10 kV SiC-Based Medium-Voltage Solid-State Transformer Concepts for 400V DC Distribution Systems
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Autor(in)
Datum
2018Typ
- Doctoral Thesis
ETH Bibliographie
yes
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Abstract
At the present time, the globalization and the digital revolution
are the main drivers of the global economic growth, which, however,
goes hand in hand with a significant increase of the world’s energy
consumption. To reduce the emission of greenhouse gases despite
the rising energy demand, there are clear trends towards an increasing
share of electric vehicles (EVs) on the automotive market and towards
the integration of more renewable energy into the utility grid. Power
electronics is one of the main enabling technologies for this fundamental
change, since the distribution of the electrical power is taking place
at medium-voltage (MV)-AC, whereas EV batteries or e.g. data centers
(on the load-side) and photovoltaic (PV) power plants as well as
wind turbines (on the generation-side) represent low-voltage (LV)-DC
loads or sources, which means that MV-AC to LV-DC interfaces are
required. The state-of-the-art solution for such MV-AC to LV-DC interfaces
are low-frequency transformers (LFTs) with subsequent (bidirectional)
AC/DC converters. There, the LFT provides the required
voltage transfer ratio and galvanic isolation.
For a further reduction of greenhouse gas emissions, the available
electrical energy should be utilized to the highest possible extent, i.e.
the energy efficiency of the entire power supply chain from the generationside
to the load-side has to be increased. In this context, Solid-State
Transformers (SSTs), i.e. power electronic converters with an MV connection
and galvanic isolation by means of a medium-frequency (MF)
transformer, are a highly attractive alternative for the realization of
MV-AC to LV-DC interfaces due to their higher efficiency, high power
density, and their additional control features compared to the state-ofthe-
art solution.
One group of high-power LV-DC loads are e.g. data centers, whose
energy demand will increase significantly in the near future due to the
exploding internet IP traffic. In data centers, the benefit of utilizing
SSTs is even higher than for other applications, since their traditional
power supply chain consists of several cascaded conversion stages with
a low total efficiency, which can be omitted by the use of MV-AC to
400V DC SSTs. There, it is intended to supply individual server racks,
which can reach power levels in the range of 20 . . . 40 kW, with separate
SSTs with the additional advantage of substantially lower cable cross
sections and/or lower losses compared to LV distribution. Therefore, a
highly efficient 25 kW, 3.8 kV single-phase AC to 400V DC SST with a
target efficiency of 98% is realized and experimentally verified in this
thesis.
Instead of interfacing the MV-AC grid with a cascaded multi-cell
AC/DC converter, which consists of several series-connected converter
cells employing e.g. 1200...1700V semiconductors, a single-cell approach
based on the latest generation of 10 kV SiC MOSFETs is selected
due to the significantly lower complexity and the higher resulting power
density. There, a bidirectional single-cell AC/DC converter faces the
MV-AC grid, whereas a subsequent isolated single-cell DC/DC stage
converts the intermediate DC-link voltage of 7 kV into 400V DC. To
utilize the full potential of these 10 kV SiC MOSFETs, in this work a
complete technology package containing all the required concepts and
circuits necessary to realize a highly efficient, highly compact, and reliable
10 kV SiC MOSFET-based MV-AC to LV-DC SST is developed.
To enable an integration of the isolated gate driver into future intelligent
MV SiC modules, which would enhance the switching behavior
and would significantly simplify the design of MV converters, the volume
of the isolated gate driver supply has to be decreased substantially
compared to state-of-the-art solutions. Therefore, a highly compact
gate driver isolation transformer with a coupling capacitance of only
2.6 pF is realized. Furthermore, the gate driver features an ultra-fast
overcurrent protection with a reaction time of only 22 ns and the capability
of clearing hard-switching faults and even flashover faults within
less than 200 ns at a DC-link voltage of 7 kV.
Since there has not been any switching loss data available for the
employed 10 kV SiC MOSFETs (especially in the case of soft-switching),
these losses have to be determined experimentally. However, an error
analysis shows that electrical soft-switching loss measurements can lead
to large errors and therefore these measurement methods are unsuitable.
To obtain reliable data for the switching losses, a highly accurate
calorimetric soft-switching loss measurement method is developed and
the results show that, compared to hard-switching, the soft-switching
losses are a factor of 30 lower.
For this reason, the goal is to operate all switches under soft-switching
conditions, since this allows for a high efficiency and enables the
downsizing of passive components by employing a high switching frequency.
Therefore, a novel bidirectional AC/DC converter topology is
developed, which enables soft-switching over the entire AC grid period
by adding a simple LC-branch to the well-known full-bridge AC/DC
converter, which internally superimposes a high triangular current on
the AC grid current to reverse the current direction in each switching
cycle. Hence, this concept is called integrated Triangular Current
Mode (iTCM) operation. Furthermore, the design of the required ACside
LCL filter is discussed in detail and a quasi lossless method to
eliminate current oscillations in MV cables independently of the cable
length is presented. Based on a theoretical analysis, which shows that it
is very important in case of MV converters of this power class to minimize
parasitic capacitances, a low-capacitive design of the magnetic
components and the PCB layout is realized. Highly accurate calorimetric
efficiency measurements show that the iTCM single-phase AC/DC
converter achieves a full-load efficiency of 99.1 %, while it features an
unprecedented power density of 3.28 kW/L.
For the subsequent isolated DC/DC back end of the SST, an LLC
series resonant converter topology is selected and operated at resonance
frequency as ”DC transformer” providing a tight coupling of the converter’s
input and output voltages. In order to achieve soft-switching
of all switches under all load conditions and especially for both power
flow directions, a special modulation scheme is developed which allows
the active sharing of the turn-off current among the MV-side and the
LV-side. Furthermore, the MF MV transformer is Pareto-optimized regarding
its efficiency and power density and special attention is paid
to its MV insulation and the selection and application of a proper insulation
material. Calorimetric efficiency measurements show that the
isolated DC/DC converter achieves an efficiency of 99.0% between 50%
rated power and full load, while it features a power density of 3.8 kW/L.
Therefore, the complete MV-AC to LV-DC SST system achieves a
full-load efficiency of 98.1% and a power density of 1.76 kW/L. Compared
to an SST with similar specifications presented in 2017 by Fuji
Electric, which achieves an efficiency of 96% and a power density of
0.4 kW/L, the SST realized in this work generates less than half the
losses and is more than four times smaller. Mehr anzeigen
Persistenter Link
https://doi.org/10.3929/ethz-b-000331208Publikationsstatus
publishedExterne Links
Printexemplar via ETH-Bibliothek suchen
Verlag
ETH ZurichThema
Power Electronics; Solid-state transformers (SSTs); SiC MOSFETOrganisationseinheit
03573 - Kolar, Johann W. / Kolar, Johann W.
ETH Bibliographie
yes
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