Modeling and Design of Medium-Frequency Transformers for Future Medium-Voltage Power Electronics Interfaces

Abstract

Newly available fast-switching Medium-Voltage (MV) Silicon-Carbide (SiC) semiconductors are setting new limits for the design space of MV converters. Unprecedented blocking voltages (up to 15 kV), higher switching frequencies (up to 200 kHz), higher commutation speeds (up to 100 kV/us), and high temperature operation can be reached. These semiconductors feature reduced switching and conduction losses and, therefore, allow for the realization of extremely efficient and compact MV converters. Moreover, the increased blocking voltage allows the usage of simple single-cell topologies for MV converters instead of complex multi-cell systems. Hence, the MV SiC semiconductors are interesting for many applications such as locomotive traction chains, datacenter power supply chains, collecting grids for renewable energies, high power electric vehicle chargers, and more-electric aircraft. Most of these applications require an isolated DC-DC converter for providing voltage scaling and galvanic isolation. However, the increased voltages and frequencies allowed by MV SiC semiconductors create new challenges for the design of Medium-Frequency (MF) transformers, which start to become the bottleneck of isolated DC-DC converters in terms of power density and efficiency. More specifically, the winding losses (due to skin and proximity effects) and the core losses (due to eddy currents and hysteresis) are rapidly increasing and mitigate the advantages (e.g., the reduced volt-second product applied to the magnetic core) obtained with the increased operating frequencies. Moreover, the MV/MF PWM voltages with fast switching transitions are also particularly critical for the insulation of MF transformers and can lead to additional losses, thermal breakdowns, and partial discharge induced breakdowns. Finally, the MF transformers of DC-DC converters should feature reduced losses (efficiencies above 99.5%) in order to match the performance offered by the MV SiC semiconductors. The main focus of this thesis is, thus, set on the design of highly efficient MV/MF transformers employed in isolated DC-DC converters. First, a theoretical analysis of MF transformers is conducted in order to extract the fundamental performance limitations of such devices. The nature of the optimal designs is examined with analytical models, scaling laws, and numerical optimizations. Afterwards, several points are identified as critical and are studied in more detail. First, the impact of model uncertainties and parameter tolerances on MF transformers is examined with statistical methods in order to highlight the achievable modeling accuracy. Then, a 2.5D numerical field simulation method is presented for assessing the impact of non-idealities on the losses produced by litz wire windings (e.g., twisting scheme and pitch length). Afterwards, the impact of MV/MF PWM voltages with fast switching transitions on the insulation is examined. The electric field pattern is analyzed inside, at the surface, and outside the insulation and shielding methods are proposed. Finally, the dielectric loss mechanisms of dry-type insulation materials under PWM voltages is examined in detail. Different analytical expressions are proposed for extracting the insulation losses and it is found that the dielectric losses can be significant for MV/MF transformers operated with MV SiC semiconductors. Design guidelines are proposed for the selection of appropriate insulation materials for MV/MF applications and silicone elastomer is identified as an interesting choice. All the presented results are verified with measurements conducted on different MF transformer prototypes. The derived models and results are applied to a MV isolated DC-DC converter, which is part of a MV AC (3.8 kV, phase-to-neutral RMS voltage) to LV DC (400 V) Solid-State Transformer (SST) demonstrator. This SST is aimed to supply future datacenters directly from the MV grid. The considered 25 kW DC-DC converter operates between a 7 kV DC bus and a 400 V DC bus. The usage of 10 kV SiC MOSFETs allows for the realization of the converter with a single-cell DC-DC Series-Resonant Converter (SRC). The DC-DC SRC is operated at 48 kHz as a DC Transformer (DCX) and the modulation scheme, which allows for Zero-Voltage Switching (ZVS) of all semiconductors, is examined in detail. The realized MV/MF transformer prototype features a power density of 7.4 kW/l (121 kW/in^3, 4.0 kW/kg, and 1.8 kW/lb) and achieves a full-load efficiency of 99.65%. The complete DC-DC converter achieves an efficiency of 99.0% between 50% and 100% load with a power density of 3.8 kW/l (62 W/in^3, 2.9 kW/kg, and 1.3 kW/lb). The results obtained with the constructed DC-DC converter, which are significantly beyond the state-of-the-art, demonstrate that MV/MF transformers can utilize the possibilities offered by the new MV SiC semiconductors.