Modeling and Design of
Medium-Frequency Transformers
for Future Medium-Voltage
Power Electronics Interfaces
DOCTOR OF SCIENCES of ETH ZURICH
(Dr. sc. ETH Zurich)
presented by
THOMAS PAUL HENRI GUILLOD
MSc ETH
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 kVμs),
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 kWin3, 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 (62W/in3, 2.9 kW/kg, and 1.3 kW/lb). The results obtained with
the constructed DC-DC converter, which are significantly beyond the stateof-
the-art, demonstrate that MV/MF transformers can utilize the possibilities
offered by the new MV SiC semiconductors.
