What keeps alternating current in sync when large power generators go offline?

In the future, Europe is to be dominantly powered by renewable energy. The expansion of wind and solar power capacity and the provision of sufficient power in winter are just two of the challenges that this presents. The general public is largely unaware of the accompanying fundamental changes in the power grid: whereas the generators of traditional large-scale power stations – that is, of hydroelectric, coal-fired and nuclear power stations – previously kept the grid stable with their simple and sluggish mechanisms, there is now a need for electronically controlled converters. Protecting these converters from grid malfunctions such as voltage dips and short circuits is by no means an easy undertaking. Now, the group led by Florian Dörfler, Professor of Complex Systems Control at ETH Zurich, has provided a solution.

First of all, it is important to know that the electricity flowing through Europe’s power grids is based on alternating current technology, meaning that the direction of the current reverses every hundredth of a second. This frequency is established by the generators in large power stations, which are synchronised with one another via the grid.

On the other hand, wind and solar power plants produce direct current, which has to be converted into alternating current by converters. Today’s converters adapt to the grid frequency and inject their power in sync with it. This approach works as long as there are enough large power plants with turbines operating in the grid. However, if an increasing number of coal-fired and nuclear power stations go offline in the future, these timing generators will be lost – and a replacement will be needed. “You can only adapt to a frequency if one has been established in the first place,” says Dörfler.

Radical protective mechanism

In the future, there will be a need for grid-forming converters – that is, converters that do not simply follow a frequency, as is the case today, but rather actively help to stabilise it. Until now, engineers did not have a viable solution for how these grid-forming converters could continue to operate in the event of a short circuit or a voltage dip in the power grid while also being protected against overloading.

Today’s converters have a protective mechanism which ensures that they disconnect from the grid in the event of a grid malfunction. This protection is necessary because, if there were to be a large voltage dip in the power grid, the converter would attempt to compensate for the missing voltage by injecting a high current. This would overload the converter and damage it irreparably in the space of milliseconds.

With new algorithms for intelligent control, Dörfler’s group has now succeeded in continuing to operate the grid-forming converters even in the event of a grid malfunction. A rigorous shutdown is no longer necessary. This approach allows a wind or solar power plant to remain online, continue supplying power, and therefore contribute to stabilising the grid frequency even in the event of a grid malfunction. Accordingly, the system can assume the role currently performed by traditional large-scale power generators.

The converter’s controller measures the grid parameters continuously and adjusts the converter in real time via a feedback loop. ETH Zurich has applied for a patent on the new algorithms.

Master’s theses in industry

The initial idea came from one of Dörfler’s Master’s students, who is now doing a doctorate at ETH: Maitraya Desai realised that, in the event of grid malfunctions, it is best to deal with the grid voltage and the frequency of the alternating current separately. As it is difficult to maintain the voltage in the event of a grid malfunction, the new control algorithm focuses on the frequency and attempts to keep it stable in the grid under all circumstances. At the same time, the algorithm limits the current to avoid overloading the converter – while allowing the voltage to vary freely.

After first carrying out calculations, the ETH researchers checked these calculations in computer simulations and finally in a small test system in the lab. As the improvements relate purely to software, there is no need for industry to build demonstration systems. Rather, it can incorporate the algorithms directly into its control software. Dörfler is planning to work closely with interested industry partners to this end. For example, the aim is for ETH students to do their Master’s theses at industrial companies, thereby helping to implement the new approach in industrial partners’ products.

“We and others have been researching this field for 15 years,” says Dörfler. “Our approach is currently the best way of solving the problem.” The new algorithms contribute to the stability of the power grid, reduce the risk of blackouts and pave the way for a transition from large, centralised power generators to a decentralised, flexible system of smaller power stations supplying renewable energy. Accordingly, they could represent a key building block in the energy transition.