Bidirectional DC-DC Converters for MVDC Applications

Abstract

Technological advancement and cost reduction of power semiconductors in past decades has significantly propelled the development and application of power electronic converters. With this advancement and specifically due to efficiency improvements on both device and topology level, a long lasting question of AC vs DC distribution is gaining renewed interest. Significant advantage of the DC system is simpler control since only the voltage is regulated, better dynamic response that simplifies the integration of renewable energy sources, and higher efficiency due to non-existence of frequency skin-effects. The enabling technology for DC transmission and distribution is a bidirectional DC-DC converter. In-depth literature review of non-isolated bidirectional DC-DC converters conducted during the course of this project has highlighted several interesting multilevel topologies for high power medium voltage energy storage applications. Using the state-of-the-art component and system models a comprehensive multi-objective optimization procedures are performed and the obtained results serve as a basis for systematic comparison. In total six different converter topologies are compared using the pareto curves graphically represented in an efficiency-power density plane. A topology that offers the highest power density and satisfies the minimum efficiency requirement can be easily recognized using the previously mentioned graphical method. Further analysis on the non-isolated DC-DC converters included identification and influence assessment of technological and operating parameters on the efficiency and power density of the considered systems. This assessment was carried out using two different methods - the sensitivity and scalability analysis. From the performed investigations several important conclusions regarding system limitations are derived. The second part of this research project focused on application of isolated bidirectional DC-DC converter as interface for energy storage system in traction. Topology of choice for this application is a dual active bridge (DAB) converter featuring wind band gap switching power devices (SiC MOSFETs) that enables unprecedented high frequency operation compared to the state-of-the-art solutions. For reaching the medium voltage requirement, a modular system consisting of four serially connected modules is utilized. Major effort focused on innovative solutions for power loss heat extraction that allowed to push the boundaries of power density in such systems. For achieving the set goals, a multi-domain analytic models are developed for describing the thermal-fluid dynamic behavior of the proposed integrated transformer cooling structure. Presented model, together with the state-of-the-art component models are used to build an optimization procedure of the considered converter topology. Procedure has two distinct steps: first being a system level optimization which results in a system design with highest power density and efficiency, while the second step takes the results of the first step and optimizes the modulation scheme at every operating point to obtain minimal system losses. The work proceeds with building a prototype system and verifying the results of the proposed and used models. In addition, a control architecture with novel generalized PWM modulator that ensures transformer voltage second balancing in transient conditions is proposed. Extensive prototype tests are conducted both on a single module and a modular system consisting of four modules. The measurement results of the conducted tests are reported for the different operating modes, thus validating the proper behavior of the system.