16th March this year was a wonderful spring day with plenty of sunshine across large parts of Germany. It wasn't only those out for a walk who were happy about that, but solar plant operators were as well. At 1 p.m. the modules provided a combined output of almost 24 gigawatts – roughly equivalent to 18 large, coal-fired power station blocks. Three hours later, though, with the sun lower in the sky, it wasn't even nine gigawatts.
“Reactive power is lubricating grease for the grid.”
As the use of weather-dependent renewable energy sources increases, such fluctuations in the supply of electricity to the grids are increasing significantly as well. The international trade in electricity is also contributing to increasing variation in the load flows of the grids. For grid operators, all this means a lot of work – because, faced with such problems, it is becoming increasingly more laborious for them to keep the voltage in the grid stable. If this were to exceed or fall below defined limits, it would pose a threat of serious damage to electrical equipment, plants and machinery.
The grid operators have several tools at their disposal to control voltage. Amongst these, an important lever is the use of so-called reactive power. Lutz Eckenroth, from the grid operator Westnetz, calls it ‘lubricating grease for the grid’. To a large extent, today’s reactive power is generated by conventional power stations. It creates the electromagnetic fields necessary for generating, transporting and consuming the electricity. It is also an indispensable corrective for grid operation. ‘We use reactive power to compensate for voltage deviations in the grid, such as can occur at certain times when renewable energy plants feed in large amounts of electricity,’ explains Eckenroth.
Maintaining voltage: renewable energy sources supply reactive power to keep the grid stable
“Wind power plants provide very high-quality reactive power – and certainly more dynamically, faster and over a larger area than conventional power stations”
The more wind and solar plants are installed, and the more the international trade in electricity grows, the more grid operators have to rely on reactive power to keep their voltage stable. But where is it going to come from, if more and more conventional power stations are disappearing from the grid?
Grid operators will meet a proportion of their needs with their own operating resources, above all with so-called reactive power compensators. But the operators of renewable energy plants are also being encouraged to play their part. ‘Technologically speaking, providing reactive power is no problem,’ says Hanna Emanuel, from the wind power plant manufacturer Enercon. ‘Wind power plants provide very high-quality reactive power – and certainly more dynamically, faster and over a larger area than conventional power stations,’ the expert explains.
This isn’t a new role for renewable energy sources. For a number of years now, operators of wind turbines and larger solar plants have been obligated to keep a proportion of their capacity in reserve as reactive power. The responsible grid operators decide how much of this output is called upon.
At present the generation of reactive power is still connected with the production of so-called active power – the energy which is commonly (and, from the point of view of physics, not quite correctly) referred to as ‘electricity’. However, wind turbines and solar systems could deliver reactive power even in periods of dead calm or darkness. This is shown by test runs that Westnetz has carried out with wind turbines from Enercon and other manufacturers. It’s made possible by frequency converters, which regulate the rotational speed and the reactive power in the case of wind energy plants, and by inverters, which convert the direct current of the solar cells to alternating current in the case of solar plants. However, these components are also able to create active power from the grid and feed it back in as reactive power – on a 24/7 basis. ‘It works absolutely superbly,’ explains Thomas Christ, Project Director at Westnetz.
Ancillary Services Platform: all players round the same table
Ancillary Services Platform dena set up the Ancillary Services Platform in 2014, with the aim of actively influencing the way in which the provision of ancillary services develops. Additionally, the platform is designed to progress the Ancillary Services 2030 Roadmap. As well as dena, electricity grid operators, project developers, plant operators and technology manufacturers are among those taking part in the Platform, alongside the Federal Ministry for Economic Affairs and Energy.
However, in order to be able to use the full potential of wind turbines and solar plants to control voltage, grid operators and the renewable energy industry must address a further set of technological, economic and organisational questions. How much reactive power must the plant operators supply in future? Should they receive remuneration for it? If so, how? And how might billing be carried out? Those are only some of the points on which players must strive to agree.
One of the places where such discussions are taking place is the Ancillary Services Platform, set up by the Deutsche Energie-Agentur (dena) – the German Energy Agency. Through this forum, dena is bringing grid operators, companies in the renewable energy industry, associations and policy representatives to the same table, so that together they can further develop the ancillary services so important for the success of the energy transition. These include all the tools by means of which grid operators keep frequency, voltage and line loads within the permitted limits and return them to normal levels after malfunctions. Reactive power is therefore only one of the many subjects that the platform is tackling. These range from frequency and voltage control to system control and system restoration. In this process, dena functions as moderator, guide and catalyst.
Fair financing of ancillary services
“The players have different interests. But the platform provides a good basis for coming together on this issue.”
At present, particular attention is being given to the question of whether reactive power and other ancillary services should be financed and remunerated and, if so, how. This is because, for plant operators, providing large quantities of reactive power equals a noticeable loss of income. As a result they then produce correspondingly less active power, for which they would receive remuneration. At present, like many operators of conventional power stations, they are not compensated for providing reactive power under the terms of the technological regulations. The objective of the Ancillary Services Platform is to work out options for solving this conflict that would be economically acceptable for all parties concerned. ‘The players have different interests. But the platform provides a good basis for coming together on this issue,’ explains Emanuel. The ball is now in the politicians’ court. They must create a legal framework that not only addresses the technical standards, but also the financing and remuneration of these services, which are vital for the energy transition.