FACTS, Flexible AC Transmission Systems · Glossary

FACTS stands for Flexible AC Transmission Systems. The IEEE understands this to mean alternating-current transmission systems with power-electronic and other static controllers. These controllers are intended to make the grid more controllable and to increase its transmission capability. Static means: the installations have no rotating machines. Their core is usually power electronics with thyristors or turn-off semiconductors. FACTS installations can support the voltage and absorb or supply reactive power. They can also change load flows, damp grid oscillations and improve the stability of the transmission grid. They do not generate additional energy, and they do not replace a power plant. Their benefit lies in influencing an alternating-current grid quickly and in a controllable way. Such a grid does not follow contractual trading relationships. Current flows according to impedances, voltage angles and switching state.

FACTS are used mainly in the extra-high-voltage and high-voltage grid. There, large amounts of power flow over long distances. Even small changes in voltage, phase angle or grid impedance can greatly change the distribution of power flows. Depending on the type of installation, its size is stated differently: as reactive power in megavolt-amperes reactive (MVAr), as apparent power in megavolt-amperes (MVA) or as influenced transmission power in megawatts. One example of scale: the German transmission system operators consider future STATCOM installations of roughly 250 to 600 MVAr per installation to be appropriate. Response time is also important in practice. Power-electronic controllers respond within milliseconds to seconds, depending on design and control task. This allows them to influence processes that are too fast for mechanically switched equipment.

Technical function in the alternating-current grid

An alternating-current grid needs more than enough lines and generation capacity. The voltage must stay within permissible limits, the frequency must be stable around 50 hertz, and the generators must keep running in synchronism even during disturbances. Further requirements follow: short-circuit currents must be manageable, equipment must be protected against overload, and after a disturbance the grid must return to a safe state. FACTS installations help with several of these tasks, above all with voltage, reactive power and power flow. Broadly, one can distinguish shunt-connected controllers (shunt), series-connected controllers (series) and combined designs.

Shunt-connected installations act on voltage and reactive power at the connection node. These include the SVC, Static Var Compensator and the STATCOM, Static Synchronous Compensator. Capacitive reactive power raises the voltage, inductive reactive power lowers it. An SVC uses reactors and capacitors that are switched or controlled by thyristors. A STATCOM works with a self-commutated, voltage-sourced converter (Voltage Source Converter). Both control reactive power continuously and can supply or absorb it. With an SVC, the available reactive power falls sharply as the voltage drops. A STATCOM, by contrast, can largely maintain its reactive current. It therefore supports the voltage better during deep voltage dips and responds very quickly. How the costs and space requirements of the two solutions compare depends on the project.

Not every compensation installation is a FACTS installation. Mechanically switched capacitor banks with damping network (MSCDN) and compensation reactors are usually switched manually and preventively. They tend to compensate slow voltage changes, for example over the day-night cycle. Rotating phase shifters are synchronous machines. They supply reactive power and in addition rotating mass, but because of their rotary motion they are not static controllers. In grid planning, all these technologies are often grouped together under the term reactive power compensation.

Series-connected FACTS installations change the effective impedance of a line. In this way they influence how much active power flows over that line. A typical example is thyristor-controlled series compensation (TCSC, Thyristor Controlled Series Compensation). Figuratively speaking, such an installation makes a line electrically shorter or longer without its actual length changing. In meshed grids, part of the power flow can thus be shifted from overloaded to less loaded paths. Converter-based designs go further still. The Static Synchronous Series Compensator (SSSC) and the Unified Power Flow Controller (UPFC) can use a converter to inject a voltage into the line in a targeted way. The UPFC combines a shunt part and a series part.

There are also devices that control the phase angle or the power flow. Whether they count as FACTS depends on their technical design. A phase-shifting transformer does not automatically belong to them, because its tap changer usually works electromechanically. However, it fulfils a similar task in operation: it influences the load flow in a meshed alternating-current grid. Designs with thyristor switches, on the other hand, are counted as FACTS.

Distinction from lines, storage and HVDC

FACTS are often described as an alternative to grid expansion. That is imprecise if it sounds as though FACTS could replace missing line capacity at will. An overhead line or a cable is a physical transport path. A FACTS installation, by contrast, changes how existing alternating-current lines are used and how stably they run. If a region permanently needs more transmission capacity, a controllable compensation installation can reduce bottlenecks or postpone them. However, the thermal limits of the conductors, protection concepts and the N-1 criterion continue to apply. Higher utilisation of existing lines also has its price: reactive power demand rises disproportionately with loading. As a result, additional compensation often becomes necessary in the first place.

Classic FACTS differ from a storage unit in that they do not absorb significant amounts of energy for later delivery. A battery storage system shifts the feed-in or withdrawal of active power over hours. A FACTS installation controls electrical quantities during ongoing grid operation. There are now, however, hybrid forms. An E-STATCOM is a STATCOM with a short-term store, for example made of supercapacitors. For fractions of a second up to a few seconds it can supply active power as instantaneous reserve. According to the grid development plan, TransnetBW plans to build all of its STATCOM installations in this form. These stores, too, serve grid stability and not the trading of energy volumes over time.

High-voltage direct-current transmission, HVDC is also clearly distinct. An HVDC link transmits active power over its own direct-current route and controls the flow between two points very precisely. FACTS operate within the alternating-current grid and change its electrical properties there. The technology is closely related: modern HVDC converters and STATCOMs use similar self-commutated converters. HVDC converters can therefore provide controllable reactive power in their vicinity. The grid development plan states in this regard that the need for STATCOM can in part also be covered by HVDC converters. However, these converters must go into operation in time.

Why FACTS are becoming more important in the power system

For a long time the transmission grid was shaped by large power plants with synchronous generators. They supplied active power, provided short-circuit power and maintained the voltage through their generators, often as a by-product, so to speak. These power plants are declining, and renewable generation is growing. As a result, these properties change. Wind and solar farms feed in via converters. Many feed-in points are far from the load centres, and the direction of power flows changes more often and more quickly.

This increases the need for controllable equipment in the grid. Voltage maintenance becomes more difficult because reactive power cannot sensibly be transported over long distances. It must be provided or absorbed close to the grid nodes where it is needed. At the same time, the regional concentration of renewable generation temporarily causes a high transport demand. Because wind and solar output fluctuates with the weather, reactive power must increasingly be controlled dynamically. In earlier grid development plans, the German transmission system operators identified an additional requirement for reactive power compensation of several tens of gigavolt-amperes reactive (GVAr). A large part of it is to be controllable. The exact values depend on the respective planning cycle and scenario.

This is particularly important for safe operation under the N-1 criterion. According to it, the grid must continue to run safely even if an important piece of equipment fails, such as a line or a transformer. In such cases a FACTS installation can avoid overloads or limit voltage dips. It does not replace the safety rule. However, it gives the grid operator additional means of intervention. If the FACTS installation itself fails, this too must be taken into account in the security calculation.

Typical misunderstandings

A common misunderstanding is to see FACTS as a "virtual line". The image can help in individual cases but obscures the physical limit. FACTS can redirect or stabilise load flows, but they do not create additional conductor cross-section. If all neighbouring lines are already heavily loaded, no free transmission path can be created by control.

It is equally wrong to equate reactive power with electricity generation. Alternating current needs reactive power because electric and magnetic fields in lines, transformers and machines have to be built up and reduced 50 times per second. Unlike active power, however, it does not perform usable work. An installation that provides MVAr thereby supplies no household and no industrial plant. It ensures that the grid can transmit active power reliably at all. Grid operators therefore like to describe reactive power as the "lubricant" of grid operation.

Another error concerns costs. Anyone looking only at the investment easily considers FACTS installations more expensive than a conventional switchgear installation. For their economic value, however, other quantities also count: less redispatch, less curtailment of renewable generation, better utilisation of existing lines, later grid expansion and greater operational security. This benefit does not arise by itself. It depends on the location, on the structure of the grid and on operational management. Regulation also matters: it determines which installations grid operators may invest in and which costs are recognised.

Grid operation, market and responsibility

FACTS installations in the transmission grid generally belong to the transmission system operators. They are not deployed on the electricity market like power plants but operated according to the requirements of the grid. Different incentives follow from this. A power plant earns from energy, from capacity or from system services. A FACTS installation owned by a grid operator is financed in Germany through grid charges if regulation recognises the investment. Its task is to keep the grid physically functional, not to exploit market prices. However, STATCOM-like technology also exists at grid users, for example in wind farms. Such installations are private customer installations and not grid equipment.

Nevertheless, FACTS affect the market indirectly. If bottlenecks in the transmission grid decrease, less redispatch is needed. If local voltage maintenance works better, renewable installations can feed in more often. If lines can be operated closer to their safe limits, the costs of grid management change. These effects are often not visible in the wholesale electricity price. They do show up, however, in grid costs, in balancing measures and in the curtailed energy volumes.

The question is therefore not only which technology is available. In Germany, the transmission system operators list reactive power compensation installations in the grid development plan as separate projects, with location or search area and planned commissioning year. In doing so they also examine other solutions such as HVDC converters or rotating phase shifters. What is decisive is who plans an installation and who assesses its benefit. Equally important are which alternatives are examined and which costs are borne through the grid charges. FACTS show well that security of supply requires more than generation capacity. It also requires controllable grid infrastructure, clear responsibilities and operating rules that also reflect technical effects economically.

FACTS are neither a source of electricity nor a store, but controllable electrical interventions in alternating-current grids. The term is precise when it concerns voltage, reactive power, stability and load flow. It becomes imprecise when it is understood as a general promise to replace grid expansion or system planning. Its significance lies in operating existing grids in a more controlled and faster way and closer to their safe operating limits.

Sources

FACTS Overview (TP 108) — IEEE Power & Energy Society / CIGRE; 1995. Beschreibung des Tutorials
Systemstabilität — Übertragungsnetzbetreiber 50Hertz, Amprion, TenneT, TransnetBW (netzentwicklungsplan.de); unbekannt. Abschnitte Blindleistung, Herausforderungen durch die Energiewende, Technologien
P90 Blindleistungskompensationsanlagen TransnetBW (NEP Strom 2037/2045, Version 2025) — TransnetBW / Übertragungsnetzbetreiber (Projektbibliothek Netzentwicklungsplan); 2026-05-19 (laut zugehörigem PDF). Projektbeschreibung und Begründung (Momentanreserve)
Notwendigkeit der Entwicklung netzbildender STATCOM-Anlagen (Positionspapier) — Die vier deutschen Übertragungsnetzbetreiber (netztransparenz.de); 2020-12. Seite 1

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Edited by eCMS on 11.11.2026 19:47