Reactive power


Reactive power is the part of the powerprovided by the grid that is generated by the interaction between voltage and current in an alternating current system and cannot be actively used by consumers. Reactive power is generated or consumed in many electrical devices (capacitors, electric motors, generators). Reactive power plays a role in building up the electric and magnetic fields of motors or capacitors. Reactive power must be distinguished from power loss, which is the energy that is lost as thermal energy due to friction losses, for example. Reactive power, on the other hand, is not "lost", but is "temporarily stored" and returned to the grid when the fields decay.

An electric motor with a total power consumption (apparent power) of 1000 watts and a reactive power factor (cos phi) of 0.9, for example, can only generate 900 watts as active power, as the rest is consumed by interactions between the voltage and current in the motor. This remainder is precisely the reactive power. It is not "lost", but "oscillates" back and forth between the motor and the mains. In order to generate 900 watts of active power, 1000 watts of apparent power must be transmitted, so the grids must be designed for this apparent power.
A detailed technical explanation of how reactive power is generated can be found at the end of the glossary entry.  

Reactive power cannot perform any useful work, but it still places a load on the network, which is why it is generally kept as low as possible. However, it is unavoidable in order to build up the electric and magnetic fields that are important not only for the operation of numerous electrical devices, but also for the transport of electricity.

How does the provision of reactive power work?

In general, it is the responsibility of the grid operators to provide the required reactive power by requesting the generation plants available in the grid.  

Up to a certain range, the provision of reactive power for generation plants is regulated in the grid connection conditions of the respective grid operator or in the Technical Connection Guidelines ('TAR') issued by the VDE. These stipulate that generation plants may only be connected to the grid if they can provide part of the power as reactive power. Depending on the grid operator, the exact proportion to be supplied is often in the order of 10% of the connected power.
For example, if a large battery storage system feeds in 10 MW of active power, it must provide an additional 1 Var (=1 MW) of reactive power for a static reactive power provision of 10%.
However, the generation plants are not additionally remunerated for this provision of reactive power, although additional costs are incurred for the generation plant due to increased line resistance and wear.

In addition, it should be mentioned that the reactive power is only required when the generators are actually feeding in. If a plant is in idle mode, it does not have to provide reactive power.

The demand that goes beyond this "forced" procurement measure of reactive power is currently often regulated via bilateral contracts between conventional large power plants and the respective grid operators. Pricing takes place in individual negotiations between the parties and is not presented to the public. It is correspondingly non-transparent. According to the BNetzA, the agreed prices differ significantly in some cases and vary from €0.08 to €2.27/MVArh (source: discussion paper "Reactive power provision for grid operation", BNetzA).

As early as 2019, the legislature legally stipulated in Section 12h of the EnWG that, as with many other electricity products such as balancing energy, trading in reactive power should also be based on a transparent, non-discriminatory and market-based procedure. However, the reality is quite different, due to non-transparent price negotiations of the network operators with a few large conventional power plants.  

But why is there still no market-based, transparent procedure so that renewables and storage systems can also participate in the reactive power market? And what needs to be done to change this?

In the course of the introduction of § 12h, the Ministry of Economics commissioned an expert opinion that examined the economic efficiency of such a procedure. The result shows that reactive power can generally be procured via a market-based, transparent procedure. The industry is currently waiting for the Federal Network Agency to specify exactly how the future reactive power market is to be structured. Unfortunately, the timetable for this is still open.

How could large-scale battery storage participate in the reactive power market?

Just as large-scale battery storage facilities can participate in the balancing energy market, it would also be possible for storage facilities to participate in such a free reactive power market without difficulty. The provision of reactive power is not only limited to times when energy is stored or withdrawn. After all, the technical connection guidelines stipulate that reactive power must be provided during these times anyway.  

In addition, large-scale battery storage systems have the technical capability to provide reactive power even when the plant is idle,when no active power is being injected or withdrawn. In these idle phases, reactive power can be provided on a contractual basis, which enables a revenue stream during the idle phases. The end customer also benefits, because the additional supply of reactive power reduces the procurement prices for the grid operators and thus the grid charges for the customers.

How exactly is reactive power generated? A technical excursion

Let's take a simplified motor as an example, which consumes a power of 1000 VA (1 VA = 1 Watt) apparent power . Due to the design of motors, they build up electric and magnetic fields during operation. Let us assume that an electric motor is operated with the frequency of 50 Hertz alternating voltage used in Europe. 50 times a second - every 20 milliseconds - the maximum voltage occurs, and likewise every 20 milliseconds the maximum current. However, since the motor is inductive, the amperage runs slightly behind the current voltage. Let's assume, given its technical specifications, that the lag of the amperage in the electric motor is about 1.4 milliseconds.  

Each cycle of 20 milliseconds represents a sinusoidal oscillation. If this were plotted on a circle, every 20 milliseconds there would be a "rotation" of 360° in the network. This is true for both voltage and current, but in our example the current lags behind the voltage by about 1.4 milliseconds. This corresponds to 1.4/20 * 360°, i.e. approx. 26°. (Phi = 26°; cos phi = 0.9)

Since the power results from voltage times amperage, it comes in view of the lagging now to the fact that a negative power always results if the signs of voltage and amperage are opposite. A certain part of positive power is therefore cancelled out by the negative power - this is exactly the reactive power! In the calculation example, the active power desired by the electric motor is reduced by the factor cos phi = 0.9. Instead of the apparent power of 1000 Volt*Ampere ( VA ) the motor delivers only an active power of 900 Watt. The inductive reactive power is sin (26°) * 1000 Volt*Ampere, approx. 440 Volt*Ampere "reactive" ('Var'). In order to provide 900 watts of active power, 1000 VA of apparent power must therefore be transmitted and the networks must be designed for this apparent power.