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A power electronic transformer (PET), also called a solid-state transformer (SST) or a smart transformer (ST), generally refers to power electronics and high-frequency transformers (as opposed to work). The frequency transformer has a higher operating frequency and a new type of power electronic equipment that has, but is not limited to, the functions of a conventional industrial frequency AC transformer. Power electronic transformers generally include at least the voltage level conversion and electrical isolation functions of traditional AC transformers. In addition, they include AC-side reactive power compensation and harmonic control, DC access to renewable energy/energy storage equipment, and fault isolation between ports. As well as communication functions with other smart devices. It should be noted that this article focuses on the classification of PET-related technologies with high-voltage AC ports. Electrically isolated DC-DC converters are also referred to in some literature as "DC transformers" or "DC power electronic transformers." Such DC-DC converters are actually compatible with many high-frequency isolation functions in PET. The structure is very similar, this article does not analyze separately.
PET is generally used in medium and high voltage power applications and can replace traditional power frequency transformers. However, compared with conventional transformers, PET is more suitable for applications that enrich system functions and improve system performance. Comprehensive analysis of existing PET literature can be found that the application of PET is currently mainly concentrated in onboard converter systems for electric locomotive traction, smart grid/energy internet and distributed renewable energy generation grid-connected systems [1-6].
Because PET adopts high-frequency transformers to achieve the electrical isolation function, it requires less iron core material compared to the power frequency transformers, which can reduce the amount of minerals such as copper and iron, and provides the possibility to increase the system power density and reduce the cost. As early as the end of the 1960s, which was the beginning of the development of power electronics technology, PET had attracted the attention and research of relevant scholars [7-8]. However, due to the limitation of the development level of power semiconductor devices at the time, the development of PET has been slow [9-17]. However, in some special occasions where the power-frequency transformer has a high space constraint, PET research has received attention and attempts have been made in practical use. One of them is the use of PET to replace the power transformers for locomotive traction in some European railway systems. The rated operating frequency of some railway power supply systems in Europe is 16.7Hz, which leads to large and heavy-duty traction transformers on locomotives and also affects the performance of the traction converter system of locomotives. Since the late 1990s, the research of PET has attracted the attention of European industry. Companies such as ABB, Bombardier, and Siemens have successively developed medium-pressure engineering prototypes [18-33]. In particular, ABB Company has developed a single-phase PET capacity for locomotive traction of up to 1.2 MW, which was first implemented on electric locomotives of Swiss Railways in February 2012 [27, 30]. In addition, special attention should be paid to the modular multilevel converters widely used in high voltage and large capacity fields, especially in the field of voltage-source converter based high-voltage direct current (VSC-HVDC) in recent years. (Modular multilevel converter, MMC) was invented for the application of locomotive haulage on-board PET [20-21].
After the United States proposed a smart grid program, and with the development of emerging technologies in the power sector, such as grid-connected renewable energy generation systems and energy internet technologies, PET has become highly controllable. The application of compatibility and good power quality characteristics in the above-mentioned future power grid technology fields has attracted widespread attention and has become a research hotspot in the field of power electronics in recent years [1,3-6, 34-95]. For the application of power grids and renewable energy, research institutes at home and abroad have successively developed several PET prototypes from several kVA to MVA levels and completed experimental verification [1,35-36,39,43,47, 49-50, 53, 55-57, 59-60, 62, 64-66, 69-70, 78-80, 83, 86, 96-99].
It should be pointed out that many functions of PET, such as DC access, variable frequency output, fault isolation, and reactive/harmonic control, etc., have exceeded the concept of conventional power frequency transformers. Therefore, it is not reasonable to compare the performance, cost, and power density of PET with power frequency transformers. A more reasonable comparison is to compare PET with an integrated power management device that integrates power frequency transformers and power quality control functions from a functionally similar point of view, rather than just comparing PET to the power frequency transformers in it.
In fact, PET should not generally be used to directly replace the AC-AC transformer and electrical isolation functions of conventional power frequency transformers, but it can also play a multifaceted role in specific situations. Take the medium voltage-low voltage distribution (microgrid) application shown in Fig. 1(a) as an example. If the low-voltage side contains a large number of photovoltaic, wind power, charging piles, energy storage equipment, and other devices that use DC as the intermediate power conversion link, In the traditional scheme, these devices need to be equipped with inverters in order to connect the AC grid. In addition, in order to improve the quality of power supply on the low-voltage AC side, power quality control equipment such as reactive power compensation and harmonic suppression may also be required. If PET is used for this occasion, as shown in Fig. 1(b), the DC port of PET can be directly connected to intermediate DC links such as photovoltaic, wind power, and energy storage equipment. That is, the front-end grid-connected inverter in these devices can be canceled. And the power quality control equipment in the original AC system; of course, the primary and secondary equipment related to relay protection and automation in the original power frequency transformer can also be eliminated. This provides the possibility to optimize the structure, efficiency, and economy of the entire low-voltage distribution (micro-grid) system, and it can also give full play to the advantages of PET. However, if PET is used as a direct replacement for the power frequency transformer in Fig. 1(a), it may be difficult to achieve a cost-effective operation of the system.
Fig. 1 PET application in low voltage distribution network
As a composite power electronic converter with practical value, the research and development of PET involves the circuit topology theory of power electronic converters, low-loss and low-harmonic modulation methods, high-performance control technologies, and fault protection methods. High-frequency transformer multiphysics optimization design method, PET soft-switching and accurate estimation of loss, high-voltage insulation and cooling technology, power circuit electromagnetic compatibility and multi-physics compact design technology, power semiconductor theory and applications and other disciplines Related theories and technologies belong to the multidisciplinary research in the field of electrical engineering. The comprehensive analysis of the current study can find that, compared with the actual needs, PET still has many theoretical and technical issues need to be resolved, especially the power conversion efficiency is low, the power density is low, high cost and poor reliability and other outstanding issues. The main reason for these problems is the limited power voltage level of power semiconductors in current power electronics. Converters in PET usually use a cascade topology, making the power semiconductors and storage capacitors and inductors in PET large. [1-2, 70]. In order to simplify the topology of power electronic converters in PET, higher voltage and lower loss power semiconductors are used. In particular, research on wide bandgap semiconductor devices based on silicon carbide (SiC) has also received widespread attention in recent years. 31-32,56-57,77,82-83,87,96,100-101].
This article reviews and summarizes key PET technology-related research literature, including: Section 1 describes the circuit topology features, classification, and advantages and disadvantages of typical topologies for PET; Section 2 describes the modulation and soft-switching techniques for converters in PET. , open-loop and closed-loop control technologies, fault protection technologies, etc.; Section 3 describes the optimized design techniques for electrical, magnetic, thermal, and insulation of high-frequency transformers in PET; Section 4 describes the power circuit of power converters in PET. Magnetic, thermal, and dielectric compact design technologies and wide bandgap power semiconductors, especially high-voltage silicon carbide (SiC) devices in PET applications and their characteristics; Section 5 summarizes the current state of PET development and its development trends induction.
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