Chapter 31 - Flexible AC transmission systems

Abstract

This chapter presents the basic operation principles of flexible AC transmission systems (FACTS) devices. The chapter provides a brief introduction on the concept and its origin, the text then focuses on the ideal behavior of each basic shunt and series FACTS device. Guidelines on the synthesis of the first generation of these devices, based on thyristors, are presented, followed by the newer generations of FACTS devices based on self-commutated semiconductor switches. The switching characteristics of thyristors – controlled turn-on and natural turn-off – are appropriate for using in line-commutated converters, like in conventional HVDC transmission systems with a current source in the DC side. In this latter application, the technology for series connection of thyristors is very important due to the high-voltage characteristics of the transmission voltage. Self-commutated switches are adequate for use in converters where turn-off capability is necessary. Thyristor-based FACTS devices use line or natural commutation together with large energy storage elements (capacitors or reactors). On the other hand, devices based on self-commutating switches like GTOs, IGCTs, or IGBTs uses gate-controlled commutation. In general, it is said that the first generation of FACTS devices is based on conventional line commutated thyristors and the subsequent generations are based on gate-controlled devices. The most important FACTS devices based on thyristors and self-commutating devices are presented.