Levy Ferreira Costa

The Smart Transformer: Impact on the Electric Grid and Technology Challenges

The increasing proliferation of renewable energy resources and new sizeable loads like electric vehicle (EV) charging stations has posed many technical and operational challenges to distribution grids [1]. Encouraged by attractive tax incentives and promotion policies, local grid end consumers are becoming not only consumers of electricity but, in many cases, also producers. The actual electric distribution system limits the use of renewable energy resources, offers poor EV infrastructure, and is based on a unidirectional information flow from sources to control centers.

Power Routing in Modular Smart Transformers: Active Thermal Control Through Uneven Loading of Cells

Increasing decentralized energy production challenges the distribution grid [1], [2], and, in many countries, power generation and consumption are spatially separated, meaning that energy must be transferred over a long distance [3]. This calls for novel ways to transfer power to the loads without overloading grid feeders and to connect new intelligent loads and storage [4], which typically form the actual electric grid hybrid (ac and dc) and couple with other energy networks (multimodal) [5]. In the current configuration, transformers are passive devices that do not enable dc systems to connect or interface the electric grid with other energy grids.

A Fault-Tolerant Series-Resonant DC–DC Converter

The series-resonant dc-dc converter (SRC) is widely used as power supply for telecommunications, wireless power transfer for electrical vehicle, and high-voltage power supplies. Recently, it became very popular in solid-state transformer application, where fault tolerance is a highly desired feature and it is obtained through redundancy. This letter proposes a reconfiguration scheme for the SRC for the case of failure in one semiconductor, which could drastically reduce the need of redundancy. Using the proposed scheme, the full-bridge based SRC can be reconfigured in a half-bridge topology, in order to keep the converter operational even with the failure [open circuit (OC) or short circuit (SC)] of one switch. As a drawback of this technique, the output voltage drops to half of its original value. Therefore, a novel reconfigurable rectifier based on the voltage-doubler topology is proposed as a solution to keep the output voltage constant after the fault. To verify the feasibility of the proposed scheme, the converter is tested experimentally in a 700-600 V prototype with 10 kW of output power. An insulated gate bipolar transistor (IGBT) SC fault is tested and the results confirm the effectiveness of the proposed approach.

Quad-Active-Bridge DC–DC Converter as Cross-Link for Medium-Voltage Modular Inverters

One of the main challenges of the solid-state transformer (SST) lies in the implementation of the dc–dc stage. In this paper, a quadruple-active-bridge (QAB) dc–dc converter is investigated to be used as a basic module of a modular three-stage SST. Besides the feature of high power density and soft-switching operation (also found in others converters), the QAB converter provides a solution with reduced number of high-frequency transformers, since more bridges are connected to the same multiwinding transformer. To ensure soft switching for the entire operation range of the QAB converter, the triangular current-mode modulation strategy, previously adopted for the dual-active-bridge converter, is extended to the QAB converter. The theoretical analysis is developed considering balanced (equal power processed by the medium-voltage (MV) cells) and unbalanced (unequal power processed by the MV cells) conditions. In order to validate the theoretical analysis developed in the paper, a 2-kW prototype is built and experimented.

Smart Transformer reliability and efficiency through modularity

The application of the Smart Transformer (ST) in the electrical distribution system follows the trend to install more intelligent devices in the grid. In competition with the traditional transformer, the winning argument of the ST can be the grid services, while the system needs to be designed for the targets of high efficiency, high reliability and high availability. This work proposes a modular ST design, which uses power semiconductors rated for lower current and voltage for high efficiency, while the reliability and the availability are targeted by directly routing the power within the modular system. An overview is given on promising modular ST architectures and the concept of power routing for improved reliability is presented.

Quadruple Active Bridge DC-DC converter as the basic cell of a modular Smart Transformer

One of the main challenges of a Solid-State transformer (SST) lies in the dc-dc conversion stage. In this work, a Quadruple-Active-Bridge (QAB) dc-dc converter is investigated to be used as the basic module for the whole dc-dc stage. Besides the feature of high power density and soft-switching operation, the QAB converter provides a solution with a reduced number of high frequency transformers, since more bridges are connected to the same multi-winding transformer. To ensure soft-switching in the full operation range of the converter, two modulation strategies are investigated: the phase-shift modulation and the triangular current modulation. The theoretical analysis is developed for both modulation strategies and a comparison between them is carried out. In order to validate the theoretical analysis, a 20 kW prototype was built and tested.

The Smart Transformer: A solid-state transformer tailored to provide ancillary services to the distribution grid

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.

Modular DC/DC converter structure with multiple power flow paths for smart transformer applications

Smart Transformers (ST), solid-state transformers with advanced control functionalities, are expected to have an important market by 2020. However, still important technological barriers, in terms of reliability and efficiency, exist and only new semiconductor devices cannot solve them. The use of advanced power converter topologies and architectures can play a role too. In this paper, a particular structure for the isolated DC/DC converter, core of a typical three-stage ST, is investigated. By employing the concept of Multiple Active Bridge, a modular and redundant architecture, a power converter characterized by multiple power routing paths is investigated.

Quad-active-bridge as cross-link for medium voltage modular inverters

The Smart Transformer, a Solid-State transformer with control functionalities, is a promising smart grid technology. One of the main challenge of this system lies in making available a Medium-Voltage dc link, which can be tackled with modular solution. Thus, several isolated dc-dc converter can be used as a basic modules. In this paper, a Quad-Active-Bridge (QAB) dc-dc converter is used as modules for the entire dc-dc conversion stage of a ST. Besides the feature of high power density and soft-switching operation (also found in others converters), this converter offers an additional power path, named “MV crosslink” through the HF transformer in the Medium-Voltage (MV) side, increasing the power controllability among the MV cell without involving the LV side. To ensure soft-switching for all operation range of the QAB converter, the triangular current mode modulation strategy, previously study for the dual-active-bridge converter, is extended to the QAB converter in this work. In addition, it is considered to use in the MV side the H-Bridge cell and the multilevel neutral-point-clamped cell, to reduce the required voltage rating of the semiconductors. The theoretical analysis is performed for both converters. The theoretical analysis is verified by simulation and experimental results.

Multilevel boost DC-DC converter derived from basic double-boost converter

This paper presents a nonisolated multilevel step-up dc-dc converter suitable for high power and high output voltage application. The main features of proposed converter are: reduced voltage across the semiconductors; low switching losses; and reduced volume of input inductor. This paper focuses on the five-level structure of the proposed converter, in which the theoretical analysis is carried out and discussed. The five-level proposed dc-dc converter has four capacitors and their voltages should be balanced for its correct operation. Therefore, a capacitor voltage balancing active control is presented and analyzed in detail herein. In order to demonstrate the performance of this converter, experimental results were obtained for an output power of 5kW. The results attest the advantages of the proposed dc-dc topology and it is reported herein.