Germany´s long-term goal in energy policy is to increase the renewable electricity (RE) generation until 2050 to a share of at least 80 % of the gross electricity consumption. This has two major consequences for the transmission system:
On the one hand, renewable generators in rural areas replace conventional power plants next to urban load centers. Due to the decommissioning of large-scale conventional power plants the transmission distance between generation and consumption sites increases.
On the other hand, most of the renewable electricity will be provided by wind and solar power plants. Their expected installed capacities are high. Taking into account their volatile generation profiles, very high but rare generation peaks are expected as well. If the feed-in priority of renewable generators remains, those peaks will present a heavy burden on the transmission system.
Following the current planning principles an oversized grid expansion would then be required. The resulting target networks would comprise long lines with low utilization factors. The following options could counteract this trend:
- the use of new more efficient transmission technologies
- the consideration of re-dispatch measures in the network planning stage
- the willingness to curtail renewable generators, even if there is enough load available
To limit the required grid expansion, different transmission technologies will have to be applied in the future transmission system. As a consequence, network planners will have to consider more decision variables than today. That’s why they need robust decision criteria (design criteria) to help them select the most appropriate technology for any transmission function of the grid.
The objective of this research activity is to develop design criteria for electrical transmission systems with a large share of renewable generation. In the process of transmission system planning these criteria shall support decision making when choosing the most appropriate transmission technology.
The following subtasks have to be completed:
- Development of performance indices for transmission networks
The transmission system has to fulfill several functions at the same time. This includes the ensuring of costumer’s supply, the facilitation of electricity trading, the ensuring of an economical dispatch of conventional power plants, the integration of renewable generation and the preservation of system integrity. It will be studied, whether a differentiated evaluation of these network functions by performance indices is possible and useful. These indices shall be used later for the evaluation of possible target grids.
- Development of an extended system planning process
Based on an economic optimization, an extended and automated planning process shall be developed. This process will be given a scenario, a start grid and a transmission technology for each additional extension measure to be performed. The process determines an extended grid, with minimum total system cost. The total system cost comprise capital costs, costs of losses, re-dispatch costs and curtailment costs for renewable generation units.
- Development of a scenario with a large share of renewable generation
To let the extended system planning process generate target grids that are able to integrate a large share of renewable generation, a suitable scenario must be defined in advance. This scenario needs to be created. It shall be based on the long-term objectives of the energy and climate policy in Germany and the EU.
- Development of a start grid
Based on publicly available grid maps and on literature values to determine the parameters of grid components, a central European transmission system model is designed. This model includes the 2013 German transmission system and those of Germanys neighboring countries. Based on current power system development plans, this network model shall be extended with all planned lines until 2023 and eventually be used as a start grid for further transmission system planning.
- Research of alternative transmission technologies
The extended planning process developed shall generate target systems for different transmission technologies. To specify the line parameters of those transmission technologies an intensive literature research will be conducted. In addition to classical overhead lines realistic component parameters for high voltage direct current transmission (LCC and VSC), gas-insulated lines and superconducting cables will be determined.
- Development of a process to optimize the grid design
After all target networks have been planned for the defined scenario, the network design shall be optimized. For this purpose a process has to be developed. This process will be given two target grids: The first target grid will comprise only classical 380-kV-overhead line technology while the second one will comprise also a new (alternative) transmission technology. By eliminating a branch of an alternative transmission technology in the second target network and replacing it by one or more 380-kV-overhead line circuits, a mixed grid model will be generated and the total system cost of it will be determined. The total cost of the mixed grid model will be compared to the total system cost of the previous target grids. It is expected that its total system cost are lower than those of the two previously specified target networks. The elimination and replacement process is repeated until the total system cost of the mixed grid models no longer decrease.
- Derivation of the design criteria
The process described above is performed for each pair of target grids applying 380-kV-technology vs. an alternative transmission technology. Thus, the most cost-effective mixed target grid models will be determined. By analyzing the resulting grid structures, design criteria will be derived. These design criteria are intended to create (approximately), the same optimal target grids in a simple way when applied by network planners.