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

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.

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.

Solid state transformer (SST), as one of the most important technologies for energy internet application, will play a crucial role in the future especially with the the development of semiconductor technology. In this paper, a simplified 5.96 MVA three-phase SST system based on 15kV silicon carbide (SIC) power semiconductor devices is shown in order to solve some disadvantages of the three-phase cascaded modular SST. The simplified topology has a great practical prospect, such as urban rail transit power supply of most cities in china. The two stages of SIC-SST, front-end rectifier and DC/DC converter are both designed in closed loop, and then a novel control strategy under unbalanced grid voltages by suppressing the negative-sequence currents at the grid side of SST is presented. Simulation results in PSIM software have verified the effectiveness of the proposed control strategy.

Along with the DC grid distribution issue, the Solid State Transformer (SST) is increasingly emerging in the field of electric power electronics, and the Hybrid SST (HSST) study is also emerging between the current AC-based Grid and future DC grid distribution. At present, several SST and HSST studies have discussed ways to implement the same topology in series to create high DC voltages. These cascade-style topologies not only make it difficult to implement controllers and designing gate drivers but also require a lot of elements, which drives up the overall system’s price. Meanwhile, IGBT and MOSFET series connections are actively being studied for high voltage topology, and various techniques are being studied to solve simultaneous turn on/off problems. This paper proposes by altering the PFC circuit used in existing LVs to switch device series-connected base for HSST and SST. In this paper, circuits were constructed using balancing resistance and passive component for voltage balancing of series-connected switches, and validity is verified through proper controller selection and formulas.

The research in semiconductor technology and power electronics gives scope to replace a conventional low frequency transformer by power electronic converter based transformer which is known as solid state transformer (SST). Reliability issue is one of the major drawbacks of SST because as it contains power electronic switching devices, the chances of occurring open and short circuit faults are more. Therefore, it requires suitable fault tolerant operation. The basic SST topology consists of AC-DC-DC-AC conversion stages. This paper presents fault tolerant operation of third stage of SST (DC-AC converter) for open circuit faults. In this paper, a novel topology of fault tolerant multilevel inverter (FTMLI) is proposed, which can sustain single switch and some specified multi switch faults. The component count also taken into consideration here and the balancing of capacitors also properly maintained. The proposed topology is giving better results in terms of efficiency, cost and THD under fault tolerant operation. The proposed circuit topology is simulated using MATLAB/SIMULINK tool.

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.

In the future AC-DC hybrid distribution system, solid state transformer (SST) will take place of conventional transformer, as it could reduce the weight and volume of transformer, and achieve voltage and current regulation, power flow control, power quality enhancement. MMC-based SST is a novel SST topology, which could provide the MVDC port. However, the excessive bulky submodule capacitors limits this topology into practical application. This paper proposes a fluctuation power control strategy for MMC-based SST to reduce the submodule capacitor voltage oscillation, which could optimize the capacitance and reduce the volume and cost of capacitor. The feasibility of this proposed method is verified by MATLAB simulation.

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.

The paper made an extensive analysis of power generation in the territory of the Republic of Crimea. A special issue is the use of renewable energy sources (RES), in particular wind (WPP) and solar power plants (SPP). The current power generation capacities of the republic are determined and the power deficit is shown, which is currently covered by the electricity of the energy system of the Krasnodar Territory. One of the most important features of the development of the modern world is the increased attention of the world community to the problems of rationality and efficiency in the use of energy resources, the introduction of energy saving technologies and the search for renewable energy sources. In 2021, the countries of the Middle East and Europe increased the capacity of wind and solar power plants several times over. In India, installed renewable energy capacity has exceeded 100 GW. In June-July 2021, solar power generation in the European Union reached a record level and amounted to 39 TWh, an increase of 28%. For most European countries, renewable energy accounts for more than 15% of electricity generation in general. This is due to the fact that in the main markets, renewable, in particular solar electricity, is significantly cheaper than the energy of gas and diesel power plants.For Russia, the southern districts and coastal zones are effective geographical areas for the use of WPPs and SPPs. Thus, the Republic of Crimea is a geographically favorable place for the development of alternative energy.The technical and economic characteristics of SPP and WPP are determined, the current own wind and solar generation is considered in detail. The ways of development and permissible capacities of renewable energy sources in the Crimea are determined. According to the requirements for the systems of regulation, control and protection of renewable energy sources (RES) during their operation as part of the Unified Energy System of Russia, the use of alternative sources of electricity together with the main grid is permissible with their total capacity equal to 20% of the maximum load of the region. Therefore, the development and distribution of renewable energy in the Crimea has a huge potential. The applied systems for integrating RES into the overall energy system are considered, their shortcomings are identified.For the further development of RES in the Crimea, it is proposed to use a model of intelligent integration of SPPs and WPPs into the energy system of the peninsula using solid-state transformers. The use of power transformers for the transmission of electricity from RES to the network significantly reduces the flexibility of the system and eliminates the control of energy parameters. To increase the control capabilities of the output power to the network, as well as to reduce energy costs during voltage transformation, it is proposed to use solid-state transformers (SST) at the inverting stage. The use of SST changes the general principle of power transfer from RES to the network to the following: the DC voltage accumulated in the batteries of WPP and SPP is fed through a high-frequency inverter to a pulse transformer, where it rises to the voltage of the power line. Then it is rectified and inverted to the required mains frequency. Modern power semiconductor components may well provide such a model for integrating RES into the grid. The proposed model is distinguished by the presence of effective control of energy parameters, as well as high efficiency.

The modern trend towards high-speed trains (HST) with distributed propulsion systems, demands high efficiency and high-power density traction systems. Line frequency transformers (LFTs) in railway traction systems are heavy and bulky, quite often necessitating power density to be compromised to achieve maximum efficiency of typically 90-92%. The advancements in power converter topologies, power switching devices and magnetic materials makes it possible to substitute massive LFTs with a new technology called solid-state transformers (SST) (also known as power electronic transformers (PET) or medium frequency transformers (MFT)) traction technology. This technology enables high power density systems with comparatively lower noise emissions which provide essential functionality without compromising efficiency. However, there are still major challenges to overcome associated with power converter connection on the high-voltage (HV) side, architecture modification and the compactness of the transformer design. This paper reviews the existing architectures and also introduces the new research possibilities especially in the power conversion stages, and the power switching devices. Finally, the design guide lines for high-power converters are presented.

An important functionality of the Solid State Transformer (SST) is the integration of renewable energy sources within the topology itself, acting as a renewable energy hub. Due to the fluctuating nature of the renewable energy source, such integration becomes a challenge and energy storage becomes a requirement. In this paper a topology variant of single phase SST with multi limb magnetic core is proposed which directly integrates the renewable energy source in the high frequency magnetic stage and uses the grid as the energy buffer eliminating the requirement of energy storage.

Solid state transformers (SSTs) are at the forefront of emerging technologies, enhancing distribution and transmission networks. These transformers facilitate the development of highly controllable and robust smart grids, seamlessly integrating renewable energy sources and accommodating connections for both alternating current and direct current components/lines at medium or low voltage levels. This research paper introduces an innovative SST incorporating the inductive power transfer (IPT) technology, currently under development within SSTAR, a Horizon Europe project. This novel device operates at a frequency of 50 kHz and provides with an output power of 50 kW. The IPT system is submerged in an ester-based, biodegradable dielectric fluid, providing effective cooling and insulation characteristics. The primary focus of this research is to highlight the guidelines for the optimal design of the forced liquid cooling system of the SST module. The analysis employs a multi-faceted simulation strategy to calculate the cooling aspects of the transformer, utilizing the commercial software ANSYS. This involves 3D coupled Electromagnetic (EMAG) - Computational Fluid Dynamics (CFD) simulations, as well as stand-alone CFD and thermal FEA simulations, ensuring robust cross-verification of the obtained results. This novel SST exhibits improved thermal performance, achieving a hot-spot temperature of 58 °C, the lowest value found in similar research studies. The findings underscore the efficiency and robustness of the proposed design, marking a significant step towards the realization of active and environmentally-friendly electrical networks.

To meet the fast-growing energy demand and manage the distributed resource, an intelligent energy management system plays a vital role in the electric power system. The next-generation electric power system, known as the smart grid, will incorporate a large number of renewable energy resources that fundamentally change the energy management paradigm. To manage efficiently the energy supply and demand in the power grid, further improvement in the reliability and efficiency of electrical grids can be achieved by utilizing dc distribution in microgrid systems. DC microgrid is an attractive technology in the modern electrical grid system because of its natural interface with renewable energy sources, electric loads (e.g. data center load, electrical vehicle charging load, residential dc load), solid state transformer and energy storage systems. In this paper we discussed the functional expectation of the dc microgrid, system architecture, communication structure and distributed grid intelligent system. This paper highlights the recent technology update in dc microgrid system. This paper also discusses benefits and challenges of using dc grid.

Medium voltage DC (MVDC) grid has quite a lot advantages for the application of large power electric ships, while the DC breaker and DC distribution are the key equipment. Solid state transformer (SST) composed by power electronics transformer and high-frequency transformer has high power density and high efficiency, so it can be used for the power conversion from MVDC to low voltage DC or AC, especially as the development of wide-band gap devices with higher voltage, higher switching frequency with less conduction and switching loss. Several SST topologies suitable for onboard DC distribution are analyzed in this paper, then the control algorithm are proposed to realize DC bus equalization. Experimental results are proposed to verify the performance of the SST and control method.

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.