Department of Electrical Engineering Graduate School, Myongji University Directed by Professor Lee, Il Oun

 Dissertation Thesis (Master's) -- Department of Electrical Engineering, Graduate School, Myongji University 2020

ABSTRACT

As Korea entered into the 2015 Paris Climate Convention, studies are being actively conducted to reduce the emission of pollutants such as carbon dioxide and fine dust in various fields. In particular, Korea is actively supporting the research and development of eco-friendly transportation means using electricity and hydrogen in the transportation industry, and research related to this is being actively conducted. In 2019, a new pollutant emission standard for railroad vehicles, which was a blind spot for pollutant emissions in Korea, was established, and research is underway to replace aging diesel railway vehicles. In particular, research on hybrid railway vehicles using hydrogen fuel cells with no combustion process and high generation efficiency is being actively conducted. However, since there are no railway vehicles using hydrogen fuel cells developed and operated in Korea, it is necessary to refer to constructing the railway vehicle system based on the successful cases operating in overseas. In this paper, the optimal topology selection and design of DC/DC Converter, which charges high-voltage battery by receiving the voltage of fuel cell in hydrogen fuel cell railway vehicle system and supplies power to inverter for propulsion of railway vehicle Deal with. Hydrogen fuel cell railway vehicles have three driving patterns of acceleration, deceleration, and grid connection. The DC/DC converter performs one-way step-up operation in all three driving patterns. Therefore, in this paper, we compare unidirectional step-up converter topology. The optimal topology is selected in a three-level Boost Converter, in which the semiconductor device has half the voltage stress of the output voltage, and in a two-level interleaving boost converter, in which the semiconductor device has half the current stress of the input current. The specification of DC/DC converter was selected by referring to the railroad cars that are developed and operated overseas because there is no commercially available hydrogen fuel cell railroad car. The final specifications were selected as input voltage 600V, output voltage 1500V, output power 250kW. In this paper, we verify the validity of hardware and controller design and analysis by developing a 20kW miniature converter with the same dynamic characteristics as a 250kW DC/DC converter. Therefore, the 20kW miniature DC/DC converter of this paper is reduced to 600V input voltage, 1200V output voltage, and 20kW output power, but has the same switching frequency as the 250kW DC/DC converter reflecting similar dynamic characteristics. To achieve over 95% efficiency, the 3-level boost converter is designed with a switching frequency of 30kHz and the 2-level interleaving boost converter with a switching frequency of 8kHz. The two-level interleaving boost converter is designed at 8kHz because a high output voltage is applied to the switch, resulting in high switching losses. Designed with switching frequency. In this paper, the three-level boost converter with 20kW capacity and the two-level interleaving boost converter were designed under the worst condition, and the expected efficiency was derived through the loss analysis of the converter according to the output voltage. Various simulation models verify the validity of the hardware and controller design. In particular, the controller design was designed considering the digital delay components of the digital controller, and the validity of the theoretical analysis on the delay components was verified through simulation. As a result of the experiment of the actual hardware, the error between the estimated loss and the actual loss through the simulation was less than 1%, confirming the validity of the loss analysis. When comparing the efficiency, power density, dynamic and static performance among the two topologies, we expect a three-level boost converter to be advantageous. Was verified. In particular, considering the current doubling effect of the three-level boost converter, the sampling frequency was 60kHz, resulting in better dynamic performance. Comparing the experimental results of the three-level boost converter and the two-level interleaving boost converter, the final topology was selected as a three-level boost converter with high efficiency, high power density, and excellent dynamic and static characteristics. Based on the experimental results, the hardware design method, the controller design method considering the digital delay component, the loss analysis and various simulation models are expected to be used to design the actual 250kW DC/DC converter.

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