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Glossary · Energy industry & energy transition

Reactive power management

Reactive power management Reactive power voltage control electricity grid STATCOM converter transformer ancillary service alternating current grid operation

Reactive power management refers to the targeted provision, absorption and control of reactive power in an alternating current grid. It serves primarily voltage control. The grid is meant to be operated within permissible voltage limits. Reactive power management does not decide how many kilowatt hours are delivered. Rather, it determines whether the electrical conditions in the grid technically allow this energy to be transmitted and distributed.

Physical fundamentals

Reactive power arises in alternating current systems because electric and magnetic fields periodically build up and collapse again. This happens in lines, transformers, motors, generators, cables and power electronic installations. The energy oscillates back and forth between the source and the equipment without being consumed on average. Reactive power has the symbol Q and is stated in var, kvar or Mvar. It must be distinguished from active power P in watts. Active power actually performs work, for example driving a motor, generating heat or charging a battery. Active and reactive power together yield the apparent power S in VA, kVA or MVA. The two quantities are not simply added. For sinusoidal quantities the geometric sum applies: S is the square root of P² plus Q². An example: 80 kW of active power and 60 kvar of reactive power yield 100 kVA of apparent power. For lines, transformers and converters, apparent power is decisive because it determines the current flowing and thus the loading.

The statement that reactive power performs no work often leads to a false picture. Reactive power is not useless but necessary for the operation of an alternating current grid. Voltage and reactive power flows are closely linked. Inductive loads such as motors need reactive power to build up their magnetic fields. Capacitive equipment such as long cables, by contrast, generates reactive power itself. In both cases the local distribution of reactive power influences the voltage. If reactive power is lacking or is provided at the wrong place, the voltage can drop. If too much capacitive reactive power is fed in, the voltage can rise. Either can overload equipment, trigger protection devices or violate the prescribed voltage quality.

Why reactive power is a local issue

Reactive power management is strongly tied to location. Active power can be traded and balanced over long distances, even though bottlenecks and losses occur in the process. Reactive power, by contrast, can only be transported over long distances to a limited extent. Its transport itself causes currents and voltage drops and stresses the equipment. Reactive power from a distant node of the transmission grid therefore often helps the voltage in a distribution grid little, or only with side effects. What matters is therefore not whether enough reactive power exists in the overall system. It must be available at the right grid point, at the right time and controllable fast enough.

In this respect reactive power management differs clearly from frequency control and balancing power. The frequency in the synchronously coupled grid depends mainly on the balance between generation and consumption of active power. It is almost identical throughout the synchronous area. Voltage, by contrast, is strongly shaped locally. It depends, among other things, on reactive power, line impedances, grid structure, transformer taps and load flows. A grid can have enough energy in the balance and still develop local voltage problems. Conversely, a grid section that transmits hardly any active power can have an excessively high voltage because of capacitive effects. This applies, for example, to lightly loaded long cables during low-load periods.

Providers and control means

In the past, reactive power was supplied mainly by the synchronous machines of conventional power plants. They are directly electromagnetically coupled to the grid and can deliver or absorb reactive power via their excitation. They also contribute to short-circuit power and voltage stability. When conventional power plants leave the grid, this capability does not disappear automatically. It must, however, be replaced by other equipment and coordinated differently. These include:

  • Converters of wind, photovoltaic and battery storage installations, which control reactive power depending on voltage, active power or a specified power factor,

  • STATCOM and SVC installations, which dynamically feed in or absorb reactive power and react faster than mechanically switched means,

  • rotating phase shifters (synchronous machines without a drive), which supply short-circuit current in addition to reactive power,

  • capacitor banks, which supply capacitive reactive power and raise the voltage, as well as reactors (shunt reactors), which compensate the capacitive reactive power of, for example, long cables and lower the voltage,

  • tap changers on power transformers and controllable local distribution transformers, which change the transformation ratio and thus shift the voltage level between grid levels without providing reactive power themselves.

Which combination makes sense depends on several factors. These include grid level, short-circuit power, load flow, cable share, generation structure and operational management. An illustrative example is a rural medium-voltage feeder with many photovoltaic installations. On a sunny midday with low load, the voltage at the end of the line rises. If the inverters there absorb inductive reactive power, they dampen this rise. The grid operator can thus often connect more installations without immediately reinforcing the line.

Converter-based installations can provide reactive power very quickly. Depending on their design, this works even when they feed in little or no active power. Their capability is limited by the converter design, the apparent power limit, the control parameters, the connection conditions and the operating state. If an inverter is already operating close to its maximum apparent power, additional reactive power can only be provided at the expense of active power. This trade-off is not a marginal detail. It affects revenue models, grid connection rules and the question of who bears the costs of ancillary services.

Actors and division of tasks

Reactive power management does not lie with a single actor. Transmission system operators need reactive power for voltage control in the extra-high-voltage grid and at the interfaces to downstream grids. Distribution system operators must keep voltage within limits in high-, medium- and low-voltage grids. Many decentralised generation installations, charging points, heat pumps and battery storage systems are connected there today. Plant operators must meet the technical requirements from grid connection rules. These may be specific reactive power characteristics, power factor ranges or voltage control methods. Legislators and regulatory authorities determine how reactive power is procured: as a mandatory connection condition, as a market-based procured and remunerated ancillary service, or via network charge regulation.

Legal framework in Germany

In Germany, Section 12h of the Energy Industry Act (EnWG) obliges transmission system operators with control area responsibility and distribution system operators to follow a specific approach. They must procure non-frequency-related ancillary services transparently, in a non-discriminatory manner and on a market basis. First among these are services for voltage control, that is, reactive power. In addition, the provision names, among others, dynamic reactive current support, a separate service for rapid support during faults. Procurement is permitted only to the extent necessary for safe, reliable and efficient grid operation. The Federal Network Agency (Bundesnetzagentur) can lay down exceptions if market-based procurement is not economically efficient. In doing so it can also exempt individual voltage levels.

The obligation has existed since 27 November 2020. With determination BK6-23-072 of 25 June 2024, the Bundesnetzagentur issued requirements for the market-based procurement of reactive power. They are addressed to the transmission system operators responsible for control areas and to distribution system operators that operate a high-voltage grid. The authority exempted operators of low- and medium-voltage grids from this obligation. As the reason, it stated that a large part of the demand there is covered by the technical connection rules. The marketable volume is small. It therefore considered an obligation for over 800 distribution system operators not economically efficient at present. The transmission system operators introduced the reactive power market under these requirements for the first time in 2025. In low and medium voltage, reactive power therefore remains for the time being primarily a connection condition. In high and extra-high voltage, by contrast, it is increasingly procured as a remunerated service.

Common misunderstandings

One misunderstanding is to see reactive power management only as compensation at the consumer. In industry, reactive power compensation has long been common, because large motors or transformers can cause an unfavourable power factor. Capacitor installations then reduce reactive power draw and relieve lines and transformers. In today's grid operation, this view is not sufficient. Many small photovoltaic installations feed in, long medium-voltage cables are added, and electromobility and heat pumps bring new load profiles. At the same time, generation is shifting into the distribution grids. All this changes the voltage conditions. Reactive power management thus becomes an ongoing coordination task between grid planning, real-time operation and connection rules.

Equating reactive power with losses is also imprecise. Reactive power itself is not consumed energy in the sense of a kilowatt hour. Its transport, however, increases the currents in lines and transformers. Higher currents lead to higher ohmic losses and occupy thermal capacity. A grid section can as a result accept less additional active power. The line is then limited not by energy consumption but by current loading. Good reactive power control therefore cannot completely replace grid expansion. It can, however, make existing equipment better usable and avoid voltage problems.

Significance through electrification

With electrification the topic becomes more important. Heat pumps, charging infrastructure, electric process heat and new industrial installations increase not only the amount of energy in the power grid. They also change load profiles, connection capacities and local voltage conditions. At the same time, photovoltaic and wind installations feed in depending on the weather. This creates grid situations in which voltage is shaped more strongly by local circumstances than by the annual balance of generation and consumption. Such circumstances include high decentralised feed-in at low load, long lines or a lot of simultaneous charging power.

The term shows that security of supply does not consist solely of sufficient generation capacity and energy. A grid must master voltage, frequency, protection concepts, equipment loading and controllability at the same time. Reactive power management concerns the voltage side of operation. It replaces neither grid expansion nor active power management nor flexibility. It does, however, help determine whether lines, transformers and converters work together stably within their technical limits. Properly understood, reactive power management denotes the organised capability of an alternating current grid to control reactive power where it actually affects voltage and equipment.

Sources

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Edited by eCMS on 11.11.2026 16:11

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.