Solid State Transformer Applications Research Articles

Solid State Transformer: An Overview of Application and Advantages

Abstract: Solid-state transformer (SST), Electronic power transformer or Power electronic transformer (PET) contain the transformer inside the AC-TO-AC converters or DC-TO-DC converters. SST is type of electric power converter that replaces a conventational transformer used in electric power distribution.SST carries the full power and provides the electrical isolation. This paper gives the information of the solid state transformers advantages and applications

An iteration-based design algorithm for high frequency transformer in SSTs and its validation by finite element analysis

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.

An Improvised Modulation and Control Approach for Dual Active Bridge DC–DC Converter System

Aimed at the control and modulation of dual active bridge bi-directional dc (DAB-BIDC) converter, an unconventional approach is applied to explore its effect in symmetrical wave-based isolated dc-dc converters. The existing control schemes of DAB-BIDC converter with unified or optimal phase shift (UPS/OPS) modulation have the constraints like complex control schemes, light load efficiency, current stress, and reactive power. Since a 50 Hz, grid-connected conventional converter system uses the sinusoidal pulse width modulation method to minimize the requirement for passive filters. Similarly, this work investigates the same analogy for the high-frequency (HF) symmetrical wave converters to get effective modulation and control scheme for the DAB-BIDC converter. Furthermore, the proposed sinusoidal pulse width-phase shift (SPW-PS) modulation scheme has been implemented by the optimally derived control parameters using Lagrange's Multiplier Method (LMM) to minimize the reactive power flow and current stress reduction at the HF link. Therefore, the DAB-BIDC converter with the proposed modulation and control approach can be used for EV charging and solid-state transformers applications. Finally, the proposed work is validated using MATLAB simulation and experimental setup having turn ratio 36/12 V, fundamental frequency 5-kHz, and switching frequency 100-kHz for the experimental validation using OPAL-RT.

Numerical analysis of partial discharge ignition in H2 bubbles floating in dielectric oils, for High-Voltage Solid State Transformer applications

Numerical analysis of partial discharge ignition in H₂ bubbles floating in dielectric oils, for High-Voltage Solid State Transformer applications

Distributed Power Management and Adaptive Coordinated Control for SST-Based Power Systems

For better utilization of DC-inherent renewable energy with high penetration and fluctuation, the solid-state transformer (SST)-based power systems have received great attention and arose increasing demand for grid supports and power management. Given the plug-and-play function of distributed renewable energy and loads, the economical and balanced power allocation as well as the flexible seamless transition among different operation modes are not considered comprehensively in existing studies. Therefore, distributed power management and adaptive coordinated control strategy are proposed for SST-interfaced power systems. 1) The application of SST as an alternative to multiple converters to interface the high-voltage grid (HG) and hybrid AC/DC microgrids (MGs) can reduce system communication. Compared with traditional transformers, the controllability and intelligence of SST-enabled power systems are improved. 2) The flexible seamless transition and adaptive coordinated control among HG-connected mode, MG interconnected mode and islanding mode are realized to adapt to different fault conditions. 3) Considering system operation limits, the economical and balanced power allocation is realized accurately, and the voltage quality is effectively ensured in various operation modes. Finally, the feasibility and effectiveness of the proposed system and control strategy are tested on the OPAL-RT-based platform.

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.

The Effects of PWM With High dv/dt on Partial Discharge and Lifetime of Medium-Frequency Transformer for Medium-Voltage (MV) Solid State Transformer Applications

Medium-frequency (MF) transformer in a phase-shift modulated dc solid state transformer (SST) is prone to partial discharge (PD) that impacts the system reliability and lifetime. Tightly packed MF transformer windings carrying PWM voltages create favorable conditions for PD inception. A peak potential condition is created between adjacent posterior windings as a result of phase-shifted modulation. This along with fast alternating electric fields incepts PD at lower than expected operating voltages. Early inception of PD lead to premature transformer failure leading to unanticipated DC SST down-time. The effects of PWM voltages containing high dv/dt switching transients on PD and lifetime characteristics of MF transformer in a dc SST is not yet understood, which is investigated in this paper. A non-intrusive PD detection strategy for MF PWM ac voltage is presented using high speed optical sensors. PD characterization is performed on Kapton polyimide insulated copper foil windings for a foil-type MF transformer. Winding samples are stress tested with steady state PWM voltages upto 2 kV and frequencies upto 50 kHz with high dv/dt upto 60 V/ns while maintaining the transformers geometric profile. Accelerated dielectric lifetime characterization is performed and novel lifetime prediction models are derived containing both frequency and dv/dt dependent variables.

Review of Solid-State Transformer Applications on Electric Vehicle DC Ultra-Fast Charging Station

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.

Bidirectional H8 AC–DC Topology Combining Advantages of Both Diode-Clamped and Flying-Capacitor Three-Level Converters

A three-level bidirectional ac–dc converter with H8 topology is proposed to combine the advantages of both neutral-point clamped and flying-capacitor (FC) three-level converters for medium-voltage (MV) ac–dc solid-state transformer (SST) applications. The innovative H8 converter features self-balanced capacitor voltage, common-mode voltage elimination, and small capacitance of the FCs. The proposed carrier-based modulation enables the H8 converter to operate under both active and reactive power conditions. A 10-kVA ac–dc converter prototype with 1.7-kV SiC MOSFETs and 3.3-kV SiC diodes verifies the feasibility and advantages of the proposed topology and the modulation method. The capacitance of the FCs in the H8 converter is reduced by 45 times compared to that of the FC converters. The H8 converter efficiency is tested up to 99% at 2-kV dc voltage. The proposed H8 converter is suitable for the next-generation PV inverter, grid-forming inverter, MV fast charger, and SST applications.

A Three-Phase Active-Front-End Converter System Enabled by 10-kV SiC MOSFETs Aimed at a Solid-State Transformer Application

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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$dv/dt$</tex-math></inline-formula> (up to 100 kV/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> 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 <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> s 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.

Four-Port, Single-Stage, Multidirectional AC–AC Converter for Solid-State Transformer Applications

Solid-state transformers (SSTs) have emerged as an interesting solution for replacing the traditional low-frequency transformers in power distribution systems owing to the capacity of performing ancillary services and providing the integration of multiple energy sources. This work proposes a four-port, single-phase, multi-directional ac-ac converter for SST applications. Considering the integration of conversion stages, it is possible to achieve higher efficiency and lower component count when compared with the typical three-stage topologies reported in the literature. The introduced structure is based on the interleaving concept associated with the well-known dual active bridge (DAB) converter. The medium-voltage (MV) side employs a multilevel neutral-point-clamped (NPC) converter. The voltages across the ac ports present five levels, whereas the operating frequency of the filter elements is twice the switching frequency. Experimental results are presented and thoroughly discussed to validate the theoretical assumptions.

Fault Resilient Soft Switching DC–DC Converter for Modular Solid State Transformer Applications

Soft-switched isolated converters are preferred for dc–dc stage in solid state transformer applications, wherein fault resilience is highly desirable. This article presents a fault resilient, soft-switching dc–dc converter and associated integrated controller. The proposed integrated controller ensures smooth and reliable fault resilient operation with simplified reconfiguration of employed voltage restorer rectifier during fault. The integrated controller promotes converter design for operation above resonance to avail soft transition of all power devices, reduced EMI, and deployment of body diode integrated switches. Further, the reconfigurable multiplier-type rectifier increases overall converter gain without increased transformer turns ratio. The converter also favors minimized resonant inductance due to lower equivalent resistance and reduced transformer size resulting from above resonant operation. The converter characteristic curves during normal and fault states are presented. Experimental investigations for 1.25-kW prototype in both dynamic and steady state are presented to demonstrate fault resilient capabilities and performance efficacy. Also, impact of postfault reconfiguration on operation frequency, efficiency, and loss distribution is demonstrated.

A bidirectional three‐phase triple‐voltage LLC (T 2 ‐LLC) resonant converter for DC/DC stage in solid state transformer and DC transformer applications

This paper proposes a novel bidirectional three-phase triple-voltage LLC (T2-LLC) resonant converter, which can be applied as the submodule of DC transformer (DCT) with fixed input and output terminal voltages. A novel series-connected three-phase circuit is adopted at medium voltage (MV) side to improve the voltage level of single submodule (SM) and decrease SM number. Therefore, the power density of DCT can be improved. Besides, the proposed converter is open-loop controlled with fixed switching frequency, and it naturally realizes bidirectional power flow without changing controlling modes according to the power direction. However, the efficiency and voltage conversion ratio of proposed T2-LLC converter are sensitive to resonant parameters, and consistency of three-phase resonant parameters should be ensured to achieve three-phase symmetrical operation. Therefore, design and optimization methods are proposed to realize ZVS turn-on, reduce power losses and ensure fixed voltage conversion ratio. The operation principle and optimization method are presented and a 1000 V/333.3 V/5 kW platform is used to validate feasibility of the proposed converter.

Investigating optimal region for thermal and electrical properties of epoxy nanocomposites under high frequencies and temperatures

This research investigates the optimal region to achieve balanced thermal and electrical insulation properties of epoxy (EP) under high frequency (HF) and high temperature (HT) via integration of surface-modified hexagonal boron nitride (h-BN) nanoparticles. The effects of nanoparticle content and high temperature on various electrical (DC, AC, and high frequency) and thermal properties of EP are investigated. It is found that the nano h-BN addition enhances thermal performance and weakens electrical insulation properties. On the other side, under HF and HT stress, the presence of h-BN nanoparticles significantly improves the electrical performance of BN/EP nanocomposites. The EP has superior insulation properties at low temperature and low frequency, whereas the BN/EP nanocomposites exhibit better insulation performance than EP under HF and HT. The factors such as homogeneous nanoparticle dispersion in EP, enhanced thermal conductivity, nanoparticle surface modification, weight percent of nanoparticles, the mismatch between the relative permittivity of EP and nano h-BN, and the presence of voids in nanocomposites play the crucial role. The optimal nanoparticle content and homogenous dispersion can produce suitable EP composites for the high frequency and high temperature environment, particularly solid-state transformer applications.

Modulated MPC for Arm Inductor-Less MVDC MMC With Reduced Computational Burden

A modulated model predictive controller is designed for an inductor-less modular multilevel converter targeting an MVDC solid-state transformer application. The underlying optimization problem is formulated such that a unique closed-form solution is derived from a highly accurate dynamic system model, which significantly reduces computation time. Unlike other closed-form methods, the proposed controller achieves inductor-less current control, has a reduced sampling speed enabled by a novel model-based circulating energy compensator, and does not require tuned PI controllers to generate current references. Simulation and experimental results demonstrate that the proposed controller achieves transient power flow control and steady state circulating energy control despite the low system inertia.

Optimal design of minimal footprint high frequency transformer

Transformer design procedure may vary essentially in respect of the transformer type and its operating frequency (ranging between 50/60 Hz and a few megahertz). This paper presents a simple and straightforward method based on the optimal choice of core geometry of a high frequency transformer (HFT) used in Solid State Transformer (SST) applications. The core of SST is the HFT which largely influences its size and overall performance. The proposed design procedure for HFT focuses on optimizing the core geometry coefficient (in cm5) with a constraint inflicted on loss density. The core geometry coefficient has direct impact on the regulation and copper loss and the procedure results in a robust overall design with minimal footprint. Also, the procedure intends to bring all the operating parameters like regulation, losses and temperature rise within permissible limits while retaining desired efficiency. Thus an energy-efficient design is achieved with minimal footprint. The optimization procedure is implemented using recently developed Moth-flame Optimization (MFO) algorithm. The results of the MFO algorithm are compared with the wellestablished PSO technique. An experimental prototype is built to validate the findings.

Multiphysics and Multiobjective Design Optimization of High-Frequency Transformers for Solid-State Transformer Applications

This article proposes a multiphysics-based and multiobjective design optimization of high-frequency transformers (HFT) for solid-state transformer (SST) applications. Achieving an efficient SST, regardless of its topology, highly depends on the design optimization of its HFT design parameters. Also, a high-power-density. The proposed algorithm (based on time-harmonic electromagnetic, thermal, and fluid physics model coupling) minimizes the volume of the HFT, total cost as well maximizes its efficiency. A case study of $20 \text{ kW}$ , $10 \text{ kHz}$ is investigated and its Pareto optimal solutions (POS) presented. The simulation results show the various dependencies of the design variables on the proposed objective functions which verifies effectiveness of the proposed algorithm. The Pareto optimal solutions (POSs) show that efficiencies above $99\%$ can be achieved with appropriate selection of the design variables. From the POS, two case studies of the HFTs (referred to as $HFT_1$ and $HFT_2$ using $AMCC-100$ and $AMCC-250$ amorphous cores, respectively) are further investigated based on multiphysics numerical models. An experimental implementation of the optimized HFTs ( $HFT_1$ and $HFT_2$ ) is integrated with a self-tuned dual active bridge converter to validate their performance. The experimental measurements from the HFTs are in very good agreement with the optimization results.

Voltage Control of Dual Active Bridge Converter for CO-Amorphous Core Material Based Solid-State Transformer Application

Voltage Control of Dual Active Bridge Converter for CO-Amorphous Core Material Based Solid-State Transformer Application

Voltage Control of Dual Active Bridge Converter for CO-Amorphous Core Material Based Solid-State Transformer Application

Voltage Control of Dual Active Bridge Converter for CO-Amorphous Core Material Based Solid-State Transformer Application

Analysis of Capacitor Charging Characteristics and Low-Frequency Ripple Mitigation by Two New Voltage-Balancing Strategies for MMC-Based Solid-State Transformers

This article investigates and compares various modulation methods and capacitor voltage-balancing algorithms of a modular multilevel converter for solid-state transformer applications. Characteristics of capacitor charging and discharging are analyzed for the existing single-step alternating voltage balancing and the conventional sorting algorithms with phase-shift (PS) modulation and nearest level control (NLC) methods. Based on the analysis, the single-step alternating algorithm leads to a low-frequency voltage ripple, while the conventional sorting algorithm introduces additional switching actions. To address these issues, two new voltage-balancing algorithms are proposed: a multistep alternating voltage-balancing algorithm, and a currentless sorting algorithm. The performance of the proposed algorithms with the PS and NLC modulation methods is comprehensively investigated and compared based on theoretical analysis and experimental results. The study results demonstrate the capability of the proposed methods to balance capacitor voltages while reducing the low-frequency voltage ripple and avoiding additional switching actions.