Protection Scheme for a Medium Voltage Mobile Utility Support Equipment based Solid State Transformer (MUSE-SST)

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

Recently, medium voltage SiC devices have been developed which can be used for grid tie applications at medium voltage. Two such devices - 15 kV SiC IGBT and 10 kV SiC MOSFET have opened up the possibility of looking into different converter topologies for medium voltage distribution grid interface. These can be used in medium voltage drives, active filter applications or as the active front end converter for Solid State Transformers (SST). Transformer-less Intelligent Power Substation (TIPS) is one such application for these devices. TIPS is proposed as a 3-phase SST interconnecting 13.8 kV distribution grid with 480 V utility grid. The Front End Converter (FEC) of TIPS is made up of 15 kV SiC IGBTs. This paper focuses on the advantages, design considerations and challenges associated with the operation of converters using these devices keeping TIPS as the topology of reference.

ISSN:2168-6777

The recent advancement in the technology of SiC MOSFETs has spurred interest in designing compact and high switching frequency (10–20 kHz) power converters. However, grid-integration of these power converters at medium voltage (MV) scale would require a conventional transformer. With the development of new high voltage (HV) 10 kV and 15 kV SiC MOSFETs, these converters can directly interface with medium voltage (MV) grids without the need for line-frequency transformers, using simple two-level and three-level topologies. The application of these devices is currently being explored in all MV Applications (8 kV to 30 kV) like Solid State Transformer, MV Drives, Power Conditioning Systems, and MVDC isolators. This paper discusses application of 10 kV SiC MOSFETs and JBS Diodes for enabling Asynchronous Microgrid Power Conditioning System (AMPCS). This medium voltage power converter is enabled by series-connection of devices, in a Three-Level Neutral Point Clamped (3L-NPC) configuration. The voltage balancing of these series-connected devices is achieved by using R C-snubbers. This paper addresses the different conduction modes and switching sequences of a 3L-NPC pole, which is used as building block for the three-phase converter. The switching loss analysis, for various snubber values, is presented for the MOSFETs and the clamping diodes along with experimental results. This research helps in providing an overview of switching losses that are disspated through the device (and heatsink) and through the snubber resistor in a 3L-NPC convertor pole.

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.

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.

Aiming at fulfilling the desirable features of compact structure, high efficiency, high reliability, and multifunctionality for the solid state transformers (SSTs), a single-cell matrix-type SST as well as its control method is proposed as the interface between the medium-voltage (MV) and the low-voltage (LV) levels for direct MV AC to LV AC conversion. To achieve zero voltage switching (ZVS) at both the MV and LV sides, a modulation scheme to construct the special resonant currents during the dead time by actively controlling the MV and LV side gate signals is implemented. Furthermore, the ZVS conditions of the MV and LV side bridges are analyzed, and system parameters are properly designed to ensure full range ZVS operation. A 1-kW SST prototype is developed to verify the functionality and effectiveness of the proposed methods. The experiments show that sinusoidal input and output, bidirectional power flow capability, and ZVS at both sides over wide input voltage and load ranges are achieved with the nominal efficiency of 96.1%. The experimental results match the theoretical analysis and the simulation results well.

This paper focuses on the development of a protection scheme for a smart distribution system containing DERs (Distributed Energy Resources) and Solid State Transformers (SSTs). First, the paper describes the fault analysis that needs to be conducted on such systems. Then, a communication assisted protection scheme is proposed, in order to overcome the shortcomings observed in the conventional protection schemes.

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 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.

Solid-State Transformers (SST) have been growing importance in the upcoming power distribution system. Currently the available circuit topologies and the maximum voltage rating supported by available power devices are the key limits in the usage of Solid-State Transformers into the Medium Voltage (MV) distribution systems. To overcome these limitations, a novel solution denoted as ROMAtrix converter has been proposed and needs to be explored. Such a configuration is able to preserve all the benefits of existing SSTs topologies, such as bidirectional power flow, isolation barrier with a medium frequency transformer and modularity. In addition, the ROMAtrix converter enables connections to a wider range of MV networks by halving the voltage drop on each power switch. However, the power flows management abilities, which would enable this structure to operate in smart grids system, has not yet been proved. To this aim, an active and reactive control strategy has been described in this paper to demonstrate the power control capabilities of the ROMAtrix structure. In order to highlight the features of the proposed topology, significant results, achieved in MATLAB/Simulink and PLECS co-simulation environment, have been shown and discussed.

This paper discusses different ways to obtain an auxiliary power supply (APS) to a solid state transformer (SST) that utilizes the modular AC-DC-DC-AC configuration. A local distributed source, that consists of two APS per module, one for medium voltage (MV) side and another one for low voltage (LV) side is proposed. For the MV side the use of a high-voltage gain static converter is necessary while for the LV side a commonly static converter may be used. Power factor correction converter is used to supply the main control scheme of SST. All APS systems employed in the SST will described in this paper.

The performance of conventional protections has deteriorated in active distribution networks (ADN) because of high penetration of distributed generator (DG) and solid state transformer (SST). In this paper, based on the features of short-circuit current fed by DG and SST, the current distribution characteristics of ADN are analyzed. Moreover, a networked protection scheme is proposed for ADN. This scheme compares the amplitudes of negative-sequence, zero-sequence and phase currents between different ports of grid component to realize fault identification. It can act rapidly and has the ability to cope with large transition resistance. Communication system only needs to transmit current amplitude, and synchronous sampling is not required. In addition, there is no requirement for voltage measurement, which will help to reduce cost. The effectiveness of the protection scheme is verified via simulation tests.

Extensive research in wide-bandgap material technology such as silicon carbide (SiC) has led to the development of medium-voltage (MV) power semiconductor devices with blocking voltages of 3.3 to 15 kV. When these devices are used in various applications, they are exposed to a high peak voltage stress and a very high $dv/dt$ (50–100 V/ns). These impose stringent requirements on the gate driving stage for these devices in terms of featuring a high isolation voltage capability along with a high $dv/dt$ ruggedness, which makes it necessary to have an ultralow coupling capacitance between primary and secondary sides of the gate drivers. One of the key issues in achieving this MV insulation pertains to the necessary clearance and creepage requirements, as defined in IEC 61800-5-1 standards. While the successful operation of these gate drivers is demonstrated in MV converter applications such as solid-state transformers, and MV grid-connected inverters, substantial research needs to be carried out to improve the gate drivers’ performance and provide a plug-and-play solution. This article aims to comprehensively review these gate drivers and consolidate various required design features concerning their galvanic isolation stage, based on normal and short-circuit operation of MV high-power converter systems. Different device short-circuit protection schemes for these gate drivers are explored in detail. Additional applications and functionalities of the gate drivers, including gate drivers used in the series-connection of MV devices and intelligent gate drivers, are also provided in brief. Based on prior research, this review aims to provide design choices and guidelines for the gate drivers, accelerating the growth and deployment of MV SiC devices for field applications.

This paper introduces a 97% efficiency medium voltage (MV) Solid State Transformer (SST) based on two-level single stage (TLSS) topology and 15 kV SiC MOSFETs. Series resonant converter (SRC) is implemented as the main stage and is identified as a current source (CS) SRC for the first time. Equivalent resonant frequency of the proposed CS-SRC is analyzed in both time domain and frequency domain. Resonant frequency calculation equation is derived and can be used to accurately design the resonant tank. Zero voltage switching (ZVS) for 15 kV SiC MOSFET in CS-SRC topology under extremely wide input voltage from 0V to 10 kV is analyzed and designed at a medium frequency (MF) of 40 kHz. Partial discharge for the 15 kV SiC MOSFET's parasitic capacitor are explained and the associated turn on loss equations are derived. Magnetizing inductance Lm and deadtime tdead are further optimized based on the trade-off between the turn on loss and conduction loss of the semiconductor devices. A fully functional and compact SST based on the proposed concept is developed and tested with an input of voltage of 7.2 kVac and from 1 kW to 12 kW. The efficiency is higher than 97% under most load conditions.