An Isolated Modular Multilevel Multifunctional DC/DC Converter Based Battery Energy Storage System with Enhanced Fault Performance

• Other Authors: Mo, Ran (author)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Electrical engineering
• Online Access: http://purl.flvc.org/fsu/fd/2018_Sp_Mo_fsu_0071E_14211

Nowadays the medium-voltage dc system (MVDC) has been proposed in the renewable energy collector fields, long distance power transmission, small-scale industrial networks and all-electric shipboards due to its relatively higher efficiency, higher flexibility and lower cost in certain applications compared to the ac grid. Batteries offer scalable energy storage solutions in these applications for high-power and long-term energy demands with high energy density. Batterers play an essential role to smooth the power fluctuations and stabilize the grid as well. As the interface between battery energy storage and MVDC bus, the battery energy storage system (BESS) converter is a key enabling technology with specific requirements. Due to the lack of mature dc circuit breakers, the BESS converter is desired to achieve superior dc fault response which benefits the MVDC system reliability and resiliency. In addition, considering the high expenses and limited lifetime of nowadays battery products, multiple services and functions are preferred for BESS. In this research, the isolated modular multilevel dc/dc converter (iM2DC) based BESS is proposed. It can achieve both fault current limiting and fault ride through functions with direct dc current control capability, so it is possible to maintain the system operation during fault to ensure fault localization and fast recovery. Besides, via the virtual impedance method, the proposed topology employs the converter cell capacitors rather than batteries to provide the ripple energy to achieve the active power filter (APF) function, which allows the energy storage system to improve MVDC system power quality without consuming battery lifetime or extra circuits. In addition, since the medium-frequency transformer operation frequency can be as high as the converter switching frequency, the whole system power density will be improved. A controller hardware-in-the-loop testbed, which consists of the iM2DC based BESS model simulated in the real-time digital simulator (RTDS) and the multifunctional control programmed in the ABB controller products, is utilized to validate the functionality of proposed technology. Furthermore, the system efficiency of proposed BESS is not most optimized with the sinusoidal modulation. Therefore, in this research, a novel phase-shifted square wave modulation strategy is proposed for iM2DC. Compared to the conventional modulation methods, the proposed technique achieves reduced dc inductance due to higher equivalent switching frequency. In addition, the required capacitor energy can be minimized, which decreases the capacitor size without sacrificing the total device rating. Detailed principles of the proposed modulation and passive components design are presented. A downscaled 2kW prototype is built in the lab and the experimental results are provided to demonstrate the proposed modulation strategy. Finally the dissertation work is summarized and the scope of future work is discussed. === A Dissertation submitted to the Department of Electrical and Computer Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. === Fall Semester 2017. === November 17, 2017. === battery energy storage system, high/medium voltage dc grid, isolated modular multilevel dc/dc converter === Includes bibliographical references. === Hui Li, Professor Directing Dissertation; Juan C. Ordonez, University Representative; Thomas A. Lipo, Committee Member; Chris S. Edrington, Committee Member; Michael Steurer, Committee Member.