The pan-European power grid is experiencing a rapid upheaval in the nature of its energy supply side. An ever-increasing penetration of Variable renewable energy (VRE), such as wind and solar energy, pose new challenges regarding the grid stability and security of supply. Without further measures, the fluctuating and non-dispatchable nature of these VRE would turn them, in most of the cases, unsuitable for providing the ancillary services (AS) needed for ensuring a safe and reliable operation of the power system.
Today’s supply infrastructure is reliant on rotating synchronous generators which play a fundamental role in ensuring the transient stability of the power system. By means of their rotating inertial mass, they inherently provide inertia to the grid. In case of sudden frequency deviations occurring, the tendency of rotating mass to maintain rotational speed is significantly contributing to stabilising the system. However, as the modern power grid is gravitating towards a converter-dominated system, these must also be able to replicate this characteristic, by means of a “synthetic inertia”.
While energy storage systems are capable of significantly satisfying the high demand for synthetic inertia, their ability to offset the intermittency of renewable energy sources is turning them into a key technology for the power grid of the future. Among the different energy storage technologies, pumped hydro storage (PHS) can be identified as particularly convenient, given its cost-effective implementation and considerable lifespan, in comparison to other technologies. Europe ranks second worldwide in installed hydro power capacity, only after Asia, thus underscoring the vast application field of using low-head PHS as a means of stabilising the system.
PHS is based on storing energy in the form of gravitational potential energy of water, pumped from a lower elevation reservoir to a higher elevation, which makes this technology only available provided suitable topographic conditions. The EU-funded ALPHEUS project (Augmenting Grid Stability Through low-head Pumped Hydro Energy Utilization and Storage) will improve reversible pump/turbine (RPT) technology and adjacent civil structures needed to make PHS economically viable in shallow seas and coastal environments with flat topography. This RPT will be coupled with a Grid-forming controlled converter, which can control voltage and frequency. Thus, contributing to stabilising the power grid.In order to maintain stable system frequency, the frequency control consists of sequential balancing reserves within a definite time frame, the typical frequency control measures nowadays in the European Network of Transmission System Operators for Electricity (ENTSO-E) area are as shown in Fig. 1.
The system inertia in Fig. 1 in today’s grid from the directly-coupled synchronous generators could be replaced by introducing a new operating reserve “synthetic system inertia” (from inverters/storage) and fFCR from the low-head PHS system developed in ALPHEUS to equalize the power difference until frequency containment reserves (FCR) is activated.
For this reason, a grid-forming-converter coupled with the low-head PHS system developed in ALPHEUS could significantly contribute to stabilising the grid stability by providing different AS, especially frequency control through the provision of synthetic system inertia, as well as fast frequency containment reserves (fFCR).
ABOUT THE AUTHORS
Bernd Engel is a Professor in the field of Components of Sustainable Power Systems at TU Braunschweig, Germany since 2011. He was previously Senior Vice President Technology at SMA Solar Technology AG, a producer and manufacturer of inverters for photovoltaic and storage systems. At the elenia Institute for High Voltage Technology and Power Systems, he and his working group are researching smart grid infrastructure and the grid integration of renewable energies. One of the main research areas is providing ancillary services through renewable energies to the grid as well as the control of inverters and their effect on grid stability. As a board member of various committees forming future national standards, Bernd Engel contributes his expertise in the field of grid integration to the project. He is the leader of the work package 6 “Grid Integration” of the ALPHEUS project.
Mohammed Qudaih received the Master of Science in Electrical and Electronic Engineering from University of Duisburg-Essen, Germany, in 2017. He is currently a research associate in elenia Institute for High Voltage Technology and Power Systems at TU Braunschweig, Germany since 2020. His research interests include active distribution grid, electric grid dynamics and stability. He is working on work package 6 “Grid Integration” of the ALPHEUS project.