Modular Current-Fed Dual-Active-Bridge DC-DC Converters for Medium Voltage System Applications

• Other Authors: Shi, Yuxiang (authoraut)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Electrical engineering
• Online Access: http://purl.flvc.org/fsu/fd/FSU_FA2016_Shi_fsu_0071E_13449

Due to environment concerns and the government incentives, renewable generations are growing rapidly these years. In particular, large-scale photovoltaic (PV) installations are increasing dramatically. However, the limited lifetime of PV converter and high power or energy fluctuation with potential grid stability issue may become barriers to their future expansion. For power converter in large-scale PV systems, cascaded multilevel inverter (CMI) integrated with high-frequency-link (HFL) based dc-dc converters (HFL-CMI) has superior advantages over the conventional centralized converter, in terms of modularity and scalability, weight and volume, direct medium-voltage ac access and distributed maximum power point tracking. In CMI converter, large electrolytic dc-link capacitors are usually required to buffer the double-line frequency energy injecting to ac grid. However, the reliability of electrolytic capacitors is very limited and cannot satisfy the 25-year lifetime expectation for PV systems. As a two-stage conversion of HFL-CMI PV system, the front-end dc-dc converter with advanced technologies can help to meet the lifetime goals. In this research, a cascaded current-fed dual-active-bridge (CF-DAB) dc-dc converter is developed to achieve a long lifetime medium-voltage PV system. The CF-DAB converter has favorite features for PV application, e.g. high step-up ratio with galvanic isolation, inherent zero-voltage-switching (ZVS) conditions, wide input voltage capability and multiport interfaces. The operation principle and ZVS conditions of CF-DAB converter are thoroughly analyzed. With the proposed power decoupling control strategy, the converter allows using small dc-link capacitors, therefore the system reliability can be improved by replacing electrolytic capacitors with film capacitors; meanwhile, low-frequency ripple-free maximum power point tracking (MPPT) can be also realized. An optimized operation mode aiming to minimize transformer root-mean-square (rms) current is also derived for high-efficiency operation. A downscaled 5 kW PV system including a CF-DAB module and an inverter module was built in the laboratory to verify the proposed PV converter with its control strategy. The high power fluctuation of PV and other renewable energy sources has aroused the concerns about their impact on power quality and grid stability, which may hinder their high penetration. Battery energy storage system (BESS) can be a valuable, fast reacting contribution, especially for smoothing the power and energy profile fed to the grids. Due to the inherent dc feature, BESSs operate more efficient when integrated into dc systems instead of ac systems. In general, dc systems are believed to be the future electric systems for better performance, in terms of system complexity and efficiency. While the dc collection concept for PV system is spreading in academia, dc collection grids for offshore wind farms have already been demonstrated in numerous projects. Beside renewable collection, BESSs can also provide energy and stability support for other dc systems, such as onboard medium-voltage dc (MVDC) system where power pulsation or pulsed loads exist. One of the key enabling technology for BESS integration to MVDC system is the dc-dc converter that is suitable for high power medium-voltage BESS. Due to high cost and limited cycle life of batteries, the efficiency and cost are critical for BESS converter. Power density is another critical measure for applications like electric ships. Dc fault rid-though capability is also desired due to the immaturity of high voltage dc circuit breakers. This dissertation proposes current-fed modular DAB (CF-MDAB) dc-dc converters for medium-voltage BESS integration, featuring high efficiency, high power density. The CF-MDAB converters exhibit favorite characteristics of CF-DAB converter including galvanic isolation and soft switching capability. Particularly, with direct dc current controllability, the CF-MDAB BESS converter can achieve dc fault ride-through operation. Other advanced functions such as active filtering and impedance shaping can be also realized to further improve system stability. A downscaled 3-kW CF-MDAB converter was built in the laboratory to verify the proposed BESS converter with its control strategy. Finally, conclusions are given and the scope of future work is discussed. === A Dissertation submitted to the Department of Electrical and Computer Engineering in partial fulfillment of the Doctor of Philosophy. === Fall Semester 2016. === August 8, 2016. === current-fed dual-active-bridge, dc-dc converter, dc fault ride-though, modular converter, soft-switching === Includes bibliographical references. === Hui Li, Professor Directing Dissertation; Juan C. Ordonez, University Representative; Jim P. Zheng, Committee Member; Michael "Mischa" Steurer, Committee Member; Thomas A. Lipo, Committee Member.