Title: Solid‐state transformers: An overview of the concept, topology, and its applications in the smart grid
Abstract: International Transactions on Electrical Energy SystemsVolume 31, Issue 9 e12996 REVIEW ARTICLEFree Access Solid-state transformers: An overview of the concept, topology, and its applications in the smart grid Hamed Shadfar, Hamed Shadfar [email protected] Electrical and Computer Engineering Faculty, Semnan University, Semnan, IranSearch for more papers by this authorMehrdad Ghorbani Pashakolaei, Mehrdad Ghorbani Pashakolaei [email protected] Electrical and Computer Engineering Faculty, Semnan University, Semnan, IranSearch for more papers by this authorAsghar Akbari Foroud, Corresponding Author Asghar Akbari Foroud [email protected] orcid.org/0000-0001-6902-8990 Electrical and Computer Engineering Faculty, Semnan University, Semnan, Iran Correspondence Asghar Akbari Foroud, Electrical and Computer Engineering Faculty, Semnan University, Semnan, Iran. Email: [email protected] for more papers by this author Hamed Shadfar, Hamed Shadfar [email protected] Electrical and Computer Engineering Faculty, Semnan University, Semnan, IranSearch for more papers by this authorMehrdad Ghorbani Pashakolaei, Mehrdad Ghorbani Pashakolaei [email protected] Electrical and Computer Engineering Faculty, Semnan University, Semnan, IranSearch for more papers by this authorAsghar Akbari Foroud, Corresponding Author Asghar Akbari Foroud [email protected] orcid.org/0000-0001-6902-8990 Electrical and Computer Engineering Faculty, Semnan University, Semnan, Iran Correspondence Asghar Akbari Foroud, Electrical and Computer Engineering Faculty, Semnan University, Semnan, Iran. Email: [email protected] for more papers by this author First published: 08 July 2021 https://doi.org/10.1002/2050-7038.12996AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Summary The development of power systems and the move to smart grid have increased the need for new technologies. In this regard, solid-state transformers have been proposed as a suitable alternative to conventional transformers. Solid-state transformers are among the equipment based on power electronic converters that in addition to better performance than conventional transformers provide a variety of other services. In this article, the concept and types of solid-state transformer topologies and configurations and their applications, especially in smart grid, are investigated. Studies show that the various characteristics of solid-state transformers have led to much consideration as potential transformers in smart grid applications, the integration of distributed generation sources, modern traction systems, and so on. 1 INTRODUCTION Nowadays the complexity of the electrical network has increased due to the increase in new energy generation and storage resources. The electrical energy output of these sources is provided at different voltages (DC and AC) with different frequencies.1 In the face of these complexities, the use of new technologies to control and improve the reliability of the power system is inevitable. Intelligent energy management (IEM) is required for the interconnection of power generation, energy storage and loads in a grid or microgrid. IEM substations must be capable of bidirectional energy flow, intelligent monitoring and control, and strong communication lines. This is where “smart grids” find meaning, and their aim is to reduce or prevent the consequences of power quality, improve reliability, increase productivity, and stability of the grid by using new technologies and equipment.2, 3 Low-frequency transformers are widely used in traditional power systems to perform tasks such as voltage change and isolation and to meet most needs related to cost, efficiency, and reliability. Obviously, they are not suitable for such requested programs. Therefore, to meet the needs of the future smart grid, new concepts of transformers have been proposed.4-8 Accordingly, as shown in Figure 1, four structures are proposed for transformers9, 10: 1. Passive or low-frequency transformer 2. Transformer with series voltage compensation 3. AC series chopper transformer 4. Solid-state transformer FIGURE 1Open in figure viewerPowerPoint Different structures of transformers: A, Passive. B, Series voltage compensator. C, AC series chopper. D, solid-state 2 CONCEPT OF THE SOLID-STATE TRANSFORMERS As said before, the solid-state transformer (SST) is offered as a tool to meet the requirements of the smart grid. Solid-state transformers are comprised of three primary parts: converter to produce high-frequency AC from input line frequency AC, isolation by a high-frequency transformer (HFT), and at last, converter to produce AC with line frequency from AC high frequency. Additionally, the isolation barrier partitions the transformer into two sections: high voltage and low voltage. In addition to being able to perform the same functions as a conventional transformer, SST provides a range of services to the grid, including reactive power compensation, power quality improvement, current limiting and voltage regulation power factor correction, etc., which can lead to improvements and establishing a connection between direct current (DC) and alternating current (AC) equipment. This equipment is introduced by various authors with different names, such as electronic transformer by McMurray in 1968,11 solid-state transformer by Brooks in 1980,12 intelligent universal transformer by EPRI in 1995,13 electronic power transformer by ABB,14 power control center by Borojevic, power router by Wang,15 MAGA cube by ETH Zurich,16 etc. It also operates as an electronic power interface between medium voltage (MV) and low voltage (LV).17 3 TYPES OF SST STRUCTURES AND TOPOLOGIES In general, the main application of SSTs is in the voltage range of distribution system. Application of the power electronic devices which are able to operate in high frequency and high voltage conditions is necessary for designing a high reliable and high efficient SST. Various structures have been proposed for the design of the SST, which for facilitating their study, can be classified according to the number of stages and the topology of the converters used in the low voltage (LV) and high voltage (HV) sides. The common element of high-frequency transformer (HFT) is observed in all structures.18 Obviously, the sustainable voltage of power semiconductor devices in LV and HV side converters (especially in HV side) must be very high. This can be an important challenge for the design of converters used in the SST structure. A suitable solution is to provide the structure in series-parallel arrangement of SST modules or to use multi-level converters with high frequency and low voltage semiconductor devices.7 Suitable power semiconductor devices for HV voltage levels include IGBT, IGCT, ETO thyristors, and power MOSFETs. These switches can tolerate very high voltages, but their switching frequency cannot exceed 1 kHz, because at higher frequencies due to their hard switching process, switching losses increase.19, 20 The small frequency range of these switches makes them less effective in reducing the volume and weight of the transformer and is not suitable for use in SST. Arrangement of a series of low voltage power semiconductor devices can also be a solution for high voltage applications. The disadvantage of this method is that it creates a large inherent inductance loop in the converter structure and induces an additional voltage in various SST components. This prevents the switches from operating properly at high frequencies. By using the integrated devices of the series connection of power switches, this defect can be reduced to a large extent and the ability to operate at high frequencies for the series arrangement can also be provided.21 However, the conduction voltage drop in the switch modules of the series switch reduces the overall efficiency of the system. Wide-band gap materials such as 4H-silicon carbide (4H-SiC) are considered to be the building blocks of the next generation of silicon switches and can operate at high voltages, tolerate higher conduction temperatures, and higher operating frequencies. In Reference 22, a research has been conducted on SiC semiconductor devices in the application of smart grids. The results indicate that SiC MOSFETs, in this application at the level of 1 kV voltage and frequency above 2 kHz and room-temperature, has the best performance. On the other hand, silicon carbide IGBTs will tolerate higher currents and lower operating frequencies. Suppose that volume and size are important parameters for the application of SST, in which case SiC MOSFET will be most suitable for voltage levels below 10-15 kV due to its majority carrier conduction mechanism.23, 24 3.1 Different types of SST configurations In general, SST configurations are classified into four categories according to the number of stages of voltage to DC conversion. Figure 2 shows the types of presented configurations for SST.18 Figure 2A shows the single-stage SST with no DC link, Figure 2B represents the two-stage SST with a DC link on the secondary side, Figure 2C demonstrates the two-stage SST with a DC link on the primary side, and finally in Figure 2D represents the three-stage with a DC link on both the primary and secondary sides. In the one-stage configuration, only one AC-AC conversion stage is performed.25 In References 26, 27, the matrix converter structure is used to convert AC to AC voltage. The arrangement of the switches is back-to-back and provides the ability to transfer power in bi-directions. The predictive controller algorithm is designed to control the output current of the system, which will optimize the overall behavior of the system by accurately predicting the circuit state variables. The authors in Reference 28 propose a new structure of the dual active bridge (DAB) converter with soft switching capability at zero voltage (ZVS) for all switches. In this configuration, 12 power switches are used that have the ability to transfer power from both sides with low electromagnetic interference (EMI). FIGURE 2Open in figure viewerPowerPoint Different types of SST configurations based on the stages In the two-stage arrangement, on the HV side, first, an AC-DC conversion stage is performed and then by transferring the power through the HFT transformer, the LV voltage will be adjusted on the secondary side. In this configuration, the losses will increase because the conditions for switching at zero voltage are difficult.25 In Reference 29, a new configuration with input three phases and four-wire three phases at the output is presented. This configuration is used in the two-stage arrangement and no electrolytic capacitor is used in the DC-link bus on the LV side. Dual bridge matrix converter is implemented in this configuration, which is controlled by the 3D-SVPWM method, and in the conditions of variable load and input imbalance voltage, the harmonic distortion of input and output will be low. In Reference 30, SST is used to drive a locomotive. In this configuration, by serializing the H-bridge of the input of several SST cells and parallelizing the output of their DC/AC converter, it is possible to transfer high power. The controlling method of converters is phase shift modulation and there is the ability to bidirectional transfer power. In a three-stage configuration, two DC-link basses are generated on both the LV and HV sides during two AC-DC voltage conversion stages. This configuration has more capabilities compared to the other two configurations. By means of three-stage SST, in addition to regulating the nominal output current and voltage in the system, it is possible to control the reactive power, compensation the voltage sag, and stabilize the power factor.25 The authors in31 present a structure consisting of a series connection of AC-DC converters at the input and a parallel connection of the DC bus at the LV side. Figure 3 shows the general arrangement of their proposed SST system. In some references, the configuration of AC-DC converters at the input is the H-Bridge converter. By serializing the input of these converters, the overall configuration expresses the cascaded multilevel converter.32 The application of series arrangement at the input and parallel arrangement at the output of several SST cells is very suitable in cases where there is a limit to the choice of semiconductor power devices because SST configuration can be used in conditions of high power and high voltage. In Reference 31, SST is not able to transfer power bidirectional but has the ability to power factor correction. In Reference 33, a new method for controlling power and voltage balance is presented for the cascaded H-bridge multilevel converter in the three-stage SST application. The suggested control method, in addition to simplifying the modulation algorithm, reduces the effect of load changes on the controller. The proposed system is resistant against an unbalanced load and is suitable for microgrid and distribution networks. In Reference 34, a configuration of SST to control the output power and voltage of a permanent magnet synchronous generator is provided. The SST output is connected to the grid and series-parallel arrangements can be used for multiple SST cells to transfer high powers. In Reference 35, a three-stage SST series-parallel arrangement configuration is presented. Its main focus is on the challenges and requirements of network-connected SST design. The overall efficiency of the system, by taking into account all losses in the laboratory sample, is 96.75%. SiC MOSFETs are used in the configuration of the converter, so it can be used in cases of high power and high voltage. FIGURE 3Open in figure viewerPowerPoint A two-stage SST with series-parallel arrangement configuration 3.2 Comparison of SST configurations 3.2.1 One stage This structure comprises of converting high voltage AC to low voltage AC with an isolation stage.36 Having low cost and low weight because of its simple structure and also having the least losses due to the presence of fewer semiconductor switches and the use of minimal copper are the advantages of this structure. Disadvantages of this structure include the following25: 5. Due to lack of DC link, compensation of reactive power and direct connection of distributed renewable generation sources and energy storage are impossible and its control is difficult. 6. Need for large filters because of lack of regulation of input and output current. 7. Also in this structure, the disturbance on one side of the transformer can affect the other side, which is one of the major problems of the traditional transformers. The following two models are proposed for this structure. 8. AC-AC full-bridge converter This topology has a simpler control method than the other one-stage SST topologies, and its main drawback is the lack of a DC link and the capabilities provided by it.37 9. AC-AC flyback This structure is the simplest topology of the isolated DC-DC converters. Disadvantages of this model include the large size of its filters and the lack of a DC link.38 3.2.2 Two stages In this structure, two models are proposed. In the first model, at first the AC voltage is converted to low voltage DC and then the low voltage DC is converted to low voltage AC. But in the second structure, first the AC voltage is converted to high voltage DC and then to low voltage AC. In these two models, there is an isolation stage (on the high voltage or low voltage side). The advantages of these two models include the ability to compensate for reactive power, support of their low voltage DC link (LVDC) from distributed energy storages (DES) and distributed energy resources (DER), support of storage resource management, having an independent frequency and independent power factor, regulating the output voltage and input current, the ability to limit the input and output current and the possibility of isolating the ancillary loads of the network. Despite the mentioned advantages, zero voltage switching cannot be used in this structure in high voltage applications. Also, the LVDC link has more current ripples due to the lack of HVDC. In general, these two models have high switching losses, which can lead to reduced efficiency in the network. The details of the two mentioned models for SST with two stages structure are as follows: 1. AC-DC isolated boost + PWM inverter This topology of SST is based on the AC-DC isolated boost version.37 The disadvantages of this model are the need to use two different controls depending on the direction of power flow and also due to the lack of HVDC connection, its LVDC link will have a high ripple and on the other hand, its bandwidth is considerably lower than that of the input current because of the cascaded control. 2. AC-DC DAB + PWM inverter This topology of SST is based on DC_AC dual active bridges (DAB) version. The large signal medium model is the same as the DC-DC version. Each version of the DAB provides uniform control in both directions and zero voltage switching (ZVS) for a wide load range. The main disadvantage of this model is its high sensitivity to currents with high ripples. Also in this structure, due to the existence of a second-order filter, the design of the controls is very limited, leading to low bandwidth for the input current loop and even lower bandwidth for the LVDC link loop.39, 40 3.2.3 Three stage This structure has a triple conversion in which high frequency isolation is performed in DC-DC stage. In this model, both HVDC and LVDC links are available. Most of the designs offered for intelligent transformers are of this type. This topology is able to compensate for reactive power and voltage regulation. In addition, it allows the connection of energy storage and renewable sources directly and the bidirectional power flow. In addition, other advantages of this model include high flexibility, better controllability, better regulation of output voltage and input current, and regulating of LVDC and HVDC links. Given the characteristics of all three models, it is easy to understand that the three stages structure has more advantages than the other two structures, and in addition, the important point is that in this model the existence of many switches in the switching circuits may lead to more losses, which reduces the efficiency of the network and impairs thermal management. Also in this structure, due to the high number of converter levels, the cost will be higher. For this structure, two models were proposed as follows. 1. PWM rectifier + DC–DC DAB + PWM inverter This topology of the SST is most popular with researchers in the Future Renewable Electric Energy Delivery and Management (FREEDM) field because of its better controllability, which allows several desirable functions for an SST. This topology consists of only one module in two isolated DC-AC and DC-DC stages. The DC-AC stage of this topology is based on the full-wave rectifier. The isolated DC-DC stage of this topology is based on DAB modules that connect the HVDC link to the LVDC link. The main drawback of this topology is its complex structure, which results in reduction of network reliability.41 2. Multilevel rectifier + DC-DC full-bridge converter + PWM inverter The DC-AC stage in this topology is based on a diode clamped multilevel rectifier. Therefore, in this structure, there is no need for modular configuration of input voltage division. A bi-directional modular version is used for the isolated DC-DC stage. In this topology, full-bridge converter supplies bi-directional power flow and injects low ripple currents into the LVDC link.18, 42, 43 In general, the technical characteristics for the three SST structures are shown in Table 1. TABLE 1. Functional capabilities supported by the SST topologies Functionality One stage Two stage Three stage Output voltage regulation Poor Good Good LVDC for DES and DER No Yes Yes DES management No Yes Yes Bidirectional power Yes Yes Yes Reactive power support to grid No Yes Yes HVDC link regulation N/A N/A Good LVDC link regulation N/A Good Very good Input current regulation No Very good Very good Input voltage sag ride through Poor Good Very good Input current limiting No Yes Yes Output current limiting No Yes Yes HVDC undervoltage protection N/A N/A Yes HVDC overvoltage protection N/A N/A Yes LVDC undervoltage protection N/A Yes Yes LVDC overvoltage protection N/A Yes Yes Independent frequency No Yes Yes Independent power factor No Yes Yes Modularity implementation Simple Simple Simple Abbreviation: N/A, not applicable. The authors in Reference 18 compare the switching losses and the number of switches in different SST topologies under the nominal voltage of the switches. For each topology, the count of switches is based on the number of 6500 V on the high voltage and the number of 600 V on the low voltage. Accordingly, all SST topologies, except for the flyback-based topology, which requires six modules due to higher voltage stresses, have three modules. The results of this simulation in Matlab software are shown in Table 2. According to the obtained data, one stage topology has the lowest switching losses among different SST topologies. TABLE 2. Switch losses for SST topologies Topology Parts Loss per switches (W) No. of switches Total loss (W) One stage HV IGBT+Diode 8.5 24 204.6 LV IGBT+Diode 8.5 24 203.2 Two stage HV IGBT+Diode 25.5 24 611.9 LV IGBT+Diode 41.7 12 500.4 Three stage HV IGBT+Diode 37.1 12 445.1 LV IGBT+Diode 13.9 4 299 3.3 HFT design principles High-frequency transformer (HFT) is the main component in SST and replaces traditional 50/60 Hz transformers in distribution systems. There are challenges in designing HFT transformers to meet high voltage, power, and frequency. First, the choice of magnetic material of the transformer core plays a critical role in achieving high power density and low magnetic losses. In Reference 44, 45, several magnetic materials such as silicon steel, ferrite, amorphous, and nanocrystalline have been investigated for high power application. In addition, transformer winding structures will have a direct effect on overall efficiency in high-frequency performance. In general, two structures are proposed for the winding: solenoidal and coaxial.44 The solenoid structure is one of the most common winding arrangements for transformers. In this structure, the magnetic flux flows parallel to the axis of the cylindrical core and the current flows in the winding around the axis, whereas, in the coaxial structure, the flux around the cylindrical axis and the current are parallel to the current axis.46 Although in the coaxial structure it will be easy to predict and control leakage flux in the system, but the solenoid structure is more often used due to its simplicity in design and construction and lower price. Also, coaxial structures are less used due to their difficulty in construction and limitations in choosing the turns ratio, especially on the LV side. In both structures, it can be used for high power and high frequency applications by connecting and combining several cores. Another challenge for the design of HFT transformers is its thermal and insulation design. Compared to traditional transformers, the insulation and temperature design process in SST is much more complex and difficult, because space is less and it is not possible to use oil for insulation. Temperature convection flow is one of the most widely used solutions in SST temperature exchange, which depends on its application. The method of cooling heat sinks with fan and water for higher power density is more used. Also, for insulation in the design of oil-free SST transformers, solid insulating materials such as epoxy are used at high voltage levels and in very compact structures.47-49 4 INVESTIGATION OF VARIOUS CONTROL METHODS The design aims of control algorithms are often related to the discussion of improving power quality in the grid. As mentioned before, in most cases, the design of the HV side structure in grid-connected SSTs uses a series–parallel arrangement of bridge dual active converter modules. Therefore, if there is an imbalance in the output voltage of each module, it causes instability problems, design complexity and unpredictable behavior in the system. Therefore, in Reference 50, a new vector control method in dq reference frame is presented for the rectifier part and a feed forward and feedback control loop for dual active bridge converter voltage is proposed. With this control method, in addition to balancing the voltage of DC links on both sides, it is possible to stabilize the power factor at 1 and the ability of bidirectional power transmission. Another disadvantage of DC link voltage imbalance is that it can increase the current and voltage stress for power switches.51 In order to tackle this problem, a voltage and power control algorithm is provided for the dq reference frame of the system. In this way, the problem of voltage balance and transmission power of each module is solved. Another algorithm for SST control is called multi-objective modulated predictive control.52 This method, while increasing the response speed of the system, solves the problems of the conventional predictive control method in creating voltage harmonic spectra and operating at very high frequencies. Another simple method is the phase shift control method. In this method, a phase shift is applied between the primary and secondary voltages of the high-frequency transformer (HFT). This provides a simple method to control the magnitude and direction of power flow in the system. However, this method can cause to the reverse power in the structure when the power factor is not unit or in low load conditions. Therefore, in Reference 53, in order to improve the efficiency of the dual active bridge structure and reduce the reverse power in the SST structure, an extended phase shift control method is presented. In this method, an internal control loop is added to apply the phase shift ratio between the gate signals of the convertor diagonal switches. By doing this, while reducing the stress caused by the reverse power flow on the power switches, it increases the flexibility of the system against loading. In Reference 54, a control method for each of the SST series-parallel modules in PV application is provided. Due to the wide power range and voltage of PV panels, the efficiency of bridge dual active converters will be greatly affected. This case is considered by modeling the system in the frequency domain, the effect of the fundamental frequency, and the harmonics frequency in the control process. The proposed method eliminates the reactive current flowing due to the different orders harmonics by controlling the phase shift of duty-cycles of the H bridge converter switches on both sides. This will improve the overall efficiency of the system. Another problem with sudden system load changes is the vast voltage changes in the DC link of the three-stage SST structure. The feed forward control method is provided to reduce voltage changes and its transient time. The conventional feed forward control method will not be possible without a separate (additional) flow sensor. Therefore, in Reference 55, a feed forward control scheme is proposed to compensate for energy changes of inductors and another feed forward scheme is proposed to control the voltage of the rectifier controller section. Both control loops improve the dynamic voltage conditions of the DC voltage of the system. In Reference 56, the structure of three-stage SST with 7-level series arrangement for rectifier is investigated. In this structure, in order to have stable operation of the system, DC voltage balance will be essential. The use of PI linear controllers, while not responding quickly enough, can also lead to saturation. This introduces a new technique of 3D space vector modulation with the ability to balance voltage. This method will be able to quickly regulate the voltage in all operating conditions while balancing the DC link voltage. 5 COMPARISON OF CONVENTIONAL TRANSFORMERS AND SSTS Conventional transformers provide a cheap and efficient method to convert voltage and insulation levels. Some advantages of these transformers are: Simple structure High reliability High efficiency Despite the widespread use of this equipment in the power system, it has the following disadvantages: High volume and high weight Oil use and related environmental issues Core saturation and harmonic production Any unwanted changes in the input directly affect the output voltage. Output current affects input current. In common, transformers are outlined to have most extreme productivity around the full