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This paper presents the control and hardware design for a Hybrid Solid-State Transformer (HSST) applied to modern distribution systems. The HSST combines the advantages of conventional transformers, such as high efficiency and low cost, with those of Solid-State Transformers (SST), such as multifunctionality and fast dynamic response. Real-time simulation using a Typhoon HIL404 device is performed to validate the proposed multifunctional control strategy. The corresponding results validate the HSST's capability to provide, while only processing partial system power, a regulated output voltage with grid voltage harmonic suppression and, at the same time, current load reactive and harmonic compensation to ensure a high-power factor and improved power quality to the grid. [ABSTRACT FROM AUTHOR]

The large size of the sub-module (SM) capacitor is a typical problem in traditional modular multilevel converter-type solid-state transformers (MMC-SSTs). The MMC-SST based on high-frequency link interconnection is an effective solution for achieving lightweight capacitance. This structure can help to eliminate the symmetric SM fluctuating power, thereby reducing the SM capacitance. In a three-phase interconnected MMC-SST with low capacitance, potential risks may arise during transient processes, especially in cases of three-phase voltage asymmetry, such as large fluctuations in the SM voltage and unstable DC bus voltage. Aiming to solve this problem, this article re-analyzes the internal power characteristics of the MMC-SST under asymmetric operation and re-derives the SM capacitance constraint suitable for different degrees of three-phase voltage asymmetry. The new SM capacitance constraint enhances the asymmetric voltage ride-through capability of the MMC-SST. The new capacitance constraint is higher than that in symmetric operation, but it still has significant advantages in capacitance compared with the traditional MMC-SST. [ABSTRACT FROM AUTHOR]

Abstract This paper proposes an iteration-based algorithm for the optimum design of a high frequency transformer for solid-state transformer (SST) applications. This algorithm minimizes the total owning cost (TOC) of a distribution type solid-state transformer. The unique features of this algorithm compared with the available algorithms in the literature are as follows: it iterates eight design variables, four constraints are defined for selecting the valid designs, and it works with different core materials and AC test voltages. The algorithm uses various user-defined data inputs to calculate loss capitalization values for TOC calculation. In every iteration, TOC is estimated, and calculated values of design constraints are compared with their threshold limits. A case study is conducted on a high-frequency transformer (HFT) incorporated in 1000-kVA, 11-kV/415-V, Dyn11 three-phase wound core SST. This is to determine the optimum design parameters. In this case study, the algorithm was iterated with 2,100,000 design data inputs, generating 258,272 designs that satisfied all design constraints. The optimum design with minimum TOC is selected from the generated 258,272 designs. The optimum design is validated using finite element analysis in ANSYS software. Comparing the results of both analyses, the deviation is less than 5%. Hence, the algorithm’s reliability is proved.

In this paper, a new linear-based closed-loop control method for a Solid-State Transformer (SST) has been proposed. In this new control method, individual current and voltage loops for each of the power conversion stages (AC-DC, DC-DC, DC-AC) are implemented. The feedback between the input and output control signals for each loop is achieved through the voltage on the DC link capacitors and the current transferred between the converters. This enables the SST to be controlled easily in a linear-based closed-loop manner without the need for complex computations. In order to evaluate the performance analysis of the proposed control system, a simulation of an SST with approximately 10 kVA apparent power was performed. Based on the obtained simulation results, the response time of the proposed control method for dynamic load variations was proved to be in the range of 40 milliseconds, and it has been observed that this method allows electrical power to be transferred from the load to the grid. The power factor value of SST under inductive load is measured to be approximately 99%, and the overall system efficiency is 96% and above, indicating that this proposed new control method has very high performance. [ABSTRACT FROM AUTHOR]

This paper proposes an iteration-based algorithm for the optimum design of a high frequency transformer for solid-state transformer (SST) applications. This algorithm minimizes the total owning cost (TOC) of a distribution type solid-state transformer. The unique features of this algorithm compared with the available algorithms in the literature are as follows: it iterates eight design variables, four constraints are defined for selecting the valid designs, and it works with different core materials and AC test voltages. The algorithm uses various user-defined data inputs to calculate loss capitalization values for TOC calculation. In every iteration, TOC is estimated, and calculated values of design constraints are compared with their threshold limits. A case study is conducted on a high-frequency transformer (HFT) incorporated in 1000-kVA, 11-kV/415-V, Dyn11 three-phase wound core SST. This is to determine the optimum design parameters. In this case study, the algorithm was iterated with 2,100,000 design data inputs, generating 258,272 designs that satisfied all design constraints. The optimum design with minimum TOC is selected from the generated 258,272 designs. The optimum design is validated using finite element analysis in ANSYS software. Comparing the results of both analyses, the deviation is less than 5%. Hence, the algorithm's reliability is proved. [ABSTRACT FROM AUTHOR]

The use of power converters in modern power systems is making the system more active. To enhance the future power grid's reliability, efficiency, interactivity, and security, digital controllers and power electronic circuits are being employed. One critical component of the power system is the 50‐Hz transformer, which is currently robust. However, there is growing interest in solid‐state transformers (SSTs) or power electronic transformers (PETs) as potential alternatives to conventional transformers. These alternatives are particularly relevant for the future grid, which will have a high penetration of renewable energy sources, electric vehicles, and nonlinear loads. To analyze the impact of faults in the power network, it is necessary to establish an accurate circuit model for SSTs, which has not been adequately addressed in the existing literature. This research paper proposes an averaged representation of SSTs and compares the results of switched and averaged models under varying load conditions for the designed SST. Additionally, an equivalent circuit model is developed for DC fault analysis using transformation techniques and a transformer analogy to shift parameters to the DC fault location. The proposed circuit equivalent model is compared with the switched model, offering accurate and efficient results for a pole‐to‐pole and pole‐to‐ground DC fault. [ABSTRACT FROM AUTHOR]

During the energy transition, new types of electrical equipment, especially power electronic devices, are proposed to increase the flexibility of electricity distribution grids. One type is the solid-state transformer (SST), which offers excellent possibilities to improve the voltage quality in electricity distribution grids and integrate hybrid AC/DC grids. This paper compares SST to conventional copper-based power transformers (CPT) with and without an on-load tap changer (OLTC) and with additional downstream converters. For this purpose, a corresponding electricity distribution grid is set up in the power system analysis tool DIgSILENT PowerFactory 2022. A DC generator like a photovoltaic system, a DC load like an electric vehicle fast charging station, and an AC load are connected. Based on load flow simulations, the four power transformers are compared concerning voltage stability during a generator-based and a load-based scenario. The results of load flow simulations show that SSTs are most valuable when additional generators and loads are to be connected to the infrastructure, which would overload the existing grid equipment. The efficiency of using SSTs also depends on the parameters of the electrical grid, especially the lengths of the low-voltage (LV) lines. In addition, a flowchart-based decision process is proposed to support the decision-making process for the appropriate power transformer from an electrical perspective. Beyond these electrical properties, an evaluation matrix lists other relevant criteria like characteristics of the installation site, noise level, expected lifetime, and economic criteria that must be considered. [ABSTRACT FROM AUTHOR]

Power grids are currently undergoing a significant transition to enhance operational resilience and elevate power quality issues, aiming to achieve universal access to electricity. In the last few decades, the energy sector has witnessed substantial shifts toward modernizing distribution systems by integrating innovative technologies. Among the innovations, the solid-state transformer (SST) is referred to as a promising technology due to its flexible power control (better reliability) and high efficacy (by decreasing losses) compared with traditional transformers. The design of SST has combined three-stage converters, i.e., the input, isolation, and output stages. The key objective of this design is to implement a modern power distribution system to make it a more intelligent and reliable device in practice. As the power converters are used in SST, they exhibit non-linear behavior and can introduce high-frequency components, making stability more challenging for the system. Besides, the stability issue can be even more complicated by integrating the distributed energy resources into the distribution system. Thus, the stability of SST must be measured prior to /during the design. To determine stability, state-space modeling, and its controller design are important, which this paper explains in detail. Indeed, the system's stability is measured through the controllability and observability test. Further, the stability analysis is performed using frequency and time-domain diagrams: the Bode plot, Nyquist plot, Nichols chart, Root locus, pole-zero plot, and Eigen plot. Finally, the SST Simulink model is tested and validated through real-time digital simulation using the OPALRT simulator to show its effectiveness and applicability. The stability performance of the proposed SST is evaluated and shows the effectiveness of the controller design of each converter circuit. [ABSTRACT FROM AUTHOR]

In the production of green hydrogen, electrolyzers draw power from renewable energy sources. In this paper, the design of Solid State Transformer (SST) for large-scale H2 electrolyzers is benchmarked. The three most promising topologies are chosen for design and comparison, including Modular Multi-level Converter (MMC) based SST, Modular Multi-level Resonant (MMR) based SST, and Input-Series-Output-Parallel (ISOP) based SST. The distance between converter towers for insulation and maintenance, the insulation system of the transformer, and the cooling system are designed with practical considerations in order to have an accurate estimation of the volume and weight of the SST. Losses in the switches are calculated based on equations, and losses in passive components are calculated based on FEM simulation. The operating frequency for each topology is optimized to minimize loss, weight, and volume. The best of each topology is then compared with each other to identify the most suitable one for large-scale H2 electrolyzers.

The safe operation of high-power solid-state transformers (SSTs) in medium-voltage grid-connected applications relies on medium-frequency transformers (MFTs). In MFTs, hybrid foil-litz windings present a cost-effective alternative to litz wire, since foil is employed as the main conductor. This innovative topology exhibits lower ohmic losses than conventional foil windings. Besides, the dielectric stress in the insulation is mitigated, facilitating a more compact design. This paper presents the experimental validation of a ${166}\; \mathrm{k}\mathrm{W}$ MFT prototype rated for the $ {17.5}\;\mathrm{k}\mathrm{V}$ AC/$ {26.3}\; \mathrm{k}\mathrm{V}$ DC insulation class. The MFT features hybrid foil-litz windings insulated with ester liquid. The design of the active parts, the insulation and the cooling system are described. The performances of alternative design concepts is evaluated. The power rating of the MFT prototype is verified experimentally by the temperature rise test at full load, operating the MFT within the cell of an SST. The insulation withstand is confirmed through dielectric tests, including AC and DC overvoltage tests up to $ {54}\; \mathrm{k}\mathrm{V}$ peak, and the lightning impulse up to $ {95}\; \mathrm{k}\mathrm{V}$.

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This article proposes a novel Solid-State Transformer (SST) power flow (PF) model which is implemented with Modified Augmented Nodal Analysis (MANA) formulation using Newton-Raphson (NR) algorithm. Any network topology can be handled by the MANA formulation, and it can be easily and methodically expanded to include other components, like SST. Since various types of buses can have various type converters and SSTs, the sum of line flows are used instead of classical bus power injections equations. The proposed approach is coded in MATLAB and tested on an IEEE 33 bus distribution test feeder. The results are compared with Holomorphic Embedding PF method (HE-PFM) and with existing methodology in literature. In line with the findings, the proposed approach achieves accurate representation of SST, better convergence characteristics with a faster simulation speed compared to HE-PFM.

During the energy transition, new types of electrical equipment, especially power electronic devices, are proposed to increase the flexibility of electricity distribution grids. One type is the solid-state transformer (SST), which offers excellent possibilities to improve the voltage quality in electricity distribution grids and integrate hybrid AC/DC grids. This paper compares SST to conventional copper-based power transformers (CPT) with and without an on-load tap changer (OLTC) and with additional downstream converters. For this purpose, a corresponding electricity distribution grid is set up in the power system analysis tool DIgSILENT PowerFactory 2022. A DC generator like a photovoltaic system, a DC load like an electric vehicle fast charging station, and an AC load are connected. Based on load flow simulations, the four power transformers are compared concerning voltage stability during a generator-based and a load-based scenario. The results of load flow simulations show that SSTs are most valuable when additional generators and loads are to be connected to the infrastructure, which would overload the existing grid equipment. The efficiency of using SSTs also depends on the parameters of the electrical grid, especially the lengths of the low-voltage (LV) lines. In addition, a flowchart-based decision process is proposed to support the decision-making process for the appropriate power transformer from an electrical perspective. Beyond these electrical properties, an evaluation matrix lists other relevant criteria like characteristics of the installation site, noise level, expected lifetime, and economic criteria that must be considered.

Electrical power grids are changing with a focus on ensuring energy sustainability and enhanced power quality for all sectors. Over the last few decades, there has been a change from a centralized to a decentralized paradigm, which is the consequence of a large-scale incorporation of new electrical technologies and resultant equipment. Considering the foreseeable continuation of changes in electrical power grids, a direction rooted in power electronics with a focus on hybrid AC/DC grids, including the support of solid-state transformers and unified systems, is presented in this paper. Converging on hybrid AC/DC grids, DC grids (structured as unipolar and bipolar) and coupled and decoupled AC configurations are analyzed. On the other hand, in the context of solid-state transformers, feasible structures are analyzed, including the establishment of hybrid AC/DC grids, and the assessment of gains for boosting power quality is presented. Unified power electronics systems are also of fundamental importance when contextualized within the framework of future power grids, presenting higher efficiency, lower power stages, and the possibility of multiple operations to support the main AC grid. In this paper, such subjects are discussed and contextualized within the framework of future power grids, encompassing highly important and modern structures and their associated challenges. Various situations are characterized, revealing a gradual integration of the cited technologies for future power grids, which are also known as smart grids. [ABSTRACT FROM AUTHOR]

This paper focuses on the study of the high frequency transformer incorporated in solid- state transformers, specifically on the development of the steps that enable the design of an optimized high frequency transformer and its equivalent model based on the desired characteristics. The impact of operating a transformer at high frequency and the respective solutions that allow this impact to be reduced are analyzed, alongside the numerous advantages that the utilization of these transformers has over traditional 50/60 Hz transformers. Furthermore, the power scheme of the solid-state transformer is outlined, focusing on the power converters, which are immediately before and after the high frequency transformer (HFT). We also investigate a control technique that allows for correct operation and the existence of power bidirectionality. In a novel approach, this paper demonstrates the systematic steps for designing an HFT according to the desired specifications of each given project, helping students and engineers achieve their objectives in power-electronic applications. Moreover, this paper aims at increasing the knowledge of this area of power electronics and facilitating the development of new topologies with high power density, which are very important to the integration of renewable power sources and other applications. Finally, a simulation is presented to validate a high frequency transformer and its control technique. [ABSTRACT FROM AUTHOR]

This paper provides an overview of an early attempt at developing a simulation model on a solid-state transformer (SST) based on input-serial and output-parallel (ISOP) topology. The proposed SST is designed as a base for a smart grid (SG). The paper provides a theoretical review of the power converters under consideration, as well as their control techniques. Further, the paper presents a simulation model of the proposed concept with a PLECS circuit simulator. The proposed simulation model examines bidirectional energy flow control between the medium-voltage AC grid and DC smart grid, while evaluating power flow efficiency and qualitative indicators of the AC grid. After the completion of design verification and electrical properties analysis by the PLECS simulation models, the synthesis offers recommendations on the optimal layout of the proposed SST topology for smart grid application. [ABSTRACT FROM AUTHOR]

Summary: The modular multilevel converter presents itself as a promising topology in medium voltage and high power applications due to its modular characteristic and flexibility in applications where DC‐AC and AC‐DC power conversion are required. Despite this, applications with electric traction present this topology in the AC‐AC conversion due to the converter at the input of a solid‐state transformer and the power system's single‐phase characteristic at the converter's input. One of the main problems characteristic of this topology is the harmonic content present in the medium frequency converter output voltage due to its square waveform. Thus, the harmonic content in this voltage is high, increasing the losses and volume in the core of the medium frequency transformer. This paper presents a multilevel modulation scheme using two frequency components in the modulating wave to reduce the harmonic content in the medium frequency converter output voltage. With the converter branch inductors, it is possible to verify the filtering of the output voltage and thus reduce the harmonic content of this voltage. A low power and voltage (1 kW/500 V) of the AC‐AC modular multilevel converter is implemented, and simulation results show some operating characteristics of the modulation scheme. Experimental results at low power and voltage (0.6 kW/208 V–125 V/1.2 kHz) present the operation of the converter and the characteristics proposed by the modulation scheme, where it is possible to verify the waveform of the sinusoidal output voltage with lower harmonic content. [ABSTRACT FROM AUTHOR]

As the core equipment of smart grid, solid-state transformer (SST) needs to have good power quality regulation capability under grid side faults and load side faults. To ensure that the high-voltage side converter works at unity power factor and smooth DC voltage. At the same time, the three-phase output voltages need to have high steady-state accuracy and strong anti-interference capability. In this paper, an improved model predictive control scheme was proposed. First, a theoretical modeling analysis of the high and low voltage side of a solid-state transformer was performed. Then, power and voltage prediction models are developed for the high-voltage side converter and low-voltage side converter of the SST, respectively. Three voltage vectors are selected for prediction by the cost function, and the equivalent voltage vector in the two-phase stationary coordinate system is synthesized and modulated to control the converter. Finally, simulation was conducted to compare and analyze various operating conditions of SST under different control schemes. The control effect of solid-state transformer under improved model predictive control is better than traditional finite control set model predictive control and proportional integral control, which can better regulate power quality.

This article presents a phase-shift control method for a converter containing a multi-winding transformer. The control strategy is developed from a power flow analysis of the converter and uses the derived power flow equations to supply constant power at a load port while the second load port experiences large load changes. This control method prevents the constant load port from experiencing transient behavior caused by magnetic coupling with the other load port. The approach is demonstrated with an experiment using a controller within a control hardware-in-the-loop set-up and then a full experimental setup within a lower power test bed. By using control to mitigate the effects of mutual flux within a multi-winding transformer, the load ports of the converter are effectively decoupled. The analytical procedure described in this article can also be applied to other converter topologies containing multi-winding transformers to achieve similar decoupled load performance, as demonstrated in the triple active bridge converter test bed.

Medium-frequency transformers (MFTs) are key components in solid-state transformers, where they provide galvanic isolation and a certain voltage transfer ratio between a MV grid and a LV bus. Typically, MFTs are operated in the kilohertz range, which results in a significantly reduced volume and material usage compared to 50/60 Hz transformers. Foil conductor is an attractive solution for improving the filling factor and the cost-effectiveness of MFTs, however, the insulation clearances increase the ohmic losses due to the current crowding effect; besides, the electric field hotspots at the winding corners require shielding by equipotential rings. In this work, a new hybrid topology is presented, where the first and last turn of the foil winding package is replaced by turns of litz wire connected in series, one above and one below the foil winding, respectively, such that they act as equipotential shielding rings; besides, the current in the litz turns deflects the magnetic field lines away from the foil edges, decreasing the losses due to current crowding. The effect of the litz rings on the losses is modeled and measured on a hybrid and a standard foil winding prototype; a loss decrease higher than 15% is observed. The verified model is used to compare the conductor and footprint saving between windings designed with litz rings, and with standard equipotential shielding rings; in this respect, a minor amount of litz wire allows reducing losses up to 30%, whereas the conductor utilization and winding height can be decreased more than 20%, improving the power density of the MFT.

This paper introduces an enhanced solid-state transformer topology for a medium-voltage (MV) utility grid. The main objective of the current study is to develop an improved flyback converter equipped with a quasi z-source converter (qZ_iFC) having a high-voltage conversion capability for the integration of low-input voltage to the DC link of an MV modular multilevel converter (MMC). The system integrates the quasi z-source and flyback converters by operating their existing switches complementary. Furthermore, the high-gain qZ_iFC allows for a reduction in the rated voltage of the input, as well as the use of a high-frequency transformer (HFT) with a unity turns ratio that provides galvanic isolation between the input and the output ports. Thus, using an HFT just for isolation purposes without voltage gain improves the system efficiency. In addition, a controller for (i) qZ_iFC which is regulating complementary switches to prevent the shoot-through current from reaching the HFT resulting in saturation; and (ii) a controller for MMC to produce MV-level AC voltage for loads are suggested. The performance of the proposed system was evaluated for several operating conditions. Results show that the proposed SST smoothly performs the power flow between the ports during steady-state and transient conditions. The power flow capabilities and efficiency values validate the viability and effectiveness of the proposed system. [ABSTRACT FROM AUTHOR]

Power grids are currently undergoing a significant transition to enhance operational resilience and elevate power quality issues, aiming to achieve universal access to electricity. In the last few decades, the energy sector has witnessed substantial shifts toward modernizing distribution systems by integrating innovative technologies. Among the innovations, the solid-state transformer (SST) is referred to as a promising technology due to its flexible power control (better reliability) and high efficacy (by decreasing losses) compared with traditional transformers. The design of SST has combined three-stage converters, i.e., the input, isolation, and output stages. The key objective of this design is to implement a modern power distribution system to make it a more intelligent and reliable device in practice. As the power converters are used in SST, they exhibit non-linear behavior and can introduce high-frequency components, making stability more challenging for the system. Besides, the stability issue can be even more complicated by integrating the distributed energy resources into the distribution system. Thus, the stability of SST must be measured prior to /during the design. To determine stability, state-space modeling, and its controller design are important, which this paper explains in detail. Indeed, the system’s stability is measured through the controllability and observability test. Further, the stability analysis is performed using frequency and time-domain diagrams: the Bode plot, Nyquist plot, Nichols chart, Root locus, pole-zero plot, and Eigen plot. Finally, the SST Simulink model is tested and validated through real-time digital simulation using the OPALRT simulator to show its effectiveness and applicability. The stability performance of the proposed SST is evaluated and shows the effectiveness of the controller design of each converter circuit.

Voltage fluctuation and power mismatch which are caused by the decentralization, randomness, and intermittence of distributed energy resource along with load variability cause severe impacts on the security and quality of power network operation. Aiming at alleviating this issue, the structure of an AC/DC hybrid microgrid based on solid-state transformer is presented in this paper. A proper control coordination is developed to guarantee stable and reliable operation of the system. An energy management strategy is proposed to coordinate the power flow among the solid-state transformer, AC microgrid, DC microgrid and energy storage and to effectively suppress the fluctuation of the DC bus voltage. A novel adaptive droop control for the energy storage is proposed to prolong the supercapacitor life and achieve an optimum economic benefit to consumers. The droop coefficient is obtained by fuzzy logic controller of which the state of charge of the supercapacitor and unit-time electricity charge are assumed to be the input parameters. Simulation attest the feasibility of the proposed control coordination and energy management strategy.

Electrical power grids are changing with a focus on ensuring energy sustainability and enhanced power quality for all sectors. Over the last few decades, there has been a change from a centralized to a decentralized paradigm, which is the consequence of a large-scale incorporation of new electrical technologies and resultant equipment. Considering the foreseeable continuation of changes in electrical power grids, a direction rooted in power electronics with a focus on hybrid AC/DC grids, including the support of solid-state transformers and unified systems, is presented in this paper. Converging on hybrid AC/DC grids, DC grids (structured as unipolar and bipolar) and coupled and decoupled AC configurations are analyzed. On the other hand, in the context of solid-state transformers, feasible structures are analyzed, including the establishment of hybrid AC/DC grids, and the assessment of gains for boosting power quality is presented. Unified power electronics systems are also of fundamental importance when contextualized within the framework of future power grids, presenting higher efficiency, lower power stages, and the possibility of multiple operations to support the main AC grid. In this paper, such subjects are discussed and contextualized within the framework of future power grids, encompassing highly important and modern structures and their associated challenges. Various situations are characterized, revealing a gradual integration of the cited technologies for future power grids, which are also known as smart grids.

This paper focuses on the study of the high frequency transformer incorporated in solid- state transformers, specifically on the development of the steps that enable the design of an optimized high frequency transformer and its equivalent model based on the desired characteristics. The impact of operating a transformer at high frequency and the respective solutions that allow this impact to be reduced are analyzed, alongside the numerous advantages that the utilization of these transformers has over traditional 50/60 Hz transformers. Furthermore, the power scheme of the solid-state transformer is outlined, focusing on the power converters, which are immediately before and after the high frequency transformer (HFT). We also investigate a control technique that allows for correct operation and the existence of power bidirectionality. In a novel approach, this paper demonstrates the systematic steps for designing an HFT according to the desired specifications of each given project, helping students and engineers achieve their objectives in power-electronic applications. Moreover, this paper aims at increasing the knowledge of this area of power electronics and facilitating the development of new topologies with high power density, which are very important to the integration of renewable power sources and other applications. Finally, a simulation is presented to validate a high frequency transformer and its control technique.

The emergence of DC fast chargers for electric vehicle batteries (EVBs) has prompted the design of ad-hoc microgrids (MGs), in which the use of a solid-state transformer (SST) instead of a low-frequency service transformer can increase the efficiency and reduce the volume and weight of the MG electrical architecture. Mimicking a conventional gasoline station in terms of service duration and service simultaneity to several customers has led to the notion of ultra-fast chargers, in which the charging time is less than 10 min and the MG power is higher than 350 kW. This survey reviews the state-of-the-art of DC ultra-fast charging stations, SST transformers, and DC ultra-fast charging stations based on SST. Ultra-fast charging definition and its requirements are analyzed, and SST characteristics and applications together with the configuration of power electronic converters in SST-based ultra-fast charging stations are described. A new classification of topologies for DC SST-based ultra-fast charging stations is proposed considering input power, delta/wye connections, number of output ports, and power electronic converters. More than 250 published papers from the recent literature have been reviewed to identify the common understandings, practical implementation challenges, and research opportunities in the application of DC ultra-fast charging in EVs. In particular, the works published over the last three years about SST-based DC ultra-fast charging have been reviewed. [ABSTRACT FROM AUTHOR]

Isolated AC-DC solid-state transformers widely use the front and rear multi-stage cascade structure of the bidirectional converter. Due to the difference in the control bandwidth of the front and rear stages, the transmission power of the two stages is different. When the power suddenly changes, the difference will seriously threaten the operational safety of this type of SST. In view of this problem, this study analyzes the mechanism of the power difference between the stages, summarizes various control strategies to reduce the power fluctuations between the stages, and derives the mathematical model of the load disturbance on the voltage between the two stages under different strategies. Using MATLAB, the closed-loop Bode plots of various typical control strategies and the corresponding step responses in the time domain are obtained, and the advantages and disadvantages of various strategies are compared horizontally. Finally, simulation and experiment results verify the correctness and validity of the analysis and comparison results. [ABSTRACT FROM AUTHOR]

The battle of currents between AC and DC reignited as a result of the development in the field of power electronics. The efficiency of DC distribution systems is highly dependent on the efficiency of distribution converter, which calls for optimized schemes for the efficiency enhancement of distribution converters. Modular solid-state transformers (SSTs) play a vital role in DC distribution networks and renewable energy systems (RES). This paper deals with efficiency-based load distribution for solid-state transformers (SSTs) in DC distribution networks. The aim is to achieve a set of minimum inputs that are consistent with the output while considering the constraints and efficiency. As the main feature of modularity is associated with a three-stage structure of SSTs, this modular structure is optimized using ant lion optimizer (ALO) and validated by applying it to the EIA (Energy Information Agency) DC distribution network which contains SSTs. In the DC distribution grid, modular SSTs provide the promising conversion of DC power from medium voltage to lower DC range (400 V). The proposed algorithm is simulated in MATLAB and also compared with two other metaheuristic algorithms. The obtained results prove that the proposed method can significantly reduce the input requirements for producing the same output while satisfying the specified constraints. [ABSTRACT FROM AUTHOR]

Operation of a three-phase three-stage solid-state transformer under variation of load is studied. A schematic diagram of the transformer is presented, and the principle of its operation and the scope of application are described. Control systems of each unit that enable switching operating modes are considered. The input inverter enables adjustment of the voltage of the first dc insert and an increase in the power factor; the double active bridge regulates the voltage at the second insert, and the output inverter controls the amplitude, frequency, and phases of the output three-phase voltage. Transformer modes have been numerically simulated using the MATLAB/Simulink package. The current and voltage waveforms confirming the correctness of the proposed approach are presented. [ABSTRACT FROM AUTHOR]

The interest of moving towards more sophisticated power grids and the possibility of having more efficient and reliable power semiconductor devices have enabled the introduction of new concepts in power systems, such as solid-state transformers (SSTs). SSTs are considered to be one of the most relevant elements in the future of smart grids since their functionalities allow for the appropriate integration of distributed generation, renewables, energy storage systems, among others, with traditional power grids. One of the main stages of an SST (a DC-DC stage) includes a dual active bridge (DAB) that enables bidirectional power flow, which is regarded as one of the main advantages over traditional power transformers. This work focuses on proposing a current control strategy based on PI passivity in order to regulate the power flow in a DAB. The proposed controller guarantees the system's stability in a closed loop, keeping its passive properties. In addition, it preserves the simplicity of a PI controller, with high performance and robustness, and it does not depend on the converter's parameters. The performance and effectiveness of the proposed controller is validated through time-domain simulations and experimental results under different power flow conditions, including bidirectional power flow. The results show that the proposed controller has a better performance than classical PI controllers.

This paper provides an overview of an early attempt at developing a simulation model on a solid-state transformer (SST) based on input-serial and output-parallel (ISOP) topology. The proposed SST is designed as a base for a smart grid (SG). The paper provides a theoretical review of the power converters under consideration, as well as their control techniques. Further, the paper presents a simulation model of the proposed concept with a PLECS circuit simulator. The proposed simulation model examines bidirectional energy flow control between the medium-voltage AC grid and DC smart grid, while evaluating power flow efficiency and qualitative indicators of the AC grid. After the completion of design verification and electrical properties analysis by the PLECS simulation models, the synthesis offers recommendations on the optimal layout of the proposed SST topology for smart grid application.

This paper introduces an enhanced solid-state transformer topology for a medium-voltage (MV) utility grid. The main objective of the current study is to develop an improved flyback converter equipped with a quasi z-source converter (qZ_iFC) having a high-voltage conversion capability for the integration of low-input voltage to the DC link of an MV modular multilevel converter (MMC). The system integrates the quasi z-source and flyback converters by operating their existing switches complementary. Furthermore, the high-gain qZ_iFC allows for a reduction in the rated voltage of the input, as well as the use of a high-frequency transformer (HFT) with a unity turns ratio that provides galvanic isolation between the input and the output ports. Thus, using an HFT just for isolation purposes without voltage gain improves the system efficiency. In addition, a controller for (i) qZ_iFC which is regulating complementary switches to prevent the shoot-through current from reaching the HFT resulting in saturation; and (ii) a controller for MMC to produce MV-level AC voltage for loads are suggested. The performance of the proposed system was evaluated for several operating conditions. Results show that the proposed SST smoothly performs the power flow between the ports during steady-state and transient conditions. The power flow capabilities and efficiency values validate the viability and effectiveness of the proposed system.

The use of high-voltage silicon carbide (SiC) devices can eliminate multilevel and cascaded converters and their complicated control strategies, making converter systems simple and reliable. A three-phase two-level voltage-source converter system serves as a simple converter system for interfacing any dc source to a three-phase grid. However, when the high-voltage devices are used in two-level converters, they are exposed to a high-voltage peak stress and a high $dv/dt$ (up to 100 kV/ $\mu$ s). Operating these semiconductor devices at these stress levels requires careful design not only of the semiconductor die and the module, but also of the gate drivers, busbars, and passive filters. This article demonstrates the operation of 10-kV SiC mosfets and discusses the design considerations, advantages, and challenges associated with the operation of the three-phase two-level medium-voltage converter system used as the active-front-end converter system. Reliable operation of the medium-voltage converter system requires the development of reliable high-voltage modules and auxiliary parts, such as gate drivers, busbars, inductors, voltage and current sensors, and proper design of the controller system. Successful tests demonstrating continuous field operation of the medium-voltage active-front-end converter at a nominal rating of 7.2-kV dc-link voltage is demonstrated for the first time in the literature. The results indicate that these devices can accelerate the growth and deployment of medium-voltage SiC devices for field operation, as demonstrated by the operation inside the mobile container. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive review of multi-port power electronics converters used for application in AC, DC, or hybrid distribution systems in an Internet of Energy scenario. In particular, multi-port solid-state transformer (SST) topologies have been addressed and classified according to their isolation capabilities and their conversion stages configurations. Non-conventional configurations have been considered. A comparison of the most relevant features and design specifications between popular topologies has been provided through a comprehensive and effective table. Potential benefits of SSTs in distribution applications have been highlighted even with reference to a network active nodes usage. This review also highlights standards and technical regulations in force for connecting SSTs to the electrical distribution system. Finally, two case studies of multi-port topologies have been presented and discussed. The first one is an isolated multi-port bidirectional dual active bridge DC-DC converter useful in fast-charging applications. The second case of study deals with a three-port AC-AC multi-level power converter in H-Bridge configuration able to replicate a network active node and capable of routing and controlling energy under different operating conditions. [ABSTRACT FROM AUTHOR]

Solid-state transformers (SSTs) enable electric energy distribution with higher flexibility, sustainability and efficiency, and the AC/DC hybrid microgrid (MG) is one of the representative applications. However, the design and analysis of SSTs and MGs were limited by lack of fast simulation tools that can accurately and efficiently handle systems with massive number of switching devices. This article demonstrates how the discrete-state event-driven (DSED) approach allows an accurate and faster simulation of the SSTs and MGs than the state of the art and benefit the practical design. The numerical prototyping of a hybrid MG composed of a megawatt SST together with multiple converters (e.g. PVs, EVs and batteries) is presented in this article. The performance is compared with existing simulation approaches. Various scenarios are investigated, including the grid-connected mode, the islanded mode, and the mode transition considering the communication delay. A time cost of several hours is decreased to tens of seconds with same accuracy using DSED, enabling faster simulations for complicated tasks. It is discussed how the speed and accuracy benefit the practical SST and MG design.

Renewable energy-based grid integrations in distribution systems cause changes in the energy structure of the grid such as load flow, short circuit and protection coordination. Today, conventional transformers are insufficient in meeting the needs of loads with different profiles and in connection with renewable energy sources such as solar and wind to the grid. Additional mechanisms are needed to overcome power quality problems, especially voltage rise and fall, fraying, or harmonics. With the advances in semiconductor technology, power electronics has emerged as a promising solution to dealing with the problems of complex power systems. Solid-state transformer (SST) is a novel technology that can affect developments in many fields such as renewable energy systems, smart grids, distribution tail systems for flexible power conversion between medium voltage distribution and low-voltage customer side. Compared to the conventional transformers, SSTs provide reactive power compensation, simple connection of renewable energy sources and battery systems, power factor correction, galvanic isolation, smart protection, harmonic control and frequency conversion. As a result, the integration of renewable energy plants to the electrical grid creates big challenges in terms of harmonics, power flow control, power factor correction, isolation and smart protection. Thus, SSTs will play a significant role in future grid topologies. In this study, modeling and analysis of a solid-state transformer for a grid-connected solar PV plant are carried out with MATLAB/Simulink. The power analyses of the proposed system are examined thanks to the simulation study. This analysis would be a significant contribution to the development and implementation of renewable energy-based applications of the SSTs.

Due to simplicity, efficiency, and the ability to accommodate energy storage devices, DC distribution networks have been seen as an optimal alternative to AC distribution networks, especially aboard future electric ships. The emerging distribution DC system entails new control and management techniques. Therefore, an integrated DC power distribution network aboard an electric ship is selected as the case study in this paper. To meet the requirements of such a large-scale mobile power system, a multiport solid-state transformer (SST) based on silicon carbide (SiC) switches/MOSFETs is proposed. Thus, the system embodiment can significantly be reduced. Moreover, at the DC distribution level, a high penetration of renewable generation with energy storage is allowed and a six-phase asymmetrical induction machine (IM) can directly be integrated. Simulations have been conducted based on a 2 MW shipboard distribution network. The effects of the propulsion system dynamics on the SST are highlighted as well. Finally, a 2 kW lab-scale prototype has been implemented to validate the theoretical findings. [ABSTRACT FROM AUTHOR]

The paper proposes an optimal planning strategy for an active distribution network (ADN) with wind energy-based distributed generation (WDG) by considering the twofold reactive power compensation (TRPC) feature of a three-stage type solid-state transformer (SST). A model for wind speed and load demand estimation for each hour of the day is formulated by using the partition around medoids (PAM) technique over an annual database of load demand and wind speed. A multi-objective mixed-integer non-linear programming (MINLP) approach for optimally locating and sizing the SST installations in the radial distribution network (RDN) is presented to address the dual objectives of voltage profile improvement (VPI) and energy loss reduction (ELR). Along with the SST parameters, the locations and number of wind turbine (WT) units to be integrated into the system are treated as optimization variables. Furthermore, the distribution power flow accounts for the distribution transformer’s (DT) operating losses as well as the approximate losses of the SST. The presented approach is tested on a 33-bus RDN and the results are reported for multiple case studies. The programming was developed in the MATLAB R2020a environment and the paretosearch algorithm (PSA) is used to address the MINLP problem. Other standard multi-objective optimization algorithms, including multi-objective multi-verse optimization (MOMVO), non-dominated sorting genetic algorithm (NSGA-II), and multi-objective salp swarm algorithm (MSSA) are used to compare the performance of the proposed method. The outcomes of the peak hour evaluation, with a 20% over-rating, show the active and reactive power demand on the MV side of the substation to have decreased by 56.3% and 30.1% respectively. The active line loss has been lowered by 76.95% and the reactive line loss has decreased by 76.14%. The absolute minimum voltage has increased by 7.314%. Furthermore, according to the annual technical evaluation, there was a 29.42% reduction in active energy served by the DTs and SSTs. [ABSTRACT FROM AUTHOR]

Three-phase unbalance issue can cause adverse impacts on the power quality of the solid-state transformer (SST) based AC/DC hybrid microgrid. In this paper, an unbalanced power control strategy for cascaded dual active bridge (DAB) converter is proposed. With the strategy, the transmitted power of each DAB module in each phase is evenly distributed in terms of the three-phase unbalance degree of the three-phase AC grid. Thus, the idea of unequal power sharing in cascaded DAB can be applied to mitigate the three-phase unbalance from the perspective of power control. Moreover, this paper proposed a three-phase unbalance power mitigation algorithm based on the unbalanced power control strategy for the cascaded DAB. The proposed strategy can help the SST realize the function of mitigating three-phase unbalance when it transmits energy to the DC microgrid. Thus, the proposed control strategy can mitigate the three-phase unbalance on the premise that the power demand of the system is met. Finally, the simulations and the experiment results have verified the effectiveness and correctness of the proposed control method.

The interaction of the smart transformer (ST) with the grid-converter-based distributed energy resources (DERs) could trigger instability. Recent research efforts have been made to study the stability issues caused by the resonances (e.g. LC or LCL filter resonances of DERs) in ST LV side grid. This paper studies the impact of the grid synchronization of the grid-converter-based DER on the stability of the ST as well as the grid. The equivalent admittance of the grid converter considering the effect of synchronous reference frame phase-locked loop (SRF-PLL) is developed and merged with the ST voltage control. Based on the generalized Nyquist criterion, the stability analysis of a ST-fed grid is carried out, showing that the PLL bandwidth is a key factor that determines the system stability. To address the stability issue, a stabilization method based on virtual impedance is proposed and can be seamlessly integrated with the ST voltage control. Simulation and experimental results are provided to validate the effectiveness of the analysis and the proposed control strategy.

Solid-state transformer (SST) is an emerging technology integrating with a transformer power electronics converters and control circuitry. This paper comprehensively reviews the SST topologies suitable for different voltage levels and with varied stages, their control operation, and different trends in applications. The paper discusses various SST configurations with their design and characteristics to convert the input to output under unipolar and bipolar operation. A comparison between the topologies, control operation and applications are included. Different control models and schemes are explained. Potential benefits of SST in many applications in terms of controllability and the synergy of AC and DC systems are highlighted to appreciate the importance of SST technologies. This review highlights many factors including existing issues and challenges and provides recommendations for the improvement of future SST configuration and development.

A solid-state transformer (SST) is being proposed for a distribution grid of 13.8 kV/380 V. The SST is based on the input-series output-parallel (ISOP) arrangement of twelve modules using the 1.2 kV SiC switches. The cost and losses analysis are discussed through a case study regarding the number of modules using the 1.2 kV and 1.7 kV SiC switches. The modules are composed of two stages: the first one is an AC-|AC| low-frequency rectifier and the second one is an |AC|-|AC| medium frequency Hybrid Switched-Capacitor LLC Series-Resonant Converter (HSCSRC). A single back-end |AC|-AC low-frequency inverter is employed to generate the low voltage AC output port of the SST. The switched capacitor ladder cell in the HSCSRC converter enables the reduction of the number of modules employed on the ISOP arrangement because the voltage stresses on the MV side's switches are halved, while the resonant stage increases the efficiency of the structure due to soft-switching on all the switches. Moreover, the front-end AC-|AC| and back-end |AC|-AC low-frequency converters enable the use of two-quadrant switches on the |AC|-|AC| HSCSRC converter, reducing the switch count on the medium frequency stage. The operating behavior and design methodology of the SST's modules are presented. A single module reduced scale prototype with the rated power of 1.67 kVA and an input voltage of 1.15 kV and an output voltage of 220 V is experimentally verified. The maximum efficiency is 97%.

An AC-AC converter with a high-frequency link employing an LLC resonant converter operating in the vicinity of the resonance frequency was studied, in which a single output power stage is used, formed by a high-frequency AC-AC converter employing four-quadrant switches. The topology, its operation and the modulation strategy are described. The high-frequency stage switches located on the primary side of the transformer operate with soft switching of the ZVS type, while the four-quadrant switches that form the output stage operate with soft switching of the ZCS type. Experimental data obtained with a 1.5 kW experimental prototype (input 220 VRMS, output 220 VRMS and switching frequency 40 kHz), which was designed, built and tested in the laboratory, are reported herein. This converter can be considered a candidate for the building block of medium voltage solid-state transformers (SST) for power distribution systems.

The smart transformer (ST), a solid-state-based transformer with control and communication functionalities, determines the voltage of the low voltage (LV) network. Due to the similar control purpose, the classic multiple feedback loop (multiloop) voltage control strategies widely utilized in the uninterruptible power supply (UPS) application, can be considered for the control of ST LV converter. However, a fundamental difference between ST and UPS is the presence in LV grid of distributed generation integrated through power electronics interfaces and the dimension of the supplied grid. Emerging issues like stability and harmonic oscillations challenge the safe operation of the ST-fed grid. In order to explore the problems associated with the grid-converter-based devices, the impacts of the grid converters on various multiloop voltage control strategies have been studied in this paper. Optimal multiloop strategy has been found to offer stable operation to a ST-fed distribution grid under various conditions. Case studies in a CIGRÉ benchmark grid and experimental results are provided to verify the correctness of the theoretical analyses.

With the high proportional renewable energy integration and rapid increase in the DC loads, such as the electric vehicle and distributed energy storage, the DC distribution system becomes a prospective solution for the urban power grid enhancement for its high-efficiency and eco-friendly nature. In most DC distribution systems, power interfaces are applied to connect low-voltage DC (LVDC) distribution systems with multiple medium-voltage (MV) systems in order to improve the operating reliability and economy. Compared to other types of multiports power interfaces, the three-port-isolated modular multilevel converter (I-M2C) has shown many advantages, including low cost, high power density, and low control complexity. However, the I-M2C cannot handle the power imbalance at the bipolar LVDC port like the other MMC-based three-port power interfaces, which limits the operating range and decreases the stability of the I-M2C in bipolar LVDC application. In order to solve the bipolar imbalance problems, a novel balance control method is proposed in this article. The proposed balance control method is based on symmetrical decomposition. By decoupling the MV power control and the LV bipolar power compensation control, the proposed method can eliminate the bipolar voltage deviation under different working conditions. The simulation results prove the validity and good control performance of the proposed method.

Summary: This paper aims to advance the recently progressing research on solid‐state transformers (SST) and examines the impact of uncertainty on load demand and distributed generation (DG) units while integrating SST into an active distribution network (ADN) in the presence of battery backed solar photovoltaic (BBSPV) and wind generation. The uncertainty related to solar irradiance, wind speeds, and load demands is modeled through kernel density estimation (KDE), generalized extreme value (GEV) distribution, and Gaussian distribution summed with the maximum likelihood estimation (MLE) approach respectively. The K‐medoid clustering procedure is utilized to group the load demand into multiple load levels, which helps in the intelligent scheduling of charging and discharging of battery energy storage systems (BESS). The distribution network planning process manages two conflicting criterion functions of real power loss (RPL) reduction and sum of square deviations of the expected voltage values, which are optimized utilizing the non‐dominated sorting genetic algorithm (NSGA‐II). The probabilistic power flow uses the direct load flow procedure utilizing bus injection to branch current (BIBC) matrix along with the forward sweep method to investigate each run of Monte Carlo simulation (MCS). Multiple case studies have been conducted on the IEEE 33 bus radial distribution network (RDN). The outcomes demonstrate the potential benefits of RPL reduction and voltage profile improvement (VPI) with increasing penetration levels of SST. [ABSTRACT FROM AUTHOR]