This book consists of three major sections. The first section covers the fundamental theories of new energy power systems. The content includes five synchronization control methods between Voltage Source Converters (VSC) and AC grids, classification of VSC and its control modes, principles and applications of versatile static synchronous machines based on Modular Multilevel Converters (MMC), definition and calculation methods of power system strength, definition and analysis methods of power system resonance stability, and the dilemma of AC power system resonance stability analysis based on impedance models. The second section focuses on the principles and applications of MMC-HVDC transmission systems. The content includes the working principles and steady-state characteristics of MMC, selection of main circuit parameters and loss calculation for MMC, control strategies for MMC-HVDC transmission systems, submodule capacitor voltage control strategies under four typical MMC operation modes, analysis of AC/DC side fault characteristics and DC side fault self-clearing capability construction, MMC-HVDC transmission systems suitable for overhead lines, MMC-HVDC transmission systems for large-scale renewable energy bases, MMC-HVDC transmission applied to offshore wind farm grid integration, voltage control principles and transient fault characteristics of MMC-HVDC grids, basic principles and implementation methods of high-voltage DC circuit breakers, full DC collection and transmission systems for renewable energy bases, insulation coordination design of MMC-HVDC converter stations, and fast electromagnetic transient simulation methods for MMC. The third section covers the principles and applications of flexible AC transmission systems based on cascaded submodule converters. The content includes principles and control strategies of Modular Multilevel Matrix Converters (M3C), principles of low-frequency AC transmission systems for offshore wind farms based on M3C, principles of Unified Power Flow Controllers (UPFC) based on MMC, principles of Static Synchronous Compensators (STATCOM) with cascaded submodules, etc. This book is suitable for advanced engineering and technical personnel engaged in research, planning, design, operation of new energy power systems and development of flexible transmission equipment, as well as teachers and graduate students in electrical engineering disciplines at universities.