Reactive power


Reactive power is the portion of what is made available by the network power, which results from 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 is to be differentiated from power loss, which represents the energy that is lost as heat energy due to frictional losses, for example. Reactive power, on the other hand, is not “lost” but is “cached” and returned to the network when the fields are dismantled.

For example, an electric motor with a total power (apparent power) of 1000 watts and a reactive power factor (cos phi) of 0.9 can only produce 900 watts as active power, since the rest is consumed by interactions between the voltage and current in the motor. This remainder is exactly the reactive power. It is not “lost” but “swings” back and forth between the motor and the network. In order to provide 900 watts of active power, 1000 watts of apparent power must therefore be transmitted, so that the networks 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.

The reactive power cannot therefore do any useful work, but it still loads the network, which is why it is generally kept as minimal as possible. However, it is unavoidable in order to build up electric and magnetic fields, which are not only important 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 grid operators to provide the required reactive power by requesting the generation plants in the network.

Up to a certain area, the supply of reactive power for generation plants is regulated in the grid connection conditions of the respective network operators or in the Technical Connection Guidelines (“TAR”) issued by the VDE. It states that generation plants may only be connected to the grid if they can provide part of the power as reactive power. Depending on the network operator, the exact proportion to be delivered is often in the order of 10% of the connected power.
For example, if a large battery storage unit supplies 10 MW of active power, it must provide an additional 1 var (=1 MW) of reactive power with a static idle power supply of 10%.
However, the generation plants are not additionally remunerated for this provision of reactive power, although there are additional costs for the generation plant due to increased line resistances and wear.

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

The demand resulting from this “forced” procurement measure for reactive power is currently often regulated by bilateral contracts between conventional large power plants and the respective network operators. Pricing takes place in individual negotiations between the parties and is not presented to the public. It is therefore opaque. According to BNetzA, the agreed prices differ significantly in some cases and fluctuate from 0.08 to 2.27 €/mVARh (source: discussion paper “Reactive power supply for network operation”, BNetzA).

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

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

As part of the introduction of Section 12h, the Ministry of Economic Affairs commissioned an expert opinion which verified the economic efficiency of such a procedure. The result shows that reactive power can generally be purchased using a market-based, transparent process. The industry is currently waiting for the Federal Network Agency to determine exactly how the future reactive power market should be structured. Unfortunately, the timetable for this is still open.

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

Just as large-scale battery storage systems can participate in the control energy market, it would also be possible for storage systems to participate in such a free reactive power market without difficulty. The provision of reactive power is not limited to times during which energy is stored or stored out. Because in these times, the provision of reactive power is required anyway in accordance with the Technical Connection Guidelines.

In addition, large battery storage systems have the technical ability to provide reactive power even when the system is at a standstill, i.e. when no active power is stored or stored. During these rest periods, reactive power can be provided on a contractual basis, which enables a flow of revenue during idle phases. The end customer also benefits, because an additional supply of reactive power reduces procurement prices for network operators and thus customer network charges.

How exactly is reactive power generated? A technical digression

As an example, let us assume a simplified motor that consumes a power of 1000 VA (1 VA = 1 watt) of apparent power. By constructing motors, they build up electric and magnetic fields during operation. Let us assume that an electric motor is operated at the 50 Hertz AC voltage frequency used in Europe. The maximum voltage then occurs 50 times a second — every 20 milliseconds — and the maximum current intensity also occurs every 20 milliseconds. However, since the motor acts inductively, the amperage lags slightly behind the voltage. Let's assume that, given its technical specifications, the lag in the amperage in the electric motor is approximately 1.4 milliseconds.

Each cycle of 20 milliseconds represents a sinusoidal oscillation. If you were to apply this to a circle, there would be a 360° “revolution” in the network every 20 milliseconds. This applies to both the voltage and the amperage, but in our example, the amperage of the voltage is approximately 1.4 milliseconds behind. This corresponds to 1.4/20 * 360°, i.e. approx. 26°. (phi = 26°; cos phi = 0.9)

Since power is derived from voltage multiplied by amperage, there is now a negative power generated whenever the signs of voltage and amperage are opposite. A certain proportion of positive power is therefore canceled out by negative power — this is exactly the reactive power! In the calculation example, the effective power required 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 only provides an effective power of 900 watts. The inductive reactive power is sin (26°) * 1000 volt*ampere, i.e. approx. 440 volt*ampere “reactive” (“var”). In order to provide 900 watts of active power, 1000 VA apparent power must therefore be transmitted and the networks must be designed for this apparent power.