The short rise time observed in the PWM voltages generated by ultrafast wide bandgap devices increases the amplitude of voltage harmonics at higher frequencies. These harmonics can excite the resonances of medium-frequency transformers (MFTs), resulting in overvoltages inside the windings during continuous operation. Without further measures, these overvoltages can lead to unexpectedly high electric fields in the insulation material, which can result in partial discharge, accelerated aging, and possible failure of the MFT. To avoid these effects, the mechanism causing the overvoltages has to be understood and quantified during the design process. Based on this, the MFT can be designed in a way that the overvoltages vanish or are tolerable. Therefore, the voltage distribution inside the MFT windings is analyzed by a fully coupled multiconductor transmission line model, which includes the damping effect of electromagnetic losses in the litz wire and the core. This method is verified by measuring the transfer functions of the voltage to ground of individual turns and their voltage waveforms during continuous operation. The waveforms indicate repeating overvoltages inside the windings. A guideline for the design verification and a simplified approach to speed-up the modeling process are presented.

The safe operation of high-power solid-state transformers (SSTs) in medium-voltage gridconnected applications relies on medium-frequency transformers (MFTs). In MFTs, hybrid foil-litz windings present a cost-effective alternative to litz wire, since foil is employed as the main conductor. This innovative topology exhibits lower ohmic losses than conventional foil windings. Besides, the dielectric stress in the insulation is mitigated, facilitating a more compact design. This paper presents the experimental validation of a 166kW MFT prototype rated for the 17:5 kV AC / 26:3 kV DC insulation class. The MFT features hybrid foil-litz windings insulated with ester liquid. The design of the active parts, the insulation and the cooling system are described. The performances of alternative design concepts is evaluated. The power rating of the MFT prototype is verified experimentally by the temperature rise test at full load, operating the MFT within the cell of an SST. The insulation withstand is confirmed through dielectric tests, including AC and DC overvoltage tests up to 54 kV peak, and the lightning impulse up to 95 kV.

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.

Purpose: The application of dry-type transformers is growing in the market because the technology is non-flammable, safer and environmentally friendly. However, the unit dimensions are normally larger and material costs become higher, as no oil is present for dielectric insulation or cooling. At designing stage, a transformer thermal model used for predicting temperature rise is fundamental and the modelling of cooling system is particularly important. This paper aims to describe a thermal model used to compute dry transformers with different cooling system configurations. Design/methodology/approach: The paper introduces a fast-calculating thermal and pressure network model for dry-transformer cooling systems, preliminarily verified by analytical methods and advanced CFD simulations, and finally validated with experimental results. Findings: This paper provides an overview of the network model of dry-transformer cooling system, describing its topology and its main variants including natural or forced ventilation, with or without cooling duct in the core, enclosure with roof and floor ventilation openings and air barriers. Finally, it presents a formulation for the new heat exchanger element. Originality/value: The network approach presented in this paper allows to model efficiently the cooling system of dry-type transformers. This model is based on physical principles rather than empirical assessments that are valid only for specific transformer technologies. In comparison with CFD simulation approach, the network model runs much faster and the accuracies still fall in acceptable range; therefore, one is able to utilize this method in optimization procedures included in transformer design systems.

Synthetic ester is a promising alternative to mineral oil for emerging applications relying on high-power static conversion, such as solid-state transformers (SST). The highly compact components used in SSTs must fulfill MV-DC insulation requirements to guarantee the safe operation of the system, therefore, their design relies on the characterization of the dielectric stress caused by the electric field. The prediction of the electric field induced by DC voltages requires analyzing the conduction phenomena in the liquid, including the effect of charge transport due to ion drift. In this scenario, one of the critical parameters that determine the transport of charge and the resulting electric field distribution is the mobility of ions μ; the value of μ in synthetic ester is available only at room temperature, however, investigations on other dielectric liquids indicate a significant temperature dependency, which therefore needs to be characterized. In this paper, the temperature dependency of μ in synthetic ester liquid is determined by comparing alternative methodologies based on the analysis of the dielectric response in the time and frequency domain, and the viscosity of the fluid. The characteristics obtained by these methodologies are consistent; the temperature dependency of μ obeys the Arrhenius law and an increase of one order of magnitude starting from 1.1 × 10 ^–10 m ² /Vs is observed in the 303K–353K temperature range.

A mesh-free semi-analytical Fourier-based method is presented to evaluate the frequency-dependent losses in the windings of solid-state transformers (SSTs) with foil-wound transformers. In the employed diffusion equation, which accounts for induced eddy currents, the imposed current density and conductivity in the transformer window area are represented using spatial Fourier series and method of images. As a result, induced current density distribution in foil conductors and ac loss in medium frequency transformers (MFTs) is calculated using the semi-analytical method. The proposed method is verified using the finite element method (FEM). It is observed that with the considered approach one can estimate the ac winding loss in the frequency range of 5–25 kHz, with 2.7% maximum absolute error and approximately 2.5 times less number of degrees-of-freedom compared to FEM computations.