Power electronic transformers: A review
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.
References (169)
- et al.
Magnetic cores made of VITROVAC and VITROPERM in separating transformers used in thyristor tripping circuits
J. Magn. Magn. Mater.
(1996) - et al.
- et al.
Electronic power transformer with supercapacitors storage energy system
Electr. Power Syst. Res.
(2009) - et al.
Dynamic average modeling of a bidirectional solid state transformerfor feasibility studies and real-time implementation
Electr. Power Syst. Res.
(2014) - et al.
Optimal regulator-based control of electronic power transformer for distribution systems
Electr. Power Syst. Res.
(2009) - et al.
Auto-balancing transformer based on power electronics
Electr. Power Syst. Res.
(2010) - et al.
Potential of solid-state transformers for grid optimization in existing low-voltage grid environments
Electr. Power Syst. Res.
(2017) - S. Balci, The analysis, design and implementation of the medium frequency power transformer with the nanocrystalline...
- et al.
A comparison of single and three phase DC/DC converter structures for battery charging
IEEE International Conference on Renewable Energy Research and Applications (ICRERA)
(2013) - et al.
A comparative study of nanocrystalline and sife core materials for medium-frequency transformers
IEEE International Conference–6th Edition, Electronics Computers and Artificial Intelligence (ECAI)
(2014)
Core material investigation of medium-frequency power transformers
An investigation of ferrite and nanocrystalline core materials for medium-frequency transformers
3. European Conference on Renewable Energy Systems (ECRES), Antalya
Thermal evaluation of a medium frequency transformer in a line side conversion system
Analysis of hysteresis and eddy-current losses for a medium-frequency transformer in an isolated DC-DC converter
IEEE International Power Electronics Conference (IPEC)
Modeling and analysis of inverter output transformers
Investigation of vibration effects caused by harmonics medium frequency nanocrystalline core power transformers
Transient thermal model of a medium frequency power transformer
Power electronic transformer (PET) converter: Design of a 1.2MW demonstrator for traction applications
IEEE-International Symposium on Power Electronics, Electrical Drives, Automation and Motion
Predictive current control of a 7-level AC-DC back-to-back converter for universal and flexible power management system
Field programmable gate array based control of dual active bridge DC/DC converter for the UNIFLEX-PM Project
A multiport power electronic transformer based on modular multilevel converter and mixed-frequency modulation
IEEE Trans. Circ. Syst. II: Express Briefs (Early Access)
High power density adjustable speed drive topology with medium frequency transformer isolation
Simplified medium/high frequency transformer isolation approach for multi-pulse diode rectifier front-end adjustable speed drives
Overview of three-stage power converter topologies for medium frequency-based railway vehicle traction systems
IEEE Trans Veh Technol
An overview of Power Electronic Transformer: Control strategies and topologies
International Symposium on Power Electronics Power Electronics Electrical Drives, Automation and Motion
Power electronics and motor drives recent progress and perspective
IEEE Trans. Ind. Electron.
Maximum efficiency operation of three level t-type inverter for low-voltage and low-power home appliances
J. Electr. Eng. Technol.
An investigation of ferrite and nanocrystalline core materials for medium-frequency power transformers
J. Electron. Mater.: The Miner. Met. Mater. Soc.
European trends and technologies in traction
IEEE-International Power Electronics Conference (IPEC)
Global loss evaluation methods for nonsinusoidally fed medium-frequency power transformers
IEEE Trans. Ind. Electron.
A new winding design method for inductors and transformers
Optimal design and implementation of high-voltage high-power silicon steel core medium frequency transformer
IEEE Trans. Ind. Electron.
Modeling and control of the isolated DC–DC modular multilevel converter for electric ship medium voltage direct current power system
IEEE J. Emerg. Select. Top. Power Electron.
Cited by (21)
Development of Fe-based diluted nanocrystalline alloy by substituting C for P in FeSiBCCu system
2023, Journal of Alloys and CompoundsDetermination of output current THD of multilevel inverter by ANN
2023, Measurement: Journal of the International Measurement ConfederationHigh B<inf>s</inf> Fe-B-P-C-Cu nanocrystalline alloy with longtime annealing stability and low heating rate sensitivity
2022, Journal of Magnetism and Magnetic MaterialsMulti-module capacitor voltage measuring technique of modular multilevel converters based on sampling instant classification
2021, Measurement: Journal of the International Measurement ConfederationCitation Excerpt :Specifically, the modular multilevel converter (MMC) [1–4], with the ability to directly access high level DC voltage, are widely used for medium or high voltage level applications such as high voltage direct current (HVDC) transmission systems [5–7], medium-voltage drives [8–9], and electric railway supplies [10]. In addition, MMC can provide both medium/high voltage AC and medium/high voltage DC interfaces for power electronic transformer that is the core equipment of the Energy Internet [11]. Currently, in the MMC control system, the voltage balancing algorithms of SMs [12–14] and the circulating current control methods [15,16] are the fundamental control requirements.
The core loss estimation of a single phase inverter transformer by using adaptive neuro-fuzzy inference system
2021, Measurement: Journal of the International Measurement ConfederationCitation Excerpt :In this study, the ANFIS technique is proposed to estimate the core losses of a single phase inverter transformer. It is of great importance to determine the core losses that the transformer will have before proceeding to the production stage [28]. However, since the determination of the power losses depends on more than one parameter, it is quite difficult to determine it completely.