TSO-DSO Flexibility: towards integrated grid control and coordination in CH
The proliferation of distributed energy resources connected at the medium and low voltage grids is essential to reach the CO2 targets. Simultaneously, large thermal generators connected at the high voltage level may be decommissioned or only available intermittently. As a result, the traditional source of power system flexibility from conventional generators (ramping up/down, active/reactive power) decreases. In the meantime, the increased stochasticity of intermittent resources such as PVs as well as newly electrified demand such as heat pumps and e-mobility requires the overall system to utilize even more flexible than today.
This project focuses on the new form of flexibility essential for future power system operation: services offered by small distributed energy resources to the TSO (excluding the service to the DSO). The flexibility aggregation is assumed to be performed at the TSO-DSO substation by the distribution utility or an aggregator. We propose an automated estimation approach of the resulting flexibility range. The benefits of utilizing the aggregated flexibility are investigated and demonstrated in the context of Switzerland for ancillary services and transmission system operation. By combining realistic operational data with real grid data, the outcome of this project contributes to the efforts by the TSO and the DSO to achieve a more integrated operational framework.
- Expected increase in availability of distributed and renewable techonology assets
- Integration of all assets is in itself challenging, but by implication there are also increasing opportunities for distributed resources to provide services to the high-voltage network
- Interest in assessing the potential of benefits achieved through various levels of coordination between the TSO and DSOs in Switzerland
The expected shift toward distributed and renewable electricity supply poses challenges to both the transmission system operator (TSO) and distributed system operators (DSOs). However, along with this shift there are also increasing opportunities for distributed energy resources and operators to coordinate with and provide services to the centralised high-voltage network. This project will assess the potential of benefits achieved through various levels of coordination between the TSO and DSOs in Switzerland.
To assess the benefits of utilizing flexibilities that can be potentially offered by distributed energy resources such as small-scale solar PVs, residential BESSs, residential electric heat pumps and personal EVs in medium- or low-voltage distribution grids, a framework is developed from two perspectives: bottom-up and top-down.
The bottom-up perspective involves three aspects.
- First, we create scenarios and utilization patterns for a representative low voltage grid and identify the availability boundaries of each distributed energy resource.
- Secondly, we calculate the maximum feasible flexibility that can be offered by each distributed energy resource, aggregated at the transformer station using AC optimal power flow and maximize the "additional'' positive/negative active and reactive power exchange at the transformer station around the operating point, while respecting the distribution grid thermal loading and voltage constraints. The result is a time-series of maximum "additional'' flexibility in each direction for active and reactive power, constituting a "flexibility region'' around an operating point at the transformer station.
- Lastly, we identify a remuneration scheme for each distributed energy resource, so that the identified "flexibility region'' is associated with a cost.
The top-down perspective refers to assessing the benefits of utilizing the aggregated flexibilities provided by a large number of distribution grids throughout Switzerland in two case studies.
- First, for frequency (e.g., secondary) reserves, we formulate a co-optimization problem for the simultaneous dispatch of energy and reserves to quantify the impacts on the generation fleet and system costs when the flexibilities are offered to the reserve market.
- Secondly, in daily operation, we study how aggregated flexibilities can relieve the required power generation by hydro dams so that the hydro plants are less constrained during the winter months. The problem is formulated as an hourly AC optimal dispatch.
The economic attractiveness and the utilization impact of aggregated flexibilities to provide services to the electricity market through offering their flexible active power capability (positive and negative) as a "capacity reserve'' and as a "generator'' in Switzerland are demonstrated with a number of scenarios covering different local constraints (e.g., proliferation levels, remuneration amounts, etc.) conditions and different international system conditions. Conclusions present the boundary conditions, and the parameters that influence the potential benefits of utilizing the aggregated flexibilities.
In summary, our take-away messages are as follows:
- The flexibilities of each distributed energy resource can be aggregated at the transformer station so that the aggregated response can be offered as a service in TSO markets if a remuneration scheme is in place, and the services respect the distribution grid constraints. The amount of the aggregated response is highly dependent on the layout of the distribution grid, the season, the time of day and the proliferation level of the small distributed energy resources, especially BESSs.
- The utilization of the flexibilities as frequency reserves decreases the overall system dispatch cost. The ramping down flexibility offered as downward reserve (provided by charging BESS, increasing conventional demand, turning on HPs, and charging EVs) has the greatest potential because these distributed energy resources together have significant available power during hours when their offer price is below the reserve price. The ramping up flexibility as upward reserve is not often price competitive, given the assumptions.
- Utilization of aggregated flexibilities at all load buses throughout the Swiss transmission grid results in more efficient network utilization, subsequently reducing required hydro dam generation requirements (relevant for "winter reserve''), especially following the phase-out of nuclear units and in presence of high shares of solar PV generation.
The overall results address the key areas of TSO-DSO interaction in general, but are particularly relevant for the future development towards an integrated power system operation in Switzerland. Increased TSO-DSO coordination using the approaches developed in this study can mitigate the impact of important current and mid-term challenges, such as the dynamic and stochastic profiles from growing PV and EV capacities across Switzerland, decreasing conventional generation, and potentially reduced transfer capacities from Europe.
[Tool: in-house FlexOPF]
