This book presents a cutting-edge co-phase traction power supply technology designed to address the severe power quality issues that hinder the development of high-speed and heavy-duty railways. Traditional traction power supply systems struggle with problems such as negative sequence currents and the need for neutral zones. The proposed co-phase power supply system (CTPSS) eliminates these issues, but its core devices, particularly those using IGBT modules, face reliability challenges due to fluctuating traction loads that cause thermal stress. To address this, the book introduces a novel reliability assessment framework that accounts for random load variations and shock profiles, and suggests several improvement strategies based on device structure and operational behavior. In addition to its theoretical contributions, the book provides practical insights drawn from extensive test data, offering detailed analysis of traction load characteristics and power quality across various railway lines. It also validates methods such as optimizing switching frequency, modifying modulation strategies, and enhancing redundancy to improve system reliability. Designed for both students and professionals in electrical engineering, this comprehensive work integrates reliability assessment, enhancement techniques, and protection strategies into a cohesive guide for understanding and advancing co-phase power supply technology.