Solid State Transformer Research Articles

A Review on Power Electronics Technologies for Power Quality Improvement

Nowadays, new challenges arise relating to the compensation of power quality problems, where the introduction of innovative solutions based on power electronics is of paramount importance. The evolution from conventional electrical power grids to smart grids requires the use of a large number of power electronics converters, indispensable for the integration of key technologies, such as renewable energies, electric mobility and energy storage systems, which adds importance to power quality issues. Addressing these topics, this paper presents an extensive review on power electronics technologies applied to power quality improvement, highlighting, and explaining the main phenomena associated with the occurrence of power quality problems in smart grids, their cause and effects for different activity sectors, and the main power electronics topologies for each technological solution. More specifically, the paper presents a review and classification of the main power quality problems and the respective context with the standards, a review of power quality problems related to the power production from renewables, the contextualization with solid-state transformers, electric mobility and electrical railway systems, a review of power electronics solutions to compensate the main power quality problems, as well as power electronics solutions to guarantee high levels of power quality. Relevant experimental results and exemplificative developed power electronics prototypes are also presented throughout the paper.

Advance technology using solid state transformer in power grids

In this paper we are going to discuss the solid- state transformer (SST) which is suitable alternative to the conventional transformer in distribution system. Further, the benefits of solid -state transformer is highlighted so as to improve problems that presently identified in distribution network. The solid state transformer (SST) is a new technology that has the potential to influence advances in a variety of fields, including smart grids, traction systems, and renewable energy systems. This research examines the SST topologies that are suitable for various voltage levels and stages, as well as their control operation. One of the most visible benefits of SST is the reduction in size and weight. Volume reduction of SST is beneficial from portability, low installation cost, and easy to assemble point of view. This study examines a variety of aspects, including current concerns and problems, and makes recommendations for future SST configuration and development.

DCSST Multi-Modular Equalization Scheme Based on Distributed Control.

As an important part of the DC micro-grid, DC solid-state transformers (DCSST) usually use a dual-loop control that combines the input equalization and output voltage loop. This strategy fails to ensure output equalization when the parameters of each dual active bridge (DAB) converter module are inconsistent, thus reducing the operational efficiency of the DCSST. To solve the above problems, a DCSST-balancing control strategy based on loop current suppression is presented. By fixing the phase-shifting angle within the bridge and adjusting the phase-shifting angle between bridges, the circulation current of each DAB converter module is reduced. Based on the double-loop control of the DAB, five controllers are nested outside each DAB submodule to achieve distributed control of the DCSST. The proposed control strategy can reduce the system circulation current with different circuit parameters of the submodules, ensure the balance of input voltage and output current of each submodule, and increase the robustness of the system. The simulation results verify the validity of the proposed method.

Robust Predictive Control for Modular Solid-State Transformer With Reduced DC Link and Parameter Mismatch

This article presents the analysis and implementation of a predictive control method for dc-link regulation and voltage balance in a cascaded modular reduced dc-link solid-state transformer (SST). Passive components like bulky dc links limit the power density of power converters, especially medium-voltage (MV) SST. Reduced dc-link or low-inertia converters can dramatically reduce the size, cost, and weight by tolerating larger dc-link ripples and improve the reliability with electrolytic capacitor-less dc link. However, a small dc link leads to tight coupling between the input and the output stages, which is a challenge for control design. In stacked low-inertia converters (SLIC), the low-inertia converter modules are stacked for MV applications, resulting in coupling between the modules and making the control more challenging. A new model predictive control method that can achieve deadbeat regulation on the dc link without weighting factors has been proposed to address this novel problem. This article focuses on analyzing the condition of the low-inertia dc link up to 80% ripple, the robustness of the control under parameter mismatches, high-order terms, and important implementation issues, such as model-based sampling and computation delay compensation. Significantly, the high-order terms are introduced because of the large dc-link ripple. These high-order terms are unique to the SLIC and negligible in conventional high-inertia converters. A discrete-time large-signal model is built to capture the dc-link's nonlinear dynamics, and the eigenvalues of a small-signal Jacobian matrix are analyzed with Floquet theory to evaluate stability, using the modular soft-switching SST (M-S4T) as an example of the SLIC. Simulation and experimental results of an MVDC M-S4T verify the analysis and the predictive control method. Finally, the general application of the predictive control to low-inertia converters is compared against a conventional PI controller using a reduced dc-link active-front-end rectifier as an example.

Adaptive integral sliding mode controller for solid state transformer based on generalized averaged model and T‐S fuzzy method

The integrated converter between distributed renewable source and main grid has been widely applied. Although the independent control strategies regarding each submodule of solid state transformer (SST) have been studied, the integrated control based on overall model has not been proposed. To this end, this paper proposes an adaptive integral sliding mode control strategy based on T-S fuzzy method for the single-phase SST to regulate the output voltage and guide the power transmission. First, the overall state-space model based on generalized average method is first built through quasi-Fourier series. It can capture switching ripple compared with conventional model. Furthermore, the built model with uncertainties and nonlinear components is switched as the linear time invariant system through T-S fuzzy method. Meanwhile, the system order can also be reduced, which reduces the complexity of the controller design. Based on this model, an integral sliding mode control approach with rapid dynamic response and strong robustness against perturbations or model uncertainties is proposed. Finally, several simulation and experimental results are provided to validate the performance of the proposed control approach.

High Frequency, High Efficiency, and High Power Density GaN-Based LLC Resonant Converter: State-of-the-Art and Perspectives

Soft switching for both primary and secondary side devices is available by using LLC converters. This resonant converter is an ideal candidate for today’s high frequency, high efficiency, and high power density applications like adapters, Uninterrupted Power Supplies (UPS), Solid State Transformers (SST), electric vehicle battery chargers, renewable energy systems, servers, and telecom systems. Using Gallium-Nitride (GaN)-based power switches in this converter merits more and more switching frequency, power density, and efficiency. Therefore, the present paper focused on GaN-based LLC resonant converters. The converter structure, operation regions, design steps, and drive system are described precisely. Then its losses are discussed, and the magnets and inductance characteristics are investigated. After that, various interleaved topologies, as a solution to improve power density and decrease current ripples, have been discussed. Also, some challenges and concerns related to GaN-based LLC converters have been reviewed. Commercially available power transistors based on various technologies, i.e., GaN HEMT, Silicon (Si) MOSFET, and Silicon Carbide (SiC) have been compared. Finally, the LLC resonant converter has been simulated by taking advantage of LTspice and GaN HEMT merits, as compared with Si MOSFETs.

Correction to: Neutral section passing strategy preventing inrush current for electric railway solid-state transformers

Due to an unfortunate oversight the acknowledgement has been given erroneously. It should read.

Inadequacies of Electrical Insulation Materials for Future, High-Frequency, “Smart Grid,” Solid-State Power Transformers

Research & Development in Material Science Inadequacies of Electrical Insulation Materials for Future, High-Frequency, “Smart Grid,” Solid-State Power Transformers Terry A Ring1*, Bjorn C Vaagensmith2 and Jesse L Reeves2,3 1Chemical Engineering, University of Utah, USA 2Idaho National Laboratory, 1955 N. Fremont Ave., Idaho Falls, ID 83415, USA 3Present Address: Northrup Grumman, 5770 S. Missile Way, Roy, UT 84067, Utah, USA *Corresponding author: Terry A Ring, Chemical Engineering, University of Utah, 50 S. Central Campus Drive-MEB 3290, Salt Lake City, UT 84112, USA Submission: November 11, 2021;Published: November 18, 2021 DOI: 10.31031/RDMS.2021.16.000880 ISSN: 2576-8840 Volume 16 Issue 1

High‐frequency‐link DC solid state transformer based on voltage self‐balancing control for MVDC power distribution application

This paper presents a novel voltage self-balancing converter (VBC) applied to DC solid-state transformer (DCSST), analyzes the operating principle of the proposed VBC, and proves the voltage balance performance. Based on this converter, a novel DCSST topology is proposed, which can be used in medium and low voltage DC distribution application, microgrid and medium and low voltage energy storage system. Compared with the previous work, under the same voltage level, it can effectively reduce the number of transformers and switches, reduce the voltage level of switches, and realize the soft switching of the VBC switches, so as to improve the system efficiency and power density, and then improve the system reliability. Meanwhile, the fault handling ability and fault tolerance ability of DCSST are improved, which makes DCSST more reliable when multiple isolated bi-directional DC/DC Converters (IBDC) are connected in series. In addition, with this topology, the input voltage self-balancing control strategy of the IBDC is not needed because of the voltage self-balancing characteristic of VBC, which simplifies the control complexity and greatly improves the control accuracy of the control system. Experimental results agree well with the accuracy and correctness of the proposed topology.

Arm Current Stress Reduction Technique for a Delta-Connected Solid State Transformer Using Zero-Sequence Current Injection

The medium voltage stage of solid state transformer (SST), as a grid-connected converter, is required to generate a negative-sequence (NS) current under certain conditions. Unfortunately, this NS current increases the arm current stress, which limits the operating range of the SST. This article proposes the concept of using a zero-sequence (ZS) current to reduce such a current stress in △-connected SST with a low voltage direct current port (SST+LVdc). An algorithm to determine the optimized ZS current is derived on basis of geometric and logic·. The theoretical analysis and the proposed control algorithm are verified by both simulation and experimental results. The proposed control method gives the △-connected SST+LVdc an advantage over the Y-connected SST+LVdc because it can only work on the △-connected cascaded H-bridge.

Soft-starting scheme for a DC solid-state transformer based on a modular multilevel converter

Direct current (DC) solid-state transformers based on modular multilevel converters (MMC-DCSSTs) have wide application prospects in DC transmission and distribution systems. However, after starting such a system, the normal operation of its MMC-DCSST​ may generate relatively large inrush and magnetizing currents on the primary and secondary sides. In this study, a soft-starting scheme for MMC-DCSSTs, which is divided into the primary-side and secondary-side charging stages, has been designed to limit these currents before the system reaches the normal operation stage. The effectiveness of the proposed method is validated experimentally, and the reliability of MMC-DCSSTs can be improved with the controllable currents.

Quasi-Two-Level Bridgeless PFC Rectifier for Cascaded Unidirectional Solid State Transformer

With the advancements in wide-bandgap power devices, the solid state transformer (SST) becomes attractive to industry due to its merits of high efficiency and power density, modularity, and scalability. However, the complexity and cost remain obstacles to widespread adoption. This article presents a unidirectional SST with a cascaded three-level bridgeless power factor correction rectifier stage and a series-half-bridge (SHB) <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> dc–dc stage, which is simpler and more cost effective than its bidirectional counterpart. The quasi-two-level (Q2L) modulation method is proposed for the three-level bridgeless PFC rectifier. In contrast with the three-level modulation, the Q2L modulation almost eliminates the neutral-point (NP) voltage ripple at line frequency, resulting in much less dc-link capacitor volume. Further, the only safe switching sequence to implement the Q2L modulation is identified and analyzed. The NP voltage shall be balanced by the SHB <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> dc–dc stage for proper operation of the system. A phase-shift-based three-level operation mode is proposed for the SHB <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> converter to balance the NP voltage. The proposed methods are experimentally verified on a 10 kW silicon carbide-based three-level converter cell and a single-phase multicell SST prototype.

Solid-State Bipolar Linear Transformer Driver Using Inductive Energy Storage

There have already a lot of circuit topologies for pulsed power generators using semiconductor switches. In this article, a novel circuit topology concept that can generate bipolar pulses based on linear transformer driver (LTD) topology is presented. Different from traditionally capacitive energy storage (CES) method, we utilize magnetic core as inductive energy storage (IES) medium to accumulate inductive energy before releasing pulses. The repetitive bipolar pulsed power generator is capable of operating in high repetition without reset circuit, because positive magnetization and negative magnetization can lead the magnetic core always back to initial state after every period. In this article, a three-stage laboratory prototype of bipolar IES-based LTD is designed and assembled using six existed CES-based LTD modules, which can generate ~2.4 kV bipolar peak pulses, 55 ns pulsewidth on a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$150~\Omega $ </tex-math></inline-formula> resistive load with charging voltage of 80 V. Finally, this proposed three-stage LTD is evaluated under burst mode to verified the high repetition capability of this bipolar IES-based LTD. As a result, this generator can operate at repetitive frequency of 400 kHz without reset.

Review of a Disruptive Vision of Future Power Grids: A New Path Based on Hybrid AC/DC Grids and Solid-State Transformers

Power grids are evolving with the aim to guarantee sustainability and higher levels of power quality for universal access to electricity. More specifically, over the last two decades, power grids have been targeted for significant changes, including migration from centralized to decentralized paradigms as a corollary of intensive integration of novel electrical technologies and the availability of derived equipment. This paper addresses a review of a disruptive vision of future power grids, mainly focusing on the use of hybrid AC/DC grids and solid-state transformers technologies. Regarding hybrid AC/DC grids in particular, they are analyzed in detail in the context of unipolar and bipolar DC grids (i.e., two-wire or three-wire DC grids), as well as the different structures concerning coupled and decoupled AC configurations with low-frequency or high-frequency isolation. The contextualization of the possible configurations of solid-state transformers and the different configurations of hybrid transformers (in the perspective of offering benefits for increasing power quality in terms of currents or voltages) is also analyzed within the perspective of the smart transformers. Additionally, the paper also presents unified multi-port systems used to interface various technologies with hybrid AC/DC grids, which are also foreseen to play an important role in future power grids (e.g., the unified interface of renewable energy sources and energy storage systems), including an analysis concerning unified multi-port systems for AC or DC grids. Throughout the paper, these topics are presented and discussed in the context of future power grids. An exhaustive description of these technologies is made, covering the most relevant and recent structures and features that can be developed, as well as the challenges for the future power grids. Several scenarios are presented, encompassing the mentioned technologies, and unveiling a progressive evolution that culminates in the cooperative scope of such technologies for a disruptive vision of future power grids.

Stochastic investigation for solid‐state transformer integration in distributed energy resources integrated active distribution network

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.

A data-driven real-time stability metric for SST-based microgrids

In this paper, a data-driven based Real-time Stability Metric (RSM) is proposed to evaluate the stability in Solid State Transformer (SST)-based modern distribution systems based on nonlinear time series analysis. This approach brings two main advantages to the analysis. First, the time series dataset is obtained from the terminal measurement; therefore, linearization is not necessarily needed as opposed to linear approaches. Second, fast dynamics in the system can be captured by an appropriate time resolution in data acquisition and it facilitates the proposed stability analysis capable in real-time. In this study, three Solid State Transformers are considered in a microgrid as the test system. Stability is evaluated by real-time determination of Lyapunov exponents through a real-time processor coordinated with the microgrid.

High-Frequency Transformer Design with Hollow Core for Solid State Transformer

Solid-state transformer (SST) is one of the new technologies that has kept pace with the development of renewable energy sources such as the solar energy and wind turbines. The SST consists of a high-frequency (HF) transformer and power electronic converter at both ends of the HF transformer. Despite the high efficiency of the traditional transformers, is they are quite large in size and also very heavy. In order to reduce the size and weight of the transformers, SST is used as an alternative for the conventional transformer which also brings along other advantages. The type of core material, the type of wire used, and the method of winding the coils on the HF transformer core affects the core losses, copper losses, the cost, efficiency, and power density of the HF transformer. In this paper a new method has been proposed to form the core of the HF transformer used in SST where a hollow core is used, thus reducing the size and weight of the HF transformer along with the reduction in the core loss. The proposed core is designed using ANSYS Maxwell 3D software in addition to calculating the inductance matrix and the coupling coefficient. Finally, the proposed model of a high-frequency transformer was simulated using MATLAB Simulink software.

A Novel Open-Circuit Fault Detection and Localization Scheme for Cascaded H-Bridge Stage of a Three-Stage Solid-State Transformer

A three-stage solid-state transformer (SST) is a preferred replacement for line frequency transformer as it provides ancillary services in addition to stepping-up/down of grid voltages. Each active switch in SST is a potential source of failure, and a quick fault detection and isolation help in reducing the downtime. In this article, a novel fault detection and localization algorithm is proposed to localize the open-circuit (OC) faults (be it a single switch fault or multiple switch faults in multiple modules) in the cascaded H-bridge (CHB) stage of the SST. The proposed method is designed such that only one voltage sensor is needed to measure the grid-side voltage of the CHB for fault detection and localization. By comparing the estimated grid-side voltage of the CHB with the measured values, OC fault is detected. Once the fault is detected, the modulation scheme is switched from phase-shifted unipolar sinusoidal pulsewidth modulation (SPWM) to phase-shifted bipolar SPWM to localize the faults. The difference in the measured grid-side voltage of the CHB before and after each module's state change is calculated and faults are localized. The presented algorithm is substantiated using experimental results from a 1-kVA SST prototype. The main advantage of the proposed algorithm is that it requires only 2n comparisons to localize the faulty modules in a CHB with n H-bridges, which takes less than a line cycle to complete.

Feedback control of pulsed-power generator based on solid-state linear transformer driver.

A pulsed-power generator based on the circuit scheme of a linear transformer driver (LTD) while using solid-state switches is referred to as a solid-state LTD (SSLTD). One of the advantages of the SSLTD compared with traditional pulsed-power generators is its flexibility in the output waveform. It has potential applications to atmospheric pressure gas discharge because this kind of discharge load usually exhibits a fast-changing impedance. In this paper, a feedback control system for SSLTDs is reported. It uses a system-on-chip analog-to-digital converter combined with a field programmable gate array. By using this system, our SSLTD has the ability to automatically adjust its output waveform based on the information obtained from the previous pulse. As a result, during repetitive operation, our SSLTD can figure out the right output waveform according to a user scenario and can respond to any variation that may occur on the load.

Current Control Strategies for a Star Connected Cascaded H-Bridge Converter Operating as MV-AC to MV-DC Stage of a Solid State Transformer

Nowadays, the increasing number of nonlinear loads and renewable energy resources pose new challenges for the standard electrical grid. Conventional solutions cannot handle most of them. The weakest component in the whole system is a conventional distribution (converting medium to low AC voltage) transformer. It should not operate with unbalanced, heavily distorted voltage and cannot control power flow or compensate current harmonics. One of the promising solutions to replace the conventional transformer and thus minimize power flow and grid distortions is a power electronics device called a solid state transformer (SST). Depending on the SST topology, it can have different functionalities, and, with the proper control algorithm, it is able to compensate any power imbalances in both low voltage (LV) and medium voltage (MV) grid sides. In the case of a three energy conversion stage SST, the LV and the MV stages can be treated separately. This paper focuses on the MV-AC to the MV-DC stage only based on a star-connected cascaded H-bridge converter. In this paper, a simple control solution for such a converter enabling different current control strategies to distribute power among the phases in an MV grid in the case of voltage imbalances is proposed. Simulation and experimental results proved good performance and verified the validity of the proposed control algorithm.