This book delves into the innovative realm of wound core transformers designed for high-voltage power grids, moving beyond the traditional laminated core structures prone to magnetic flux leakage and energy dissipation. Through a novel enclosed wound core design that eliminates joints and corners, it presents a transformative approach that minimizes leakage and enhances the electromagnetic performance of transformers. This book not only addresses the technical challenges and solutions in transformer design but also marks a significant step forward in the pursuit of energy efficiency and performance optimization in electrical power systems. Targeting a specialized audience of professors, scholars, and graduate students engaged in transformer energy-saving optimization and related research fields, it offers an in-depth exploration of the complete loss calculation, innovative manufacturing processes, and testing technologies of large-scale wound core transformers.