10 kV SiC-Based Medium-Voltage Solid-State Transformer Concepts for 400V DC Distribution Systems
- Doctoral Thesis
Rights / licenseIn Copyright - Non-Commercial Use Permitted
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. Show more
External linksSearch print copy at ETH Library
SubjectPower Electronics; Solid-state transformers (SSTs); SiC MOSFET
Organisational unit03573 - Kolar, Johann W. / Kolar, Johann W.
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