Query: Medium-Voltage Solid-State Transformer: Technology for a  and Resilient Grid

Candidate papers:

abstract: The most important impact of power electronics on our society in the last 50 years has been the elimination of the 60 Hz ac power delivery system for consumer electronic products. Central to this achievement is the use of silicon (Si) power devices and pulsewidth modulation (PWM) techniques in delivering regulated ac and dc powers to low voltage (LV) loads such as light emitting diodes and computers. These solid state power electronic converters have provided our society numerous benefits, including high quality power and substantial energy savings. They also form the core technology for integrating renewable energies such as wind and solar into our power grid. Figure 1 shows a typical power delivery architecture commonly found in computer supplies and data centers. The incoming universal ac grid power is converted by a power factor correction circuit to 400 V dc before it is stepped down to a lower voltage dc intermediate bus, such as 12 V, and then it powers the digital loads at voltages as low as 1 V by a point of load converter. Si power metal oxide semiconductor field effective transistor (MOSFET) transistors from 20 V to 700 V are almost exclusively used in this application with switching frequencies from tens of kilohertz to one megahertz. Emerging devices based on gallium nitride (GaN) heterojunction field effect transistors reduce the switching and conduction losses when compared with Si power MOSFETs and are, therefore, poised to compete in these applications, driven by the need for higher energy efficiency and higher power density.
author_names: Alex Q Huang
title: Medium Voltage Solid State Transformer: Technology for a Smarter and Resilient Grid
venue: IEEE Industrial Electronics Magazine
year: 2016

abstract: As an eventual replacement of the century old line frequency transformer (LFT) the Solid State Transformer (SST) has been investigated intensively in the past decade due to its ability to offer many smart functionalities such as voltage regulation and power flow control in the distribution power grid. For the medium voltage and high voltage SSTs, the availability and cost of high voltage power devices are the major limitations. One possible solution is to use modular SST architecture in an Input Series Output Parallel (ISOP) configuration to reach high input voltage and high power capacity. However, modular SST introduces additional challenges in control due to the need for many voltage and current sensors, multiple control loops to achieve cell to cell level voltage and power balancing. To address these problems, this paper introduces a novel modular SST solution utilizing single stage AC AC resonant converter as the modular cell with distributed open loop control. The proposed modular SST is therefore capable of reaching medium and high voltages easily with simple and robust control. The proposed architecture eliminates the DC DC stage hence the associated bulky DC link capacitors. Higher power density can therefore be achieved. 10 kW modular LLC converter prototype is developed based on 1.2kV SiC MOSFETs. Peak AC AC efficiency up to 97.3% is achieved and demonstrated. Voltage and power balancing among modular cells are analyzed against cell to cell variations, including resonant frequency, switching frequency and losses. Simulation results show that the proposed modular SST has no performance degradation even considering those variations in a practical system. An ISOP architecture using two LLC converters is also demonstrated with preliminary experimental results, showing that with LLC converters working close to resonant frequency, the input voltage balancing can be achieved.
author_names: Xin Zhao

Yang Lei

Haoming Wang

Xiangjun Quan

Alex Q Huang
title: Design of A Medium Voltage Solid State Transformer based on Modular AC AC Resonant Converter and an Input Series Output Parallel Architecture
venue: 2019 IEEE Energy Conversion Congress and Exposition (ECCE)
year: 2019

abstract: Due to much higher achievable blocking voltage and faster switching speed, power devices based on wide bandgap (WBG) silicon carbide (SiC) material are ideal for medium voltage (MV) power electronics applications. For example, a 15 kV SiC MOSFET allows a simple and efficient two level converter configuration for a 7.2 kV solid state transformer (SST) for smart grid applications. Compared with multilevel input series and output parallel (ISOP) solution, this approach offers higher efficiency and reliability, reduced system weight and cost by operating at medium to high switching frequency. However, the main concern is how to precisely implement this device in different MV applications, achieving highest switching frequency while maintaining good thermal performance. This paper reviews the characteristics of 15 kV SiC MOSFET and offers a comprehensive guideline of implementing this device in practical MV power conversion scenarios such as AC DC, DC DC and AC AC in terms of topology selection, loss optimization and thermal management.
author_names: Alex Q Huang

Qianlai Zhu

Li Wang

Liqi Zhang
title: 15 kV SiC MOSFET: An enabling technology for medium voltage solid state transformers
year: 2017

abstract: Solid state transformers (SSTs) are evolved as an emerging technology which offer several key features in integrating different grids, storage devices, and renewable energy sources, etc. In this paper, a 10 kV SiC MOSFET enabled Mobile Utility Support Equipment based SST (MUSE SST) is presented for integrating a medium voltage (MV) AC grid (4.16 kV L L, 60 Hz) and a low voltage (LV) AC grid (480 V L L, 60 Hz) The MUSE SST consist of three power conversion stages: MV side active front end converter (AFEC: MV) dual active bridge (DAB) and LV side active front end converter (AFEC: LV) In order to protect MUSE SST from various abnormal operating conditions, different protection schemes are discussed. Dedicated gate drivers incorporating device short circuit protection are implemented for both the MV and the LV SiC MOSFETs. It is observed that the MV short circuit faults are found to be severe and resulting in very high inrush currents from the MV grid which can lead to the impairment of entire SST system. A bi directional switch in parallel with a high impedance resistor (BSPHR) circuit based protection scheme is presented to limit the grid currents during MV dc link short circuit conditions. The BSPHR scheme and efficacy of gate drivers are validated through simulations and experimental results, respectively. The avalanche ruggedness of 10kV, SiC MOSFETs is established experimentally using a single shot unclamped inductive switching (UIS) test and the critical avalanche energy for the MOSFET failure is obtained.
author_names: Venkat N Jakka

Sayan Acharya

Anup Anurag

Yos Prabowo

Ashish Kumar

Sanket Parashar

Subhashish Bhattacharya
title: Protection Design Considerations of a 10 kV SiC MOSFET Enabled Mobile Utilities Support Equipment Based Solid State Transformer (MUSE SST)
venue: IECON 2018 44th Annual Conference of the IEEE Industrial Electronics Society
year: 2018

abstract: The solid state transformer (SST) is a promising technology in the future smart grid. Compared to the line frequency transformer, the SST has more control flexibilities and the plug and play ability, which allow it to interconnect a hybrid ac dc multiterminal grid for reliable and flexible power distribution. The dc bus voltage stability of an SST enabled hybrid ac dc multiterminal grid is a critical issue due to the dynamic interactions of power converters, which has been investigated in this article. The output impedance modeling method of the dual active bridge (DAB) dc/dc converters considering both the control schemes and bidirectional power flow effect is proposed. Depending on the control strategy and power flow direction, the DAB converters have three different output impedance characteristics at the low frequency range, which are capacitive, inductive, and resistive. It is found that the dynamic interaction between the inductive impedance and the capacitive impedance of DAB converters could cause instability issues when the system damping ratio is low. A hardware in the loop based impedance measurement testbed is used to extract the DAB converters' output impedance, which validates the theoretical derivation. The real time simulation results are also provided to demonstrate the dc bus voltage instability.
author_names: Qing Ye

Ran Mo

Haoyu Li
title: Impedance Modeling and DC Bus Voltage Stability Assessment of a Solid State Transformer Enabled Hybrid AC DC Grid Considering Bidirectional Power Flow
venue: IEEE Transactions on Industrial Electronics
year: 2020

abstract: Solid State Transformers (SST) are attracting considerable attention due to their great application potential in future smart grids. It is an essential technology capable of promoting the modernization of the electric power distribution system and it is considered a key element for interfacing future microgrid systems to medium voltage utility grids, allowing plug and play integration with multiple renewable energy sources, storage devices and DC power systems. Its main advantages in relation to conventional transformers are substantial reduction of volume and weight, fault isolation capability, voltage regulation, harmonic filtering, reactive power compensation and power factor correction. A three stage modular cascaded topology has been considered as an adequate candidate for the SST implementation, consisting of multiple power modules with input series and output parallel connection. The modular structure presents many advantages, e.g. redundancy, flexibility, lower current harmonic content and voltage stress on the power switches, however component tolerances and mismatches between modules can lead to DC link voltage imbalance and unequal power sharing that can damage the solid state transformer. This paper proposes a decentralized strategy based on adaptive droop control capable of promoting voltage and power balance among modules of a modular cascaded SST, without relying on a communication network. The behavior of the proposed strategy is assessed through a MATLAB/Simulink simulation model of an 100 kVA SST and shows that power and voltage balance are attained through inner power distribution of the SST modules, being transparent to elements connected to the transformer input and output ports. Besides that, real time simulation results are presented to validate the proposed control strategies. The performance of embedded algorithms is evaluated by the implementation of the SST in a real time simulation hardware, using a Digital Signal Processor (DSP) and high level programming.
author_names: W A Rodrigues

Thiago Ribeiro Oliveira

Lenin Martins Ferreira Morais

Arthur Hermano Rezende Rosa
title: Voltage and Power Balance Strategy without Communication for a Modular Solid State Transformer Based on Adaptive Droop Control
year: 2018

abstract: In today circumstances, due to the rapid development of smart grid network the occurrence of fault become high. The levels of fault current in many places have often exceeded the withstand capacity of existing power system equipment. As a result, security, stability and reliability of power system will be negatively affected. Thus, limiting the fault current of the power system to a safe level can greatly reduce the risk of failure to the power system equipment due to high fault current flowing through the system. Because of that, there is no surprise to fault current limiting technology has become a hot spot of fault protection research since this technology can limit the fault current to a low level. In power system design view, limiting the fault current to a low level can reduce the design capacity of some electrical equipment in the power system. This will lead to the reduction to the investment cost for high capacity circuit breakers and construction of new transmission line. Consequently, from both technical and economical points of view, fault current limiting technology for reducing short circuit current is needed.
title: Solid State Fault Current Limiters (SSFCL) in Medium Voltage Systems
year: 2019

abstract: The rapid evolution of power electronic solutions in all around the globe brings a common problem, which is the adoption of nonlinear loads. This fact carries out a strong impact over the quality of power systems and consequently on energy efficiency, since nonlinear loads act as sources of harmonic currents that flow to other loads or even sources, causing non optimal performance in their operation. Nowadays, conventional transformers are limited to just manage (increase or decrease) voltage level, but they are not able to deal with power quality events, such as harmonics, sag, swell, among others. Hence, there is a need to incorporate a versatile smart device to deal with the challenges previously described for a smart grid environment. This chapter introduces a solid state transformer (SST) with topology of multilevel cascade H bridge converter as a solution. SST is an emerging technology that has the advantages of low volume, low weight, fault isolation, and other management features. Within its fundamental operation, this chapter presents a detailed description of a SST system comprising communication and control, highlighting their main advantages in comparison with conventional transformer such as mitigation of waveform harmonic distortion, allowance of integration of distributed generation, and bi directional power flow.
author_names: Fernando Vaca-Urbano

Manuel S Alvarez-Alvarado

Angel A Recalde

Felix Moncayo-Rea
title: Solid State Transformer for Energy Efficiency Enhancement
year: 2019

abstract: Large scale grid connected photovoltaic energy generation systems are progressing remarkably benefiting from the latest developments in solid state semiconductors technology. In such systems, the photovoltaic arrays can be connected directly to the medium voltage grid without employing a bulky line frequency transformer to step up the voltage. Nano crystalline cores with a small size and a high permeability operating at medium or high frequency can be installed in the power conversion stage. Hence, the necessary isolation as well as voltage boosting features can be provided. However, only a few power converters allow this type of isolation. This paper proposes a new modular converter structure suitable for medium voltage grid connected systems with high frequency isolation. The output voltages of the series connected modules are added in order to provide the necessary voltage boosting. Four different power converter topologies with small input capacitors can be used as submodules for the presented medium voltage configuration having different advantages and drawbacks. These different topologies are analyzed in terms of power losses, footprint, and functionality. To validate the mathematical analysis and the computer simulations, a scaled down 5 kVA three phase, 1 kV prototype is built and tested with four modules for each phase.
author_names: Ahmed Darwish

Mohamed A Elgenedy
title: Current Source Modular Medium Voltage Grid Connected System With High Frequency Isolation for Photovoltaic Applications
venue: IEEE Transactions on Energy Conversion
year: 2019

abstract: The solid state transformer (SST) was conceived as a replacement for the conventional power transformer, with both lower volume and weight. The smart transformer (ST) is an SST that provides ancillary services to the distribution and transmission grids to optimize their performance. Hence, the focus shifts from hardware advantages to functionalities. One of the most desired functionalities is the dc connectivity to enable a hybrid distribution system. For this reason, the ST architecture shall be composed of at least two power stages. The standard design procedure for this kind of system is to design each power stage for the maximum load. However, this design approach might limit additional services, like the reactive power compensation on the medium voltage (MV) side, and it does not consider the load regulation capability of the ST on the low voltage (LV) side. If the SST is tailored to the services that it shall provide, different stages will have different designs, so that the ST is no longer a mere application of the SST but an entirely new subject.
author_names: Levy Ferreira Costa

Giovanni De Carne

Giampaolo Buticchi

Marco Liserre
title: The Smart Transformer: A solid state transformer tailored to provide ancillary services to the distribution grid
venue: IEEE Power Electronics Magazine
year: 2017