Solid-state transformers (SSTs) are power electronic converters that provide isolation between a medium-voltage and a low-voltage (LV) system using medium-frequency transformers. The power electronic stages enable full-range control of the terminal voltages and currents and hence of the active and reactive power flows. Thus, SSTs are envisioned as key components of a smart grid. Various SST concepts have been proposed and analyzed in literature concerning technical aspects. However, several issues could potentially limit the applicability of SSTs in distribution grids. Therefore, this paper discusses four essential challenges in detail. It is found that SSTs are less efficient than low-frequency transformers (LFTs), yet their prospective prices are significantly higher. Furthermore, SSTs are not compatible with the protection schemes employed in today’s LV grids, i.e., they are not drop-in replacements for LFTs. The limited voltage control range typically required in distribution grids can be provided by competing solutions, which do not involve power electronics (e.g., LFTs with tap changers), or by hybrid transformers, where the comparably inefficient power electronic stage processes only a fraction of the total power. Finally, potential application scenarios of SSTs (ac-dc, dc-dc, weight/space limited applications) are discussed. All considerations are distilled into an applicability flowchart for SST technology.

Applicability of Solid-State Transformers in Today’s and Future Distribution Grids

This paper describes a novel, model-based, and design optimization methodology for high-power medium-frequency transformers (MFTs) for medium-voltage high-power electronic applications, namely emerging solid-state transformers. Presented procedure enables the designer to interactively select the most optimal design in a simple and intuitive way, while inherently offering flexibility in terms of various design alternatives, depending on the component availability. The core of the design algorithm is explained in detail together with all of the relevant modeling and technical challenges associated with realization of a prototype. A $\text{100 kW}$ , $\text{10 kHz}$ MFT prototype has been designed, according to the presented algorithm, practically realized and thoroughly tested in order to verify the theoretical design.

/pdf/100-kw-10-khz-medium-frequency-transformer-design-568hlys16z.pdf

100 kW, 10 kHz Medium-Frequency Transformer Design Optimization and Experimental Verification

The characterization of soft-switching losses (SSL) of modern high-voltage SiC mosfet s is a difficult but necessary task in order to provide a sound basis for the accurate modeling of converter systems, such as medium-voltage-connected solid-state transformers, where soft-switching techniques are employed to achieve an improved converter efficiency. Switching losses (SL), in general, are typically measured with the well-known double pulse method. In the case of SSL measurements, however, this method is very sensitive to the limited accuracy of the measurement of the current and voltage transients, and thus is unsuitable for the characterization of fast-switching high-voltage mosfet s. This paper presents an accurate and reliable calorimetric method for the determination of SSL using the example of 10-kV SiC mosfet modules. Measured SSL curves are presented for different dc-link voltages and switched currents. Furthermore, a deeper analysis concerning the origin of SSL is performed. With the proposed measurement method, it can be experimentally proven that the largest share of the SSL arises from charging and discharging the output capacitance of the mosfet module and especially of the antiparallel junction barrier Schottky diode.

Accurate Transient Calorimetric Measurement of Soft-Switching Losses of 10-kV SiC mosfets and Diodes

Recently, more occasions with high input voltage and/or high output voltage applications are emerging. However, the existing switching devices are limited by the voltage stress, e.g., the available maximum blocking voltage of insulated-gate bipolar transistors is 6.5 kV. Therefore, several methods are provided to solve this issue, including series connection of switches, multilevel converters, and modular series–parallel structures, where the modular series–parallel systems are the most popular choice due to the advantages of low cost, simple design process, high modularity, reliability, and redundancy. The main objective of series–parallel systems is to ensure the sharing of input/output voltage/current. Based on this, a comprehensive review on the available voltage/current sharing methods for series–parallel systems, which include input-series output-parallel, input-parallel output-series, and input-series output-series structures is provided in this article. Moreover, the relationships between the input voltage/current sharing and output voltage/current sharing of series–parallel systems are explained.

A Review of Voltage/Current Sharing Techniques for Series–Parallel-Connected Modular Power Conversion Systems

Power electronic transformer (PET) technology is one of the promising technology for medium/high power conversion systems. With the cutting-edge improvements in the power electronics and magnetics, makes it possible to substitute conventional line frequency transformer traction (LFTT) technology with the PET technology. Over the past years, research and field trial studies are conducted to explore the technical challenges associated with the operation, functionalities, and control of PET-based traction systems. This paper aims to review the essential requirements, technical challenges, and the existing state of the art of PET traction system architectures. Finally, this paper discusses technical considerations and introduces the new research possibilities especially in the power conversion stages, PET design, and the power switching devices.

Evolution of Power Converter Topologies and Technical Considerations of Power Electronic Transformer-Based Rolling Stock Architectures

This paper presents the design and development of a three-phase 10-kV/400-V 500-kVA electronic power transformer (EPT). The power circuit is designed in a modular fashion, i.e., the main circuit consists of many identical ac–dc–dc–ac modules (abbreviated as power modules). Each power module consists of a high-voltage power cell, a low-voltage power cell (LVPC), a medium-frequency isolation transformer, and a filter. The corresponding control and protection system is developed. A special three-stage startup strategy is designed to shorten the startup time and reduce the startup inrush current. The negative-sequence current compensation is introduced in the input stage to handle the unbalanced loads. To keep the dc-link voltages balanced, an individual dc voltage balancing controller based on regulating the output power of each parallel LVPC is proposed. The detailed control hardware design and software implementation are discussed. The functions of this 10-kV EPT prototype are verified through the laboratory and field tests. The results are shown in this paper. Currently, the prototype is operating in the industrial power grid.

A 10-kV/400-V 500-kVA Electronic Power Transformer

A 1.5 kV, 35 kW, 1 kHz silicon steel core medium-frequency transformer is designed and prototyped for a 10 kV, 0.5 MW electronic power transformer. This transformer uses 0.18 mm silicon steel as core material due to the advantages of easy processing, high saturation flux density, low noise, and low cost. The detailed design considerations and an optimal design method are presented in this paper. Different from the previous work on medium-frequency transformer design, the proposed approach takes ripples into account. Core loss model under square wave excitation with ripple and winding loss model considering sideband harmonics are established. Besides, two-dimensional finite-element simulations are adopted to obtain ac/dc resistance factors. Finally, the proposed approach is verified by experiments on prototype. The test results show performance better than expected, with desirable no-load loss and power density of 2.9623 × 106 W/m3.

Huihong Cai

Papers

A 10-kV/400-V 500-kVA Electronic Power Transformer

Optimal Design and Implementation of High-Voltage High-Power Silicon Steel Core Medium-Frequency Transformer