Overview

Flexible AC Transmission Systems, called FACTS, got in the recent years a well-known term for higher controllability in power systems by means of power electronic devices. Several FACTS-devices have been introduced for various applications worldwide. A number of new types of devices are in the stage of being introduced in practice. Even more concepts of configurations of FACTS-devices are discussed in research and literature.

In most of the applications the controllability is used to avoid cost intensive or landscape requiring extensions of power systems, for instance like upgrades or additions of substations and power lines. FACTS-devices provide a better adaptation to varying operational conditions and improve the usage of existing installations. The basic applications of FACTS-devices are:

• power flow control,

• increase of transmission capability,

• voltage control,

• reactive power compensation,

• stability improvement,

• power quality improvement,

• power conditioning,

• flicker mitigation,

• interconnection of renewable and distributed generation and storages.

In all applications the practical requirements, needs and benefits have to be considered carefully to justify the investment into a complex new device. Figure 1.1 shows the basic idea of FACTS for transmission systems. The usage of lines for active power transmission should be ideally up to the thermal limits. Voltage and stability limits shall be shifted with the means of the several different FACTS-devices. It can be seen that with growing line length, the opportunity for FACTS-devices gets more and more important.

Figure 1.1: Operational limits of transmission lines for different voltage levels

The influence of FACTS-devices is achieved through switched or...