Grid restoration with renewable energy
The issue of security of supply is a recurrent one in public discourse. Specifically, discussions about the consequences of the energy transition in public reporting and the major blackouts in recent years explain its presence in the public eye. The report of the Office of Technology Assessment at the German Bundestag (TAB) shows the serious consequences a prolonged power cut would have. It highlights an acute need for action. In addition, grid restoration is becoming more complicated due to the fluctuating, uncontrollable feed-in from distributed generation systems in the event of a power cut. This leads to the need for an adjustment of current grid restoration strategies.
The aim of the research and development project LINDA is to develop strategies whereby some effects of the TAB report could be avoided. In the event of a major failure, decentralised black start-capable systems with a secured minimum power output could initially restore the critical infrastructure, which includes operational centres, hospitals and prisons — as well as the self-supply of important transformer stations in local 20-kV isolated grids. The problem: during this initial restoration of the grid, whole medium voltage ring networks and any connected consumers and generators such as biogas plants and photovoltaic systems are supplied with power. While this may sound good, overproduction in particular can lead to a situation in which the power balance becomes uncontrollable. An instable grid is the result. Another problem could be that communication fails to work in the event of a failure, in which case energy generation management no longer works either.
One possible solution is to raise the fundamental frequency of the balancing power plant in isolated network operation in a targeted manner. Thus, small plants would not start up or operate within the control range of the P-f characteristic curve. It is important to check whether power plants could do this permanently at varying loads. In principle, the additional production could even have a stabilising or relieving effect on supply due to its low inertia. However, systems with an accordingly hard shutdown at different frequencies are less well suited, as is the share of older systems exempted from the obligation to retrofit required by the System Stability Ordinance (SysStabV).
The connection behaviour of systems with the power gradient limit is also to be considered, as they may cause power oscillations with corresponding stability problems. The critical size is the unknown number of generation systems of different generations and therefore with differing generation behaviour.
Robust control for the entire grid
As part of the LINDA project and in an effort to research a suitably robust control concept, the engineers are reproducing the overall dynamic grid behaviour along with the different mechanisms of electricity generators and consumers in a simulation. In addition to the control properties of hydroelectric power plants, the behaviour of any distributed electricity generation systems that might be connected to the isolated grid, such as biomass and cogeneration plants and photovoltaic systems, is replicated in the simulation. Following that, they will have to develop a new control concept and integrate it into the controllers of the plants and systems. The control concept ought to be stable over a wide working range to avoid any need for individual adaptations to the local grid, or regular corrections of the control parameters when boundary conditions change. These could be, for example, grid expansion, a rising number of RES plants, or the installation of storage systems, including those in electric vehicles.
The respective technology should be designed in a way so that it complies with the technical guidelines set forth in FNN and ENTSO-E even during normal operation, thereby making a full contribution to system stability. Furthermore, protection concepts must be developed for isolated grid operation due to the greatly reduced short-circuit power. The foundation is an analysis of a non-selective, or only partially selective, protection against risks arising from such a situation. These protection concepts need to be activated by the grid control system upon detection of isolated grid operation.
Core areas of the project
- Rules for the reliable operation of isolated grids without own communication infrastructures in the event of major failures in order to supply critical infrastructure via the existing grid with a high proportion of decentralised, grid-coupled generation systems.
- Application of the findings to the further development of grid restoration concepts for grids with a high penetration of distributed generation.
An essential component of the project is the field demonstration of findings gained in the simulation. For this purpose, an existing hydropower plant of Bayerische Elektrizitätswerke was adapted to isolated grid operation. There are several biogas plants and villages with many small PV systems and disconnectable consumers in the immediate vicinity.
Using this particular grid, three consecutive experiments are supposed to yield a demonstration of stable isolated grid operation in an emergency event. The findings of each experiment will be added to the simulations. They serve to improve and validate the theoretical models.
08/2015 – 07/2018
LEW Verteilnetz GmbH - Netzplanung
"Modern grids as a key element of a sustainable energy supply" (German)