Design and Performance Analysis of Solid-State Transformer

A Solid-State Transformer as Enabling Technology for Microgrids for Integration into the Power Distribution System: An Overview

The solid-state transformer (SST) can be a promising solution to replace the conventional 50- or 60-Hz main transformer (MTr) for railway vehicles. Unlike the existing SST technologies for stationary power applications, the SST applied to railway vehicles requires severe constraint of installation space and intensive management of heat dissipation. To optimally design the SST module applied to the railway vehicles, which is one of the major components in the whole SST system, the optimal magnetic component designs for high-efficiency and thermal stable operations in the confined installation space are proposed in this article. In order to meet the SST system requirements for ac 25-kV-to-dc 1.5-kV power conversion, an overall SST topology and its control schemes are established. Based on the proposed SST topology, considering the operating drive profile of the urban railway vehicles, the operating sequence of the proposed SST is introduced and verified by simulation. By virtue of the proposed magnetic component design, the optimal number of turns for the inductor and transformer in dual-active-bridge converters can be appropriately determined for 50-kV high-voltage insulation and high-efficiency operation when the installation space for the magnetic components is given. For targeting the 3.0-MW maximum power transfer capability of the proposed SST system, the 300-kW SST modules, operating at 500 Hz and 10 kHz for ac/dc converters and dc/dc converters, respectively, were fabricated and verified by experiment. By two experimental SST module sets, the static performance of the power converters in the SST module and the dynamic operating sequence has been successfully implemented. For the 300-kW full-load operation, 99.7% of power transfer efficiency for the proposed magnetic components and 97.2% of total power efficiency were obtained. As a result, the measured temperature characteristics for the proposed SST module, including the magnetic components, were managed below 125 °C under a severe experimental environment.

A review on solid-state transformer: A breakthrough technology for future smart distribution grids

Due to its high functionality, the solid state transformer (SST) represents an emerging technology with huge potential to replace the conventional low-frequency transformer (LFT) in a wide range of applications, including railway traction, smart grids, and others. On the other hand, model predictive control (MPC) has proven to be a highly promising control approach for several power electronics systems, especially those based on multiple power converters. Considering these facts, over recent years, different MPC techniques have been proposed for different types of SSTs. In addition to that, numerous MPC strategies have also been investigated for various power converters topologies that can be used in SSTs. However, a paper summarizing and discussing MPC strategies in the framework of SSTs has not yet been proposed in the literature, being the main goal of this work. In this paper, all the existing MPC techniques in complete SST topologies will be presented and discussed. In addition, for the sake of the example, an overview of MPC strategies in converter topologies typically used in SSTs will also be presented.

In this paper, design of a compact high frequency four-port transformer for a Solid-State Transformer (SST) arrangement is presented. Unlike other SSTs, the four-port system integrates three active sources and a load port with galvanic isolation via a single transformer core. In addition to this feature, one of the three source ports is designed to operate at Medium Voltage (MV) 7.2kV for direct connection to 4.16kV AC grid, while other ports nominal voltages are rated at 400V. The transformer is designed to operate at 50kHz and to supply 25kW/port. Thus, the proposed system connects the MV grid, Energy Storage System (ESS), PV, and DC load to each other on a single common transformer core. Based on the system power demand and availability of renewable energy resources, utility and energy storage ports can either supply or draw power, while PV port can only supply power, maintaining the required demand for the load. This work focuses mainly on the High Frequency Transformer (HFT) design. An extensive study is carried out to obtain the optimal, compact, cost effective, and high efficiency model. Modeling, mathematical, and simulation results are derived and presented to demonstrate the viability of this design.

Solid State Transformer (SST) is a new type of power transformer based on power electronic converters and high frequency transformers. The SST realizes voltage transformation, galvanic isolation, and power quality improvements in a single device. In the literature, a number of topologies have been introduced for the SST. In this work, employing a modular multilevel converter, a new SST topology is introduced which provides not only high-voltage AC (HVAC), low-voltage AC (LVAC) and low-voltage DC (LVDC) ports, but also high-voltage DC (HVDC) port. Besides, the proposed topology is easily scalable to higher voltage levels.

Currently, the existing grids are no longer able to cope with the requirements of the modern digital economy. In this regard, the role of unconventional sources of electricity, such as solar panels and wind farms, has increased. The growing role of direct current occurs due to the transition of the automotive industry from an internal combustion engine to an electric motor, as well as due to an increase in the service life and capacity of batteries. However, existing grids do not integrate well with receivers and sources using DC for the operation, which leads to a decrease in the quality, reliability of power supply and economic benefits. The way out of this situation is the transition from traditional to smart grids, which need to search for new methods, devices and solutions that meet the following rules: coordinated operation between DC and AC circuits with the ability to integrate batteries and charging stations; maintaining the quality of electricity in various operating modes, including off nominal ones; collection, conversion into digital form and transmission of information about the state and operating mode of the substation technological equipment and systems in real time operation; flexibility and reliability in the distribution and generation of electricity; high economic feasibility; self-diagnostic functions to ensure the detection of equipment and systems failure with an accuracy of a single module (block). At the same time, a classical transformer cannot cope with the assigned tasks; its disadvantages are given in this article. One of the solutions to this problem is the use of solid-state transformers - semiconductor AC power converters. The paper presents topologies and areas of application of classical AC and DC networks, their disadvantages. It is shown that with the development of direct current sources and consumers, problems arise with the integration of different types of current and voltage levels associated with the number of conversion steps and the quality of electricity. A block diagram of smart power supply networks based on a solid-state transformer is given with a description of what increases the overall efficiency of the network, the quality and flexibility in the distribution of electricity while reducing the number of conversion steps. The topologies of single-stage, two-stage and three-stage solid-state transformers are given and the areas of their application in different systems are shown. It has been established that for smart power supply networks, the most suitable is a three-stage topology of a solid-state transformer, which makes it possible to implement galvanic isolation with two DC-links, which will provide flexibility in switching power supply from DC to AC and vice versa, as well as increase the efficiency of the entire system as a whole.Mathematical modeling of the operation of a three-stage three-phase solid-state transformer for a symmetrical three-phase active-inductive load is presented. The system has output current and voltage feedback and automatic voltage regulation on the second DC-link. The tracking loop is implemented by changing the angle of block modulation in the controlled inverter and rectifier, which are part of the dual active bridge of a three-stage solid-state transformer. The controller is a proportional-integral controller. The graphs of the current and voltage changes when the load is switched on and off are given. It is shown that the voltage drop is insignificant and is almost completely compensated by the stored reactive energy in the reactive elements of a solid-state transformer.The applications of solid-state transformers, their advantages and disadvantages, as well as ways to eliminate these disadvantages are described in the article. The obtained results have shown the feasibility of using solid-state transformers in smart grids, in alternative energy, charging stations for autonomous devices of medium and high power, for energy distribution and in other areas.

The three-phase Solid State Transformer (SST) is one of the key elements in the Future Renewable Electric Energy Delivery and Management (FREEDM) System. The SST consists of an input rectifier, Dual Active Bridge (DAB) bidirectional dc-dc converter and followed by an inverter for ac voltage output. The DAB converter is a bidirectional dc-dc converter using high frequency transformers to step up or down the voltages at high frequency for a reduction in size while maintaining high efficiency and reliability. The single phase high-frequency high-voltage transformer for two-level DAB has been reported by the authors [2]. In this paper, high frequency three-phase transformer for multi-level (3-level) and multi-phase (three-phase) DAB is designed for three-phase multilevel SST. There are two proposed topologies and for each topology the high voltage and high frequency transformer is designed at 3 kHz and 20 kHz. The proposed transformer designs for electromagnetic, thermal and mechanical analysis are validated with the simulation of Finite Element Analysis software.

Solid state transformers (SSTs) have found widespread power applications in traction locomotives, distribution systems, smart grids, etc., where they are going to replace conventional transformers and associated power enhancement equipments to provide better power quality features and functionalities. An eminent structure of SST comprises of three power conversion stages: input (AC-DC), isolation (DC-DC), and output (DC-AC). Multilevel converters are incorporated in several SSTs for operating at high voltage levels. This paper presents a three phase multilevel converter based SST (MCSST) which contains Cascaded H Bridge (CHB) multilevel converters in its input stage. Most of the reported topologies of MCSST have a common low voltage DC (LVDC) bus and a common control block for the entire SST structure. The presented MCSST consists of three independent phase modules and each phase module is controlled separately. Therefore, the outputs of these modules can be applied to both three phase loads as well as three independent single phase loads. Such a configuration can also be used to operate effectively under unbalanced load conditions. The discussed analysis of the reported work is verified using PSCAD/EMTDC simulation results.

The use of nonconventional power supplies featuring unstable quality of the electricity produced and fluctuations in the output power, on the one hand, and the availability of various consumers connected to power supply grids, on the other hand, are factors that give rise to phase unbalances in the grids. These circumstances complicate the operation of smart grids, especially in microgrids. One of possible ways to improve the quality of smart power supply grids is to use a three-stage solid-state transformer with galvanic isolation and two DC links, an arrangement that makes it possible to connect power supply sources and consumers, and to arrange backup storage of electricity in storage batteries. The aim of the study is to analyze the electric power quality at the output of a three-stage solid-state transformer powered by an unbalanced three-phase voltage source. The study was carried out by numerical modeling using the Matlab/Simulink software package. The modeling parameters are given, and the current and voltage waveforms at the solid-state transformer output are obtained. Phasors of the input and output voltages are constructed with expanding them into zero-, positive-, and negative sequence harmonic components. The harmonic distortion coefficients for current and voltage in each phase, and the voltage unbalance coefficients for the zero and negative sequences at the solid-state transformer input and output are determined using the method of symmetric components. It has been shown that by using solid-state transformers it becomes possible to obtain noticeably better quality of electric energy and eliminate the unbalance to the levels stipulated by the relevant state standard. It is shown that solid-state transformers are a promising technology as part of smart power supply grids, which opens the possibility to control and adjust power supply parameters, improve the power supply quality, distribute power flows between the sources and consumers, and integrate DC and AC grids of various voltage and power levels.

Guest Editorial: Situational awareness of integrated energy systems

The solid state transformer (SST) is a power electronic transformer that combines power electronic conversion technology with high frequency power conversion technology based on the principle of electromagnetic induction. Modulation strategies based on the input stage of modular multilevel solid state transformers currently have the following types: nearest level modulation (NLM), carrier phase shift modulation (CPS) and hybrid modulation. This paper focuses on the advantages and disadvantages of the nearest level modulation and hybrid modulation. Considering comprehensively, the hybrid modulation method is adopted in the input stage of the modular multilevel solid state transformer to realize the control of the solid state transformer rectifier stage. The simulation results show that the hybrid modulation method reduced the harmonic content of the grid side output current and the arm circulation to a certain extent.

A Prognostic Energy Optimization Technique with unified power quality conditioner for energy stabilization in grid system

The paper proposes a simple structure of high-power solid-state transformers (SSTs) able to control the energy flow in critical lines of the medium-voltage (20 kV) distribution grid. With an increasing number of renewable intermittent sources connected at the nodes of the meshed distribution grid and a reduced number of nodes connected to large power plants, the distribution grid stability is more and more difficult to achieve. Control of the energy flow in critical lines can improve the stability of the distribution grid. This control can be provided by the proposed high-power SSTs operating a 20 kV with powers over 10 MW. This function is difficult to achieve with standard SST technologies that operate at high frequencies. These devices are made with expensive magnetic materials (amorphous or nanocrystalline cores) and a limited power by SST cells. The required total power is reached by assembling many SST cells. On the other hand, existing SST designs are mainly aimed at reducing the equipment’s size and it is difficult to design small objects able to operate at high voltages. The authors propose to use cores made with grain-oriented electrical steel (GOES) thin strips assembled in wound cores. Experimental results obtained, with GOES wound cores, show that the core losses are lower for a square voltage than for a sine one. This counterintuitive result is explained with an analytical calculus of eddy currents and confirmed by a non-linear time-stepping simulation. Therefore, simple converter structures, operating with rectangular voltages and low switching losses, are the best solutions. Experimental results also show that the core losses decrease with temperature. Consequently, high-power SST cells can be made with transformers whose GOES cores are hotter than coils for reducing core losses and keeping copper losses at low levels. The paper proposes an appropriate transformer mechanical structure that avoids any contact between the hot GOES wound core and the winding, with a specific cooling system and thermal insulation of the hot GOES wound core. The proposed design makes it possible to build SST cells over 1MW and full SSTs over 10 MW at moderate costs.

A Solid State Transformer model for power flow calculations