Design and Prototyping Medium-Frequency Transformers Featuring a Nanocrystalline Core for DC–DC Converters

Abstract

Medium frequency transformers (MFTs) are a key component of DC–DC dual active bridge (DAB)-type converters. These technologies are becoming a quintessential part of renewable energy solutions, such as photovoltaic systems and wind energy power plants, as well as in modern power grid interfaces functioning as solid-state transformers in smart-grid environments. The weight and physical dimensions of an MFT are key data for the design of these devices. The size of an MFT is reduced by increasing its operating frequency. This reduction implicates higher power density through the transformer windings, as well as other design requirements distinct to those used for conventional 60/50 Hz transformers; therefore, new MFT design procedures are needed. This paper introduces a novel methodology for designing MFTs, using nanocrystalline cores, and tests it using an MFT–DAB lab prototype. Different to other MFT design procedures, this new design approach uses a modified version of the area-product technique, which consists of smartly modifying the core losses computation, and includes nanocrystalline cores. The core losses computation is supported by a full analysis of the dispersion inductance. For purposes of validation, a model MFT connected to a DAB converter is simulated in Matlab-Simulink (The MathWorks, v2014a, Mexico City, Mexico). In addition, a MFT–DAB lab prototype (1 kVA at 5 kHz) is implemented to experimentally probe further the validity of the methodology just proposed. These results demonstrate that the analytic calculations results match those obtained from simulations and lab experiments. In all cases, the efficiency of the MFT is greater than 99%.

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

“…The choice of the design method is fundamental in defining the MFT performance. Hence several design methods have been recently proposed especially for the medium-frequency range (600 Hz to 20 kHz) [9,[17][18][19], featuring different core materials such as silicon steel, ferrites, nanocrystalline and amorphous ones. Silicon steel-based designs are usually specified at 1 kHz, while nanocrystalline cores are more commonly used at 5 kHz.…”

“…(3) it presents a comparison of the performance of a silicon steel-MFT, designed following a classical method [10], with a nanocrystalline MFT designed using a cutting-edge method [19] considering the same conditions. The main purpose of this comparison is to determine which of the two MFTs achieves higher power density, higher efficiency and overall better performance at lower construction cost.…”

“…Last but not least, silicon steel alloys are employed in efficient MFTs designs at 1 kHz. [9] 0.8 0.6 0.6 1.29 99.00% Nanocrystalline [17] 50 5 0.9 11.5 99.48% Nanocrystalline [19] 1 5 0.9 15.01 99.41%…”

“…Currently, the maximum operating frequency of silicon steel MFTs is 1 kHz [18]. In the case of nanocrystalline MFTs, the typical operating frequency is 5 kHz [17,19]. These two MFT designs have both high-power density and high efficiency.…”

“…For comparison purposes, Table 1 shows the latest proposals using nanocrystalline and silicon steel cores. Nanocrystalline MFTs are designed at 5 kHz/0.9 T [17,19], whereas silicon steel MFTs are designed at 1 kHz/0.5 T [18]. From other points of view, if nanocrystalline MFTs were designed at 1 kHz/0.9 T, then its power density would be greater than that obtained with silicon steel MFTs.…”