Solid State Transformer Research Articles

A Cascaded Modular Isolated Back-to-Back Solid State Transformer Scheme for AC/DC/AC Interconnection With Improved Performance and Simple Control

This paper proposes a cascaded modular isolated back-to-back solid-state transformer (CMIB-SST) for electrical isolated interconnection of two distribution AC grids, which consists of modular multilevel converter (MMC)-stage, cascaded H bridge (CHB)-stage, and six-terminal active bridge (STAB)-stage. A switching synchronization hybrid phase-shift modulation (SSHPSM) for the STAB-stage is utilized to suppress the low-frequency voltage ripple of submodule (SM) both in CHB-stage and MMC-stage to reduce the SM capacitance significantly with a simple control scheme of the CMIB-SST. The proposed CMIB-SST is friendly to the cost under the new capacitance constraint, and the big size electrolytic capacitor could be replaced by film capacitor, which is beneficial to security and life of the SST module. The voltage gain, mathematical model of the proposed SST is analyzed to illustrate the principle of SM low-frequency ripple-current decoupling with the SM capacitor and cancellation mechanism in STAB-stage. The corresponding transformer design are carried out under considering the parasitic leakage inductance value and the constraints of its magnetic fields, thermal, electric field strength and insulation. The scheme is evaluated by cost and volume of switches, SM capacitors and magnetic unit. Finally, the processed topology and SSHPSM method are verified by simulation and experimental results.

Solid-State Transformers of Smart High-Power Battery Charger for Electric Vehicles

There is no doubt that society will not turn back: most thermal vehicles will switch to electric ones, not only for cars but also for bigger vehicles. Fast chargers will become more and more essential parts of the global system, they will provide a charge control of batteries and galvanic insulation from the grid. At the present times, compact realizations are based on Solid-State Transformers (SSTs) operating at high frequencies with their well-known advantages and disadvantages, including power limits and reasonably high costs. The authors propose a medium-frequency transformer (MFT) structure using Grain Oriented Electrical Steel (GOES) wound cores and aluminum foil windings. Thanks to this solution, the cost of the device can be reduced. GOES has losses that increase with frequency but decrease with temperature; therefore, the authors propose a transformer structure allowing a hot core inside coils at usual temperatures. This idea is applied to a 400-kW compact battery charger connected to a DC smart grid.

A Study on the Distributed-Control Architecture of a DSP-Based Solid-State Transformer System with Implementation

This article proposes a Distributed-control Architecture (D-CA) and an operation sequence with start-up strategies for a Digital Signal Processor (DSP)-based Solid-State Transformer (SST). Although various control techniques for SSTs have been reported in earlier studies, there is still a lack of research covering comprehensive content, including hierarchical control architectures and operation sequences with start-ups considering the implementation of DSPs. Therefore, this article addresses the following factors of SST. First, the D-CA is described for the design of the hierarchy between control boards. With the D-CA, because sub-boards are in charge of their corresponding DC-link voltage balancing control individually, the computational burden on the master board can be reduced. Second, the operation sequence of the SST system is explained based on the SST with D-CA. The step of DC-link voltage balance is considered throughout the entire operation sequence for safe driving. Furthermore, the PWM start-up strategies for a Cascade H-bridge Multilevel (CHM) converter and Dual Active Bridge (DAB) converter are proposed to prevent switching pulse errors caused by DSP operating characteristics. These start-up strategies reduce the current surges. The validity of the proposed D-CA and operation sequence with start-up strategies are verified by experimental results.

High-Frequency Transformer Design With Medium-Voltage Insulation for Resonant Converter in Solid-State Transformer

A Solid-state transformer (SST) can directly convert medium voltage to low voltage (e.g. 400 V) with minimized power-conversion stages. The transformer in the DC-DC module not only provides medium voltage insulation but also significantly affects the power density of the module. In this paper, an innovative compact transformer structure is proposed to handle medium voltage insulation using an insulation coordination with epoxy, bobbin, shielding layer and stress-control layer. A brand-new bobbin design and related fabrication process ensures a partial-discharge-free insulation. Besides traditional core loss and winding loss, high-frequency related core loss and shielding loss is modeled in detail for an accurate loss evaluation. A simple and comprehensive multi-parameter, multi-objective optimization process is proposed to achieve the size and loss trade-off. The transformer prototype passes the related insulation tests, following the standard IEEE Std. C57.12.01. Finally, the design is demonstrated on a 15 kW, 200 kHz CLLC converter with 98.9% peak efficiency and 3.8 kW/L power density.

Temperature Dependency of Ion Mobility in Synthetic Ester by Dielectric Response Analysis in Time and Frequency Domain

Synthetic ester is a promising alternative to mineral oil for emerging applications relying on high-power static conversion, such as solid-state transformers (SST). The highly compact components used in SSTs must fulfill MV-DC insulation requirements to guarantee the safe operation of the system, therefore, their design relies on the characterization of the dielectric stress caused by the electric field. The prediction of the electric field induced by DC voltages requires analyzing the conduction phenomena in the liquid, including the effect of charge transport due to ion drift. In this scenario, one of the critical parameters that determine the transport of charge and the resulting electric field distribution is the mobility of ions μ; the value of μ in synthetic ester is available only at room temperature, however, investigations on other dielectric liquids indicate a significant temperature dependency, which therefore needs to be characterized. In this paper, the temperature dependency of μ in synthetic ester liquid is determined by comparing alternative methodologies based on the analysis of the dielectric response in the time and frequency domain, and the viscosity of the fluid. The characteristics obtained by these methodologies are consistent; the temperature dependency of μ obeys the Arrhenius law and an increase of one order of magnitude starting from 1.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">–10</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs is observed in the 303K–353K temperature range.

Multidimensional Design of DC-Link for Two-Stage Solid-State Transformer Cell Considering Second-Order Harmonic Current Distribution

In a two-stage solid-state transformer (SST) cell, the grid power pulsates at the double-line frequency, yielding second-order harmonic current (SHC) across the unit. SHC into the DC/DC stage causes extra losses in the semiconductors and magnetics, resulting in reduced efficiency and lifetime at the gain of lower DC-link requirement. This paper investigates how the DC-link design affects the SHC distribution using a proposed impedance model, which in turn impacts losses, volume and overall reliability. Particularly, the overall lifetime is evaluated considering the reliability interactions between components resulting from varied DC-link design. The newly introduced reliability dimension provides further insight into the DC-link design, in addition to the voltage ripple requirement and efficiency-power density tradeoff. The model and design concept are verified by the simulation and experiment.

SiC Three-Level Neutral-Point-Clamped Converter With Clamping Diode Volume Reduction Using Quasi-Two-Level Operation

Medium voltage (MV) SiC MOSFETs have recently garnered considerable attention in the medium-voltage high-power areas like high-frequency solid-state transformers and multilevel converters. While direct series connection of these MOSFETs is an option for higher voltage levels, it requires complex voltage balancing approaches for device voltage balancing during fast switching transients. Alternatively, converter-level solutions such as the three-level (3L) neutral-point-potential (NPC) converter can be used. This paper proposes a new modulation strategy, combining 3L and Quasi-two-level (Q2L) modulations, to minimize the rating and volume of clamping diodes by tightly controlling the thermal stress on the diodes. This approach achieves better efficiency, higher power density, and a simpler converter bus-structure to stack two SiC MOSFETs effectively in series. We present a real-time clamping diode loss estimation to improve the effectiveness of the proposed modulation strategy. To verify the proposed converter-level approach and modulation strategy, we test a 20 kV rated phase-leg with two 10 kV SiC MOSFETs and 3.3 kV SiC diodes.

Cuckoo search firefly aided optimization framework for design of medium voltage micro-grid connected soft switching solid-state transformer (MS4T)

A single phase Soft Switching-Solid State Transformer (SS-SST) design is proposed with H-bridge topology as an alternative solution to fulfil the demand of low (or) medium grid power applications. A medium/low frequency transformers fed with H-bridge circuit are incorporate without DC-voltage link, and it’s provided sinusoidal output voltage into the grid. An optimization of Cuckoo Search Firefly (CSF) algorithm was proposed in this research to find optimum switching angle and duty cycle in bridge circuit unit. At present optimum grid power is achieved a maximum efficiency of medium/low power frequency with the help of proposed SS-SST (MS4T) model. For proposed design is used to electric aircraft, ship power systems, battery energy storage systems (BESS) and fast charging electric vehicles (EV). Which are appealing the networks of medium-voltage DC (MVDC). Proposed MS4T design is based on soft-switching transformer with low conduction loss, low EMI and high efficiency via H-bridge converter circuit. The capacitor voltage balancing control between cascade module and design of the component including a medium level voltage frequency transformer that is implement a 1 kV to 0.25 kV MS4T described. Therefore, the efficacy of the present investigations are established with MATLAB platform. The medium voltage Micro Grid (MG) output is estimated under different operation load conditions. A simulation result of the grid power is measured minimum harmonics level by using optimum switching angle, switching frequency and duty cycle arrangements.

Solid-State Transformer Based on Naturally Cell Balanced Series Resonant Converter With Cascaded H-Bridge Cells Switched at Grid Frequency

This paper proposes a closed-loop control architecture for a three-stage (MV, isolated DC/DC, LV) Solid State Transformer (SST) system and a modulation technique for the MV stage that shows better performance over existing solutions in terms of control simplicity, semiconductor loss, and cost. A simple and effective closed-loop control architecture is proposed for the entire SST system using the natural cell balancing ability of the Series Resonant Dual Active Bridge (SRDAB) converters in the DC/DC stage. In order to reduce semiconductor losses and cost, Si IGBT and SiC MOSFET devices are introduced properly in the MV stage of the SST power architecture, and the proposed modulation technique switches these devices appropriately according to their merits. The proposed modulation scheme ensures more than 50% semiconductor loss reduction in the MV stage. A circulating logic is proposed to achieve equal power sharing among cells in the MV stage and associated components. Detailed dynamic modeling, closed-loop control architecture, and modulation scheme are discussed in this paper. The proposed control technique and modulation scheme are verified experimentally in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{600}~V/100~V, \text{5}~kW$</tex-math></inline-formula> SST system.

Model predictive control for solid state transformer

The solid-state transformer (SST) is being presented as a technology that enables the construction of new power systems due to its many advantages. These benefits include reduced weight and volume compared to traditional transformers, power factor compensation, accurate output voltage regulation, harmonic mitigation, short-circuit current limitation, and voltage dip immunity, under certain conditions. Unlike other studies that only demonstrated the effectiveness of Model Predictive Control (MPC) in individual parts of the SST, this study shows the control of an entire SST using the MPC strategy. The performance of the SST during power system transients is evaluated to showcase the features of the SST by predictive control.

Passive CM Filter Configuration for a Multistage Grid-Tied Solid State Transformer

<bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">This work proposes a common mode (CM) filter configuration for a grid-tied two-stage AC/DC solid state transformer (SST). The proposed CM filter utilizes passive components along with the heat-sinks of the individual power converter stages to mitigate the overall conducted emissions (CE) injected by the SST system into the grid. The working principle of the proposed CM filter is analyzed and elucidated using CM equivalent circuit models. In addition to CE reduction with the proposed CM filter, it is also demonstrated that the heat-sinks of the SST system can be closer to ground-potential while being impedance grounded. The effectiveness of the proposed CM filter is validated by experimentally measuring the CE of the grid-tied SST system using a Line Impedance Stabilization Network (LISN).</b>

Accelerated Model Predictive Control Based on a Greedy Algorithm With Round-Robin Priority for N-Level Single-Phase Flying Capacitor Converter in Solid-State Transformer

Accelerated Model Predictive Control Based on a Greedy Algorithm With Round-Robin Priority for <i>N</i>-Level Single-Phase Flying Capacitor Converter in Solid-State Transformer

Active Control Strategy of Partial Discharge for Insulation of High-Power High-Voltage High-Frequency Transformers (H3Ts)

The insulation coordination design against electric-originated complex operation conditions is one of the critical challenges for high-power, high-voltage, high-frequency transformers (H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> T) in solid-state transformers (SSTs). The connection between the insulation performance and the operational parameters of H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> T are tightly coupled, but their interaction has been ignored till now. In this letter, an active control method is proposed by adjusting the voltage amplitude, frequency, and temperature to decrease partial discharge (PD) to improve the high reliability of H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> T. At first, PD versus voltage level, frequency, and temperature in existing typical insulation are identified. Then, the PD rules of different parameters are utilized to provide feedback information and guideline for the control of H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> T. This letter aims to improve the reliability of H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> T through active control of PD activities.

Modulation-Based DC-Bias Elimination of DAB Converter Under Second Harmonic

As a basic unit of solid-state transformer (SST), the single-phase system composed of H-bridge and dual-active-bridge (DAB) is becoming an exciting topic for its high-efficiency, flexible power flow. However, the inherent second-harmonic power brings dc-link voltage ripple and phase shift oscillation, especially for the back-to-back systems with bilateral harmonics. In this paper, the behavior of DAB's high-frequency link under second harmonic is modeled and described, revealing the inevitable dc-bias and current-stress increase. A modulation-based dc-bias eliminating strategy is presented, which uses two sawtooth carriers to generate a transitional period without any calculations. To reduce magnetic material's waste, a second-harmonic correction to transformer design is proposed. Moreover, the optimal inductance for single-phase shift (SPS) with minimal current stress and full-range ZVS is obtained. These strategies enhance the ripple immunity of DAB, enabling the reduction of dc-link capacitance. Comprehensive simulations and experiments are performed to validate the accuracy and effectiveness of these optimizations.

The Future of Electrical Power Grids: A Direction Rooted in Power Electronics

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.

High Frequency Transformers for Solid-State Transformer Applications

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.

Development and Reliability Assessment of Si and SiC Power Devices-Based Solid-State Transformer for Urban Railway Vehicle

A Solid-state transformer (SST) is regarded as a promising alternative to conventional low-frequency (LF) transformers in railway vehicles. Through SST, efficiency improvement and reduction of weight, and volume can be achieved. On the other hand, the cost and reliability are the main concerns of an SST over an LF transformer. Nevertheless, there is a lack of study on the reliability of SST in state-of-the-art research. In this paper, a solid-state transformer for urban railway vehicles is designed by considering different aspects such as cost, efficiency, weight, and volume depending on the number of SST modules. Then, a full-scale prototype of Si and SiC power devices-based SST modules is developed and its operation is experimentally verified. Finally, the system-level reliability of the designed SST is assessed based on two real mission profiles of urban railway vehicles by focusing on the lifetime of different power devices.

Auxiliary Power Supply for Input-Series Output-Parallel Medium-Voltage Solid State Transformers

Auxiliary power supply (APS) is an essential component of medium voltage (MV) solid state transformers (SSTs) feeding the auxiliary units, such as gate drivers, controllers and sensors. Usually APSs are obtained from the distributed bus, which may cause the input voltage imbalance before the main power transmission. This paper proposed a new design for isolated APS of an input-series output-parallel (ISOP) SST. The designed distributed MV APSs are galvanically isolated by flybacks, and the centralized APS on the low voltage (LV) side is supplied from the MV APSs via a decoupling four-port medium frequency transformer (MFT) with high isolation voltage. To solve the issue of input voltage imbalance posed by the ISOP system, a dual-loop control scheme based on APS-fluctuating is proposed, which can realize output voltage regulation (OVR) and input voltage sharing (IVS) simultaneously. The small-signal model, start-up strategy and parameter design of the proposed system are investigated. Finally, a 500 V/2.2 kW lab prototype of SST with APS has been built and the overall system efficiency reaches 95.1%.

Pulsed Gas Discharge Driven by Bipolar Solid-State Linear Transformer Driver

Pulsed gas discharge has been studied using a pulsed power generator with a bipolar solid-state linear transformer driver (SSLTD) scheme. Taking advantage of the output flexibility of the bipolar SSLTD, we have studied the residual charges phenomenon by the coaxial discharge load. The experiment result shows that for two consecutive positive pulses discharge, the current of the second positive pulse is usually lower than that of the first pulse for the same voltage amplitude. However, a middle negative pulse applied between the first and second pulse can increase the current of the second positive pulse. An explanation for this phenomenon has been explored using a 1-D model that considers the effect of residual charge left by the previous pulse discharge. According to the explanation of our 1-D discharge model, this phenomenon may be caused by the residual charge distorting the discharge electric field strength. The findings obtained by this article will help us understand the fundamental characteristics of pulsed atmospheric pressure gas discharge.

H5-CFDAB: A Current-Fed Dual-Active-Bridge Converter for Application in Modular DC Solid-State Transformer

Isolated DC-DC converters in DC Solid-State Transformers (DCSSTs) must conform to stringent requirements such as high voltage conversion ratio, high efficiency, and reliable operation with grid fault handling capabilities. This paper proposes a novel current-fed Dual-Active-Bridge (CFDAB) converter, named H5-CFDAB, for modular DCSST application. Compared to the existing topologies, the proposed converter provides a single-stage power conversion with reduced component count and semiconductor Volt-Ampere (VA) rating. The proposed modulation scheme makes the pulse widths of the transformer winding voltages independent of the duty ratio. This allows further optimization of the high-frequency-link (HFL) characteristics like minimized HFL conduction losses and full range Zero-Voltage-Switching (ZVS) at turn ON for the switches, further increasing the converter efficiency. Several modules of the proposed H5-CFDAB converter connected in Input-Series-Output-Parallel (ISOP) configuration makes the system suitable for Medium-Voltage (MV) high-power applications. The operation of the proposed converter is analyzed in detail, including the HFL characteristics and derivation of the ZVS conditions for the switches. In addition, a closed-loop control scheme is proposed for achieving tight regulation of the converter outputs during transients. A 1.2 kW laboratory prototype is built to verify the effectiveness of the proposed converter, along with the control scheme.