The Evolution of Flexible Alternative Current Transmission Systems Design from Thyristors to Modular Multilevel

Abstract

This article systematically reviews the technological evolution of power circuit topology in Flexible AC Transmission System (FACTS) devices, from thyristor semi controlled circuits to IGBT fully controlled architectures, and then to the distributed structure of modular multilevel converters (MMC), revealing the changes in circuit design concepts during technological iterations. This article compares the topological characteristics of each stage and finds that thyristor circuits have simple driving but limited dynamic performance due to passive commutation. IGBT circuits improve response speed through PWM control but face switching losses and electromagnetic interference problems. Although MMC structures achieve high voltage and low harmonic output, they need to solve problems such as capacitor balancing and isolation control. The evolution of key aspects in circuit design is closely related to device characteristics. The driving circuit has evolved from pulse triggering to multi-level active driving, the protection mechanism has been upgraded from fuses to multi-level locking, the voltage balancing technology has developed from passive balancing to dynamic algorithms, and the heat dissipation scheme has shifted to liquid cooling systems due to the increase in power density. In summary, the current technological bottlenecks mainly focus on high losses and electromagnetic compatibility issues. The future breakthrough direction should integrate wide bandgap semiconductor devices, artificial intelligence optimization control algorithms, and multi physics field collaborative design to improve system economy and reliability and meet the needs of new power system construction.