Elsevier

Measurement

Volume 171, February 2021, 108848
Measurement

Power electronic transformers: A review

https://doi.org/10.1016/j.measurement.2020.108848Get rights and content

Abstract

The medium frequency transformer is a key component for the design of input–output isolated converter design when the isolation and/or voltage matching is needed. These kinds of converters are used in different applications such as battery based energy storage systems, the high voltage DC conversion, grid interfaces of renewable energy sources, etc. Design at the high frequency significantly decreases the size and improves the efficiency of the power transformer. With the recent advancements of soft magnetic core materials and switching devices, the high frequency transformers become more interesting not only as a part of power converters but also as a replacement of conventional line frequency transformers. In this detailed review study, studies on the design of the power transformers used in power electronic converters are examined and their application areas, operating frequency values, core material types are investigated and classified. In addition, the design methodology is proposed with Finite Element Analysis (FEA) software and a power electronic transformer is designed with the different core materials.

Introduction

Use of medium frequency transformers have been extended by the realization of improved power switches and core materials. New generation power switches are designed to operate under higher voltage and frequency compared with the former ones. New core materials and sizing methodology also help to minimize the transformer design. Medium and high frequency transformers embedded into power electronic converters are required to provide isolation and and/or voltage matching and used in different applications such as renewable energy systems, electric vehicles, uninterruptible power supplies and energy storage systems [1], [2], [3], [4], [5], [6], [7]. In this context, the designs of medium / high frequency transformers have been discussed in many studies conducted in recent years, and effort is continuing intensively to reach higher power capacities and smaller volumes along with higher efficiency values.

Thanks to silicon steel (SiFe) alloy core materials, conventional transformers are designed for grid-frequency (<100 Hz) power systems and operate at a flux density value which is very close to its saturation point. The minimum core area and volume become quite large, and thus, resulting transformer size becomes bulky. However, an increase in operating frequency significantly reduces the volume of transformers used in power electronic circuits [3], [4], [8]. Regarding transformers designed for power electronics circuits, if the operating frequency values are in the range of 400–25 kHz, it is called “medium frequency transformer”, if it is higher than 25 kHz, it is called “high frequency transformer”. On the other hand, transformers used at operating frequency values below 100 Hz are called “low frequency transformer” or “line frequency transformer” [9].

The power transformers are the key components of the isolated DC-DC power converters with high voltage gain which has become a popular topic in recent years [1], [7], [10], [11]. In the isolated DC-DC converter applications, power transformers have three main tasks [12], [13], [14]. First one is to ensure galvanic insulation through magnetic coupling between the low voltage and high voltage side. The second is to provide voltage conversion ratio that is voltage-matching function. Thus, the electrical energy obtained from different types of energy sources can be matched at the same voltage value by the conversion ratio of the transformer. Third is the leakage flux inductance on the primary side that can be used to achieve soft switching [5], [15].

Apart from these, power electronic transformers provide bidirectional power flow, compensation of short term power quality problems and improved power quality, ability to reach very large power levels by using modular power converter structures as well as many other advantages [13], [14], [15], [16]. Basically, the block diagram of the DC-DC converter with the power transformer is shown in Fig. 1. In this block diagram, the power flow can be provided in single or two directions, or it can be used only as an isolated DC-DC converter.

As can be seen in Fig. 2, power converters which use transformers can be designed in two different structures called direct conversion and indirect conversion [17]. In the direct conversion structure, shown in Fig. 2(a), there is an AC/AC frequency converter circuit on both sides of the transformer. The AC/AC converter on the left side of the topology is required to convert the low-frequency grid voltage to medium- (or high-) frequency voltage. The AC/AC converter on the right side is used to convert medium- (or high-) frequency AC voltage to grid-frequency AC voltage. In the structures of indirect converter circuits shown in Fig. 2(b), the DC-DC converter topology, which can be designed as single-phase or three-phase and can operate in two-direction with isolation, is used as the main component [14], [18], [19]. At the input side of this topology, there is a rectifier circuit to rectify a particular AC voltage. The DC voltage obtained at the rectifier output is converted into a medium-frequency AC voltage. Here, the transformer provides voltage matching and galvanic isolation. At the transformer output, the medium-frequency AC voltage at the desired amplitude value is converted back to the DC voltage. Finally, the AC voltage is obtained with the DC/AC inverter in order to be connected to the low-frequency medium-voltage grid or to supply conventional AC loads.

The full bridge converter topology is commonly employed with the indirect power converter circuit. However, limitations of switching elements such as rated current/voltage values and switching losses in high power and high frequency applications introduce some problems and enable research studies on different converter topologies. Especially at high power and medium voltage applications, multi-level converter topologies are increasingly becoming important. These topologies can reach higher current and voltage values by using lower voltage and current rated switches, and remove the serial and/or parallel switch connection requirements [9]. In addition, an increase in number of converter level improve the output voltage and/or current quality [17], [26], [27], [28], [29], [30], [38]. In Fig. 3, three different multi-level converter topologies, the neutral-point clamp (NPC) converters, the capacitor clamped (CC) converters and the cascaded H-bridge (CHB) converters are shown [17], [20], [21], [22], [23]. Among them, cascaded H-bridge converter topologies which can be used at different voltage ranges especially at medium voltage applications and provide higher power thanks to its modular structure are becoming widespread [24], [25].

The CHB converters consist of several single-phase full-bridge (H-bridge) converters connected in series. This topology is very useful for applications which is inherently compose of partial DC sources such as PV and fuel cell systems. Unlike NPC or CC converters, this topology does not have any clamping diodes or capacitors. Each H-bridge unit can be designed as modules and they are in the same structure [20]. Thus, it provides better and more homogeneous heat distribution [26], [27], [28], [29], [30].

In general, in the determination of the core material for transformer design, operating frequency (f) and peak flux density (Bm) values are two main variables. While 3% silicon steel (SiFe) alloys are used in grid-frequency transformers, soft magnetic materials such as 6.5% non-oriented silicon alloy, amorphous, nanocrystalline and ferrite are mostly used at medium/high frequencies [6]. Ferrite materials are preferred for high frequency applications due to their low specific core losses. However, due to their brittle structure, they have limitations in the core sizing. For this reason, the tendency of alloy strip core materials with low specific core loss at high frequencies such as amorphous and nanocrystalline is increasing in the design of high power transformers [31], [32], [33], [34], [35]. Thus, the two most important factors affecting the size and efficiency of the transformer are the operating frequency and the specific core loss value which varies according to the saturation flux value for the corresponding frequency of the selected core material. In addition, when different core materials which have different power density (kVA/kg) values are used for reducing dimensions according to the operating frequency value, mechanical differences occur [36], [37]. The studies on the design of medium/high frequency transformers from the past literature are summarized in Table 1 according to the determined operating frequency values and core material type. This table also gives an idea about the core materials to be selected according to the operating frequency.

However, the availability of the core material should be considered at the design stage, and it can also be considered as a factor affecting the rate of use. With this overview, national and international literature has been extensively searched and about 100 studies on medium/high frequency transformers for the last 25 years have been analyzed [9]. In this comprehensive examination, considerations such as power level, operating frequency, preferred type of core material, weight/volumetric size and application areas are taken into consideration. When an examination is made as application areas, as given in Table 2, the DC-DC bidirectional converters, the isolated DC-DC converters, the resonant converter circuits, the motor drive circuits are generally used in areas such as railway application, submarine applications, aviation and renewable energy sources.

In addition, in [17], a chosen core material map for medium frequency transformers based on power capacity and operating frequency has been produced [60]. For instance, two designs have been presented for the power systems of electric trains. In the first design, the transformer operating below 1 kHz is designed with 6.5% SiFe core material. In the second design, another transformer operating above 5 kHz is designed for the same power system by using nanocrystalline core material. The most important difference among these designs is the size: The second design reduces the volume and weight of the electric train power systems by eight times. This is a good example to show the effect of switching frequency and core material [39].

In the past literature, many studies have been presented on the medium/high frequency transformers. While some of these studies have focused on power electronics and converter topologies, some studies have focused on the transformer design and parameter analysis of the transformers. Since the medium/high frequency transformers are an important part of the system they employed, their design is also important and attracts researchers’ attention. In Table 3, past studies have been grouped and listed. As seen from the table, design of power converters and transformers are two hot topics.

Section snippets

Core materials of medium/high frequency transformers

The power transformers, one of the most important elements used in power electronics applications, can operate with the high efficiency, high power density, low loss at high frequencies, low vibration and acoustic noise. In addition, power density requirements of the applications such as electric vehicles, ships and railway systems where space and weight are important makes the core material critical. Conventional laminated electrical steels used in grid frequency transformers are insufficient

Determination of power handling capacity of the power transformer

The core sizes of the transformers vary depending on the operating frequency. However, another important factor in sizing of the core is the type of core material. This is because the specific core losses of the core material are different depending on a certain operating frequency and flux value. Thus, the power losses (Pc) in the core for the unit core volume (Vc) of the transformers vary depending on the flux amplitude (Bm), operating frequency (f) and specific characteristics (α and β) of

Power transformer design procedure

In general, C.W.T. Mclyman' s methods, who worked at NASA and has many books and publications in this field, is used in medium/high frequency transformer design, sizing and relating procedures. The fundamental variables in sizing are the operating frequency and the flux density amplitude of the core material. Here, the most important factor limiting the designer is the specific core loss with respect to the operating frequency and flux amplitude. Therefore, while the flux amplitude close to the

Conclusion

In this study, a review study has been proposed for medium/high frequency power transformers. Nowadays, with the increase in both switching frequency and current/voltage capacities of power semiconductors, the power levels of power converters are increasing. Besides, the power capacities of the magnetic components to be used at the high frequency designs are also increasing. At high-power levels, the core material plays an important role for the overall system to operate as desired. Therefore,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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