine foil windings. The winding loss and magnetic energy in MFTs in the medium-frequency range are calculated utilizing a finite element method (FEM) using common triangular and alternative rectilinear mesh elements. Additionally, in order to improve the computational efficiency of the calculations, a spectral element method (SEM) is coupled with a FEM, thus creating a hybrid FEM–SEM formulation. In such a hybrid approach, the FEM is used to calculate the current density distribution in the two-dimensional (2D) cross-section of the foil conductors to achieve reliable accuracy, and the SEM is adopted in the nonconducting clearance distances of the winding window to reduce the system of equations. The comparative analysis of the calculated resistance and reactance of the under-study models showed that the FEM with rectilinear mesh elements and the FEM–SEM model outperformed the FEM with triangular mesh elements in terms of accuracy and computational cost. The hybrid FEM–SEM model enables a reduced system of equations for modeling the electromagnetic behavior of MFTs. This research provides valuable insights into both the computational approaches and meshing challenges in the analysis of MFTs and offers a foundation for future research on the design and optimization of MFTs.

Analytical Double-2D Frequency-Dependent Leakage Inductance Model for Litz-Wired High-Frequency Transformer

Accurate and fast calculation of frequency-dependent leakage inductance is critical for the optimization of the high-frequency transformer with Litz wire winding. The analytical method is widely used due to its high computation speed. However, its computation accuracy is limited due to the following factors. i) One-dimensional (1D) or two-dimensional magnetic field model (2D) has unsatisfying accuracy for the transformer with a rotationally asymmetric structure. ii) The skin and proximity effect factors based on the multilayer foil winding are inaccurate for the Litz wire winding in the calculation of frequency-dependent magnetic energy. This paper proposes a double-2D frequency-dependent leakage inductance model for the transformer with Litz wire winding. It uses the image method to calculate the 2D leakage magnetic field inside and outside the core window, respectively, which considers the rotationally asymmetric structure in leakage inductance calculation. The round conductor model is proposed for the frequency-dependent magnetic energy calculation. The single strand in a Litz wire is taken as the basic modeling object of the round conductor model, which is more detailed than the multilayer foil winding model for the Litz wire winding. The comparison between the experiment, FEM, state-of-the-art models and the proposed model is conducted for eight transformers with E-core, U-core, round or rectangular Litz wire winding. The proposed model shows satisfying computation accuracy in the whole frequency band. And the computational speed of the proposed model is 100 times that of the FEM.

Eddy-Current Reduction Structure of Copper Foil Winding for Medium-Frequency Transformer in DC-Grid Applications

Eddy-Current Reduction Structure of Copper Foil Winding for Medium-Frequency Transformer in DC-Grid Applications

Anodized Aluminum Foil Winding Axial Flux Machine for Direct-Drive Robotic Applications

There is a need for higher specific torque electric actuators in novel direct-drive robotic applications. Since the specific torque is inherently limited by material properties, designers have to look at novel materials to push the specific torque limit further. In this article, anodised aluminium foil is considered as an alternative to enamelled copper wire in a yokeless and segmented armature axial flux machine. This machine topology has a high specific torque and is particularly suited for an anodised aluminium foil winding. The DC resistance, thermal properties and eventually the specific torque of a prototype test case machine are compared for both winding materials. A 3D thermal finite element model is used to analyse the influence of the thermal interface between winding body and housing, and the influence of the cooling performance on the specific torque. To conclude, anodised aluminium foil winding offers a higher specific torque in direct-drive yokeless and segmented armature axial flux actuators where the winding body thermal resistance is dominant in the thermal path from heat source to heat sink. It is experimentally shown that for the prototype machine, the torque per kg active stator mass increases 13% through the use of anodised aluminium foil.

Research on Magnetic Characteristics and Short-Circuit Force of 3D Wound Core Transformer

The problem of eddy current loss in power transformers directly affects the technical economy of the transformer, which is a problem that cannot be ignored in the design and calculation of power transformers. Studying how to reduce the eddy current losses caused by leakage magnetic fields is a difficult point in transformer design. The load loss of low-voltage foil copper windings in a 10 kV/400 kVA 3D wound core transformer is calculated, and the three-dimensional finite element models of the transformer are established by Using Magnet software and COMSOL Multiphysics software, and the calculation results are compared. At the same time, the short-circuit force of foil windings is shown in the simulation. The results show that the three-dimensional finite element models are correct.

Solid-State Transformers of Smart High-Power Battery Charger for Electric Vehicles

There is no doubt that society will not turn back: most thermal vehicles will switch to electric ones, not only for cars but also for bigger vehicles. Fast chargers will become more and more essential parts of the global system, they will provide a charge control of batteries and galvanic insulation from the grid. At the present times, compact realizations are based on Solid-State Transformers (SSTs) operating at high frequencies with their well-known advantages and disadvantages, including power limits and reasonably high costs. The authors propose a medium-frequency transformer (MFT) structure using Grain Oriented Electrical Steel (GOES) wound cores and aluminum foil windings. Thanks to this solution, the cost of the device can be reduced. GOES has losses that increase with frequency but decrease with temperature; therefore, the authors propose a transformer structure allowing a hot core inside coils at usual temperatures. This idea is applied to a 400-kW compact battery charger connected to a DC smart grid.

The Effects of PWM With High dv/dt on Partial Discharge and Lifetime of Medium-Frequency Transformer for Medium-Voltage (MV) Solid State Transformer Applications

Medium-frequency (MF) transformer in a phase-shift modulated dc solid state transformer (SST) is prone to partial discharge (PD) that impacts the system reliability and lifetime. Tightly packed MF transformer windings carrying PWM voltages create favorable conditions for PD inception. A peak potential condition is created between adjacent posterior windings as a result of phase-shifted modulation. This along with fast alternating electric fields incepts PD at lower than expected operating voltages. Early inception of PD lead to premature transformer failure leading to unanticipated DC SST down-time. The effects of PWM voltages containing high dv/dt switching transients on PD and lifetime characteristics of MF transformer in a dc SST is not yet understood, which is investigated in this paper. A non-intrusive PD detection strategy for MF PWM ac voltage is presented using high speed optical sensors. PD characterization is performed on Kapton polyimide insulated copper foil windings for a foil-type MF transformer. Winding samples are stress tested with steady state PWM voltages upto 2 kV and frequencies upto 50 kHz with high dv/dt upto 60 V/ns while maintaining the transformers geometric profile. Accelerated dielectric lifetime characterization is performed and novel lifetime prediction models are derived containing both frequency and dv/dt dependent variables.

Investigations of Microstructure and Mechanical Properties of Lap Jointed Dissimilar Metals by Friction Stir Spot Welding Process

The joining of different materials is required for industrial application to utilize the hydride structures with attractive advantages such as superior strength to weight ratio, low cost, high tensile strength and less weight of the component. Specifically, Al-Cu composite material is widely used in foil conductors of a transformer, electrical connectors, foil windings in capacitors and tubes in heat exchangers. However, joining or spot welding the Al parts to Cu parts are significant challenge owing to variation in mechanical properties and chemical compositions of joining materials. In this present research work, the friction stir spot welded process (FSSWP) is carried out to join the Al and Cu materials. Further, microstructure and mechanical properties of friction stir spot welded specimens (FSSW) are studied at different tool rotational speeds likely from 1000rpm to 1500rpm. The microstructural study is carried out using scanning electron microscope images at the interface and overall welded region. The tensile strength of both single and double spot-welded specimens is analyzed using a universal tensile test machine. The output of this study states that the optimal tool rotational speed is 1500rpm for both single and double spot-welded specimens. Moreover, the double spot-welded specimen exhibits more tensile with a crack-free spot-welded surface than that of the single spot-welded specimen. The tensile strength double spot-welded specimen has a 6.8% higher strength than that of a single spot-welded specimen. Based on the present study, it is concluded that the double spot-welded specimen can be used for different industrial applications to replace Cu material with this Al-Cu material that gives added advantages to those components.

Hybrid Foil-Litz Windings for Highly Efficient and Compact Medium-Frequency Transformers

Medium-frequency transformers (MFTs) are key components in solid-state transformers (SSTs), where they provide galvanic isolation and a certain voltage transfer ratio between a medium-voltage (MV) grid and a low-voltage (LV) bus. Typically, MFTs are operated in the kilohertz range, which results in a significantly reduced volume and material usage compared to 50/60 Hz transformers. Foil conductor is an attractive solution for improving the filling factor and the cost-effectiveness of MFTs, however, the insulation clearances increase the ohmic losses due to the current crowding effect; besides, the electric field hotspots at the winding corners require shielding by equipotential rings. In this work, a new hybrid topology is presented, where the first and last turn of the foil winding package is replaced by turns of litz wire connected in series, one above and one below the foil winding, respectively, such that they act as equipotential shielding rings; besides, the current in the litz turns deflects the magnetic field lines away from the foil edges, decreasing the losses due to current crowding. The effect of the litz rings on the losses is modeled and measured on a hybrid and a standard foil winding prototype; a loss decrease higher than 15% is observed. The verified model is used to compare the conductor and footprint saving between windings designed with litz rings, and with standard equipotential shielding rings; in this respect, a minor amount of litz wire allows reducing losses up to 30%, whereas the conductor utilization and winding height can be decreased more than 20%, improving the power density of the MFT.

Frequency-Dependent Inductance and Winding Loss Model for Gapped Foil Inductors

For comprehensively optimizing high-frequency foil inductors, calculating the increased inductance and the additional eddy current losses due to a two-dimensional magnetic fringing field caused by air gaps in the center leg is mandatory. This article proposes an analytical model that accurately calculates the inductance and the additional eddy current losses in gapped inductors with foil windings. The analytical field expressions are combined into closed-form winding loss and inductance formulas and verified by measurements. Furthermore, the frequency dependency of the inductance due to the shielding effect of the foil conductors is being discussed.

Mitigation of Circulating Currents in Parallel Foil Windings for Medium Frequency Transformers

The practicality of parallel foil windings assembled on medium frequency transformers (MFTs) is analyzed and experimentally verified. The efficiency improvement in case of equal current sharing in parallel windings in contrast to single foil is numerically validated. The presence of circulating currents between parallel windings is investigated using the impedance matrix of the foil windings where the impedances are numerically computed. For experimental validations, an MFT prototype with parallel foil windings is manufactured and the effect of additional external impedances on reducing the circulating currents is addressed. A simple and practical technique is proposed to ensure equal current sharing between parallel windings of MFTs coupled with series resonant converters (SRCs).

Time-Domain Homogenization of Foil Windings in 2-D Axisymmetric Finite-Element Models

In this paper, an approach for the time-domain homogenization of foil windings in axisymmetric finite-element (FE) models is presented. The homogenized formulation is characterized by an axial current redistribution due to the skin effect and a radial inter-turn voltage given by the insulation in-between the turns. The method is successfully applied to an axisymmetric 20-turn foil-winding inductor. At low frequencies, the local and global results present an excellent agreement with those obtained by an accurate but expensive FE model in which all turns are explicitly discretized. The air-gap effect and the frequency limitations are studied as well.

АНАЛІТИЧНИЙ ТА CFD-РОЗРАХУНОК ТЕПЛОВОГО СТАНУ ФОЛЬГОВИХ ОБМОТОК МАСЛЯНИХ РОЗПОДІЛЬНИХ ТРАНСФОРМАТОРІВ

An analytical method of calculating the thermal condition of foil windings of lower voltage oil distribution transformers has been developed. At known oil temperatures in the tank, this technique provides the identification of heat-recoil ratios on winding surfaces, taking into account their design features and heat loads, as well as calculating the excess esexcesses of the average temperature of the winding and its most heated point above the oil and over the cooling environment. In order to calculate the excess temperature of the winding over the oil by the method of separating variables using the final cosinus-conversion Fourier obtained the solution of the edge problem for the Poisson equation with heterogeneous boundary conditions on the surfaces of the rectangular section winding with anisotropic properties and with its uneven distribution of losses. In addition, an alternative approach has been developed to determine the thermal state of an axisymmetric transformer model by numerical CFD modeling of the system of equations of motion and continuity of the Navier-Stokes coolant. This allows you to obtain the distribution of the oil velocity field and absolute temperatures, both of the oil in the tank, and of the foil and ball windings of the transformer using the minimum empirical data on the physical properties of the oil and the heat transfer of the tanks. The methods are verified by known experimental data for transformers TM-1000/35 and TM-630/10. References 11, figures 4, tables 2.

Critical stress of oval foil winding with epoxy coated insulation determined using measured equivalent modulus of elasticity

In the distribution transformers design oval windings are used due to economic advantages. On the other hand, such windings are more susceptible to radial forces in a short circuit. A diamond dotted paper with an epoxy coating is used in order to increase the stiffness of the winding. Despite that, winding failure may occur during the short circuit, e.g. buckling of inner winding. Because of a very thin foil conductor (typically 0.5–2 mm), the most critical is inner low voltage foil winding which can collapse due to radial forces at stresses far below the elastic limit of conductor material. This paper shows an analytical approach to the calculation of critical stress in inner oval foil winding with epoxy coated insulation. Critical stress was calculated using the equation for free buckling of round winding. Equivalent Young's modulus of elasticity was obtained experimentally from the testing of the sample model loaded with bending force on a tensile test machine. A total of 12 test samples were formed from aluminium foil conductor and diamond dotted paper and cured at the temperature of 105°C. The results were successfully verified on distribution transformers subjected to short circuit withstand tests.

Eddy Current Loss Effect in Foil Winding of Transformer Based on Magneto-Fluid-Thermal Simulation

The foil winding temperature distribution is crucial to the life of power transformer since local heating can accelerate aging of insulation materials. However, based on the traditional 3-D electromagnetic model and the principle of heat transfer, the non-uniform losses and temperature distribution of foil windings are difficult to estimate because of complex magnetic leakage flux. In this paper, the magneto-fluid-thermal coupling model has been established to calculate the temperature rise and predict the potential hotspots of a simplified dry-type power transformer rated at 2500 kVA. Moreover, the influence of insulation barrier is considered to reshape the air-flow path and to affect the radiative effect between the high-voltage and low-voltage windings. Experimental temperature rises are measured with infrared thermography to verify the simulation results, which achieved reasonable hotspot predictions and provide the theoretical method for transformer designers.

Calculation of Leakage Reactance in Transformers With Constructive Deformations in Low Voltage Foil Windings

Foil-winding transformers with high currents in low voltage winding normally have copper/aluminum bars as leads of the coils. A practical solution for locating bar leads without increasing the size of the active part is to flatten the coil in the leads zone causing a deformation. Abnormalities in the leakage reactance have been identified in this type of transformers. In order to take this effect into account, this paper proposes a correction to the mathematical formulation for calculating the leakage reactance in transformers with deformations in the low voltage winding. Three methods for calculating the leakage reactance have been compared for four real case studies: the proposed formulation, the commonly used formulation, and detailed simulations with the finite element method in three-dimension (FEM3D). The leakage reactance calculated with the proposed method has been found to have an improved accuracy compared with the one calculated using the formulation without correction in all case studies. The main contribution of this paper is that the proposed correction in the formulation makes it possible to increase the accuracy in the calculation of leakage reactance, avoiding the need to resort to detailed simulations with FEM3D.

Thermal Performance Modeling of Foil Conductor Concentrated Windings in Electric Machines

While the use of foil windings in transformers and air core inductors is not uncommon, their use in electric machines is just emerging. Although prototype realizations have been reported in the literature, a definitive analysis of their thermal performance is necessary to evaluate their potential for broad applications. This paper is devoted to presenting a comprehensive model for evaluating their thermal performance including loss modeling and temperature rise prediction in realistic designs. An overview of foil windings in electric machines and their advantages in improved fill factor is presented, along with experimental results that compare the ac losses and temperature distribution under identical steady-state operating conditions with a prototype round conductor coil and foil conductor coil. A lumped parameter thermal model that predicts the heat transfer behavior of the foil conductor coil is presented along with supporting experimental results.