Research on Topology and Economy of High-Frequency Isolated AC/AC Converter in Fractional Frequency Transmission System

Abstract

As a new transmission mode， fractional frequency transmission has increasingly prominent advantages in the field of renewable energy， especially in offshore wind power. As the key equipment of fractional frequency transmission， high-voltage and high-power AC/AC converter with electrical isolation function has attracted more and more attention. Based on modular multilevel technology. Based on modular multilevel technology， a high-frequency isolated modular multilevel converter （MMC） is proposed by combining high-frequency transformer and voltage-source converter models. A mathematical model in a synchronous rotating dq coordinate system is established， and a decoupling control strategy based on grid voltage feedforward and a voltage equalization control strategy within the bridge arm are designed to ensure stable operation of the system. Based on the method， the economic indexes of four MMC schemes are compared and summarized in the same engineering context. The advantages of small size and low economic cost of high-frequency isolated converters in fractional frequency transmission systems are demonstrated. A simulation model of the dual-terminal converter integration is built in MATLAB/Simulink environment， and the operational performance of the converter is simulated and verified.

Keywords

• fractional frequency transmission system
• ac/ac converter
• high-frequency isolation
• offshore wind power