The power grid is changing dramatically with the energy transition. Kiel-based scientists are working together in a DFG priority programme to make the power grid of the future flexible, stable and intelligent.
As fossil-fuel-based electricity generation is being phased out, renewable energies are becoming increasingly important. Even if their development has stalled somewhat recently, society will almost certainly not be able to manage without photovoltaics and wind farms if the worldwide hunger for energy is to be satisfied. For the power grid, this means the greatest change in over 100 years. After all, not only is there increasingly a move away from large coal-fired and nuclear power stations towards small and local power generating systems such as wind farms and solar parks, but the grid must also be capable of handling both green energy, more and more of which is being supplied as DC, and the AC produced by power stations. Digitisation allows electricity to be controlled intelligently.
However, nobody actually knows exactly what the power grid of the future will really look like. Scientists from 15 research institutes have come together to develop concepts and models for this, including TU Dortmund University, which is responsible for coordination. In the Priority Programme 1984 of the German Research Foundation (DFG), which is scheduled to run for six years, they are searching for solutions for an intelligent, decentralised, hybrid DC-AC power grid that is capable of incorporating various energy systems. Professor Marco Liserre, an internationally renowned specialist in power electronics, and his doctoral researcher, Zhixiang Zou, are lending their expertise on behalf of Kiel University here.
In a German-Swiss cooperation with the EPFL in Lausanne, Liserre and Zou are focusing on the interfaces between the wind farms/solar plants and the power grid, the so-called rectifiers, which convert DC to AC and vice versa. These represent a potential weak point in terms of grid stability. The problem revolves around potential interactions between the rectifiers and conventional grid infrastructure components, such as transformers, which are not only difficult to predict but also have a negative impact on voltage quality in the power grid. "Rectifiers require a far more complex control system and communication," explains Liserre. "They therefore interact differently than we are used to seeing in the conventional power grid." To counteract these adverse effects on grid quality caused by green energy, we first need to gain a better understanding of the characteristics displayed by rectifiers and what they are capable of.
This is precisely the task that researchers set themselves in the first three-year phase of the project, which is now coming to an end. Using a virtual island grid that Liserre and his team set up in their real-time simulation laboratory at the Faculty of Engineering (referred to as a "microgrid"), they modelled various problems, which were then tested on a genuine small grid in Lausanne. They have already been able to celebrate several interim successes: "We now have a better understanding of the interaction between rectifiers and the power grid. As such, we can more accurately predict how the devices will react, for example to harmonics," comments Liserre. Harmonics are additional frequencies that are integer multiples of the mains frequency of 50 Hertz. They occur in transformers, yet also in the power supplies of televisions and computers, where they can compromise the functionality of these devices or even completely destroy them. The researchers are also working on the problem of voltage drops caused by grid perturbations or lightning strikes.
Liserre and his Swiss colleagues are planning active measures for the next three years of the priority programme. The key objective here is to be able to manage the control options for rectifiers. The Kiel-based researcher compares the work with that of a mobile phone: "We are not looking to change the smartphone in itself, but rather to develop an app that allows its full potential to be exploited.” At the end of the programme, it should be possible to make the transition from a previously passive energy system to a controllable, flexible system. Another objective is to make this system more resistant to extreme weather episodes, natural disasters and terror attacks, as the components should be able to react both actively and quickly in such cases. Marco Liserre explains the importance of an intelligently controlled – or even self-controlling – power grid: "This would not only allow the energy transition to deliver environmentally friendly electricity, but also offer greater security for society at large.”
Author: Denis Schimmelpfennig