Analysis of the Performance Limits of 166 kW/7 kV Air- and Magnetic-Core Medium-Voltage Medium-Frequency Transformers for 1:1-DCX Applications

Abstract

Solid-state transformers (SSTs) are employing compact high-power medium-voltage (MV) medium-frequency transformers (MFTs) and enable the power transfer between galvanically isolated DC systems. Considering a typical SST isolation stage, i.e., a 166 kW unregulated series-resonant DC-DC converter acting as a DC transformer (DCX) with equal input and output MV DC voltages of 7 kV (1:1-DCX), we derive component-level and system-level performance limits of aircooled realizations with either an air-core transformer (ACT) or with a magnetic-core transformer (MCT). We describe the design of two fully rated MFT prototypes in detail and provide a comprehensive experimental characterization to validate the derived performance limits, including also the dielectric losses and the analysis of stray magnetic fields. The realized ACT and MCT prototypes achieve measured efficiencies of 99.5 % and 99.7 % at gravimetric power densities of 16.5 kW/kg and 6.7 kW/kg, respectively. Considering 10 kV SiC MOSFETs, calculated system-level efficiencies of the ACT-based and MCTbased 1:1-DCX result in 99.0 % and 99.2 % at the nominal operating point, with similar part-load efficiency characteristics. The paper concludes with an application-oriented qualitative evaluation of the two concepts.

New single‐stage bidirectional three‐phase ac‐dc solid‐state transformer

“…The total rectifier semiconductor losses sum up to roughly 4.7 kW or about 0.5% of the nominal power P N . Assuming a typical MFT realization with 0.5% losses (5 kW) [41,42], the estimated overall system efficiency is ≈98.1%. This is a competitive value for state-of-the-art MVac-LVdc SSTs [15], leaving some headroom for losses of the grid-side input filter.…”

166 kW Liquid-Insulated Medium-Frequency Transformer With Hybrid Foil-Litz Windings

“…High-power MFTs are often realized considering either the core-or the shell-type configuration [20]. Compared to the core-type, the shell-type construction requires fewer winding terminations and lead connections, which simplifies the insulation of the primary winding [9]. Additionally, it allows limiting the stray magnetic field.…”

“…The higher losses reduce the maximum acceptable current in the winding, and hence its power density. Litz wire is normally preferable in such scenario, since the magnetic field curvature does not raise the proximity losses as dramatically as in a foil winding [9], [10]. However, litz is more expensive and has a lower fill factor than foil conductor.…”

Design method for high-power high-frequency magnetic-resonance air-core transformer

“…Figure 7). The soft switching loss is positively correlated with the frequency (Czyz et al , 2022b; Rothmund et al , 2018). When the frequency exceeds 100 kHz, the efficiency of CLLC resonant converter may decrease because of the increase of frequency.…”

“…The windings of this structure does not require magnetic materials to guide the magnetic field and can achieve more than 99% efficiency at 166 kW transmission power. References (Czyz et al , 2022a; Guillod et al , 2022) achieved a weight power density of 16.5 kW/kg and 31 kW/kg for different cylindrical coil structures through multi-objective optimization design, which is more than twice the MCT at the same capacity (Czyz et al , 2022b). However, the shield designed to suppress the leakage magnetic field reduces the volume power density of ACT.…”