Researchers from the Fraunhofer Institute for Solar Energy Systems (ISE) create a replica small-scale power grid in order to analyse how synchronous machines and grid-forming converters are affected by voltage stability.
Large power stations with synchronous generators provide stability in the power grid. However, through the new clean energy transition that is underway, they are being successively powered down.
To address this issue, researchers from the Fraunhofer Institute for Solar Energy Systems (ISE) are analysing how grid-forming converters can ensure a future supply of sinusoidal alternating current and stable grid frequency.
The power gird, that helps supply energy to all of our household actually has a complex design, and its typical equilibrium levels are a delicate matter. The standard form for energy following through the power lines in Europe is a sinusoidal alternating current with an almost constant frequency of 50 hertz. This is made possible by the stability of the physical properties of synchronous generators in large power stations.
Synchronous generators in power stations.
These large power plants use their rotating mass to bring inertia to the system, thus reacting with an inertial response to grid disturbances. Potential deficits in energy generation can be compensated using the stored kinetic energy in the short term, therefore bridging the time until further protection measures can be implemented. Using this system, immediate far-reaching losses of power in critical situations such as in the unplanned outage of large generation capacities are mostly able to be avoided.
However, nuclear and coal-based power plants are now being removed from the grid and instead replaced by renewable forms of energy generation. “This means that the synchronous generators, which play a central role in grid control, will be lost,” explained Dr Sönke Rogalla, Head of the Power Electronics and Grid Integration Department at Fraunhofer (ISE). In grid-forming converters, he and his research team discovered a promising solution for ensuring grid stability.
Converters
Converters are electronic power devices that convert direct currents into alternating currents. They can vary in their appearance, depending on their power class, from a small battery up to a large megawatt power plant. Their electric behaviour must be set and regulated through specific control algorithms. At present, converters are programmed in order for them to successfully feed the desired amount of power into a stiff assumed power grid, provided by robust large power plants. Grid-forming converters, on the other hand, are programmed to behave as a voltage source. Similar to conventional power plants, grid-forming converters respond to short-term demand in the grid and provide inertial responses.
“For example, it is important that the devices in special cases such as overload situations, defective lines, or system splits respond in a reflexive manner and keep the grid stable,” said Roland Singer, Group Manager for Converter-Based Grids. “For this reason, we are performing research on the development of devices and algorithms. We can test various application scenarios with the aid of simulations as well as with the test infrastructure in our institute’s in-house Multi-Megawatt Lab in Freiburg.”
Grid-forming converters research project
According to Dr Rogalla, there is a consensus among the transmission system operators that grid-forming converters are essential for a large share of the new plants being put onto the grid. Due to this, the Fraunhofer ISE is collaborating alongside diverse power grid operators in an advisory role and, since 2017, and have worked with various scientific and economic cooperation partners in the “VerbundnetzStabil” project. “Here we have managed to put together a unique arrangement of skills from the power electronics and control division as well as from grid dynamics and grid integration. This allowed us to gain a comprehensive view of the use of and the precise requirements for grid-forming converters on a larger scale,” added Rogalla.
During the initial phase of this project, the requirements for future power grids were clarified and critical situations were defined. This laid the concrete groundwork for developing and programming devices alongside the converter producer KACO new energy. In the Multi-Megawatt Lab, the researchers were then able to replicate a small-scale power grid, using it to examine how the proportion of synchronous machines and grid-forming converters affected the voltage stability.
The results
“Our research clearly shows once again that a changeover from synchronous generators to grid-forming converters is a viable solution and the increasing urgency of such a changeover,” emphasised Singer. “At the same time, we were able to clearly lay down what the grid of the future will actually require and, with the help of a test guideline that we drew up, provide suggestions for important technical details for which no clear standard yet exists,” added Rogalla. “With the upcoming market launch of grid-forming converters, we want to thus help the industry in the technical evaluation of suitable devices.”
The final report for the project is still currently being completed. Simultaneously, the researchers intend to monitor their devices and findings on a real functioning power grid held in one of the institute’s office areas. In a further research project, which is currently being developed, the established technology will be implemented in a large photovoltaic storage power plant and grid interactions are to be examined under real conditions.
