**6. Conclusions**

The implemented model of the DRA and the performed simulations showed that distributed resources aggregated into the DRA structure are able to provide ASs together with CDGUs and therefore contribute to the improvement of the power system's operation. The establishment of the DRA structures, as part of the modern power system, enables small resources to participate in wholesale markets. The flexibility of the DRA resources facilitates load-generation balancing and allows for further RES development due to the mitigation of their intermittent operation. The variety of the provided services is strictly dependent on the DRA composition and legal regulations (e.g., the possibility of renewables curtailment).

The performed studies indicate that further research should consider how to determine the optimal set of DRA participants. The e ffectiveness of the service provided has an impact on the reduction of the volume of produced electricity and therefore reduces the basic market income for the aggregated resources' owners. The properly designated revenues for participants should encourage them to share their capabilities for the AS provision. Additionally, other types of services, e.g., operating reserves, could be considered.

The MILP formulation of the optimization model resulted in a short computation time.

**Author Contributions:** All authors developed the initial idea of providing ancillary services (ASs) by the distributed resource aggregator (DRA). A.L. and D.C. designed the DRA structure, relations with other entities and the optimization model. R.D. provided critical comments and research suggestions. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to express the gratitude to the FICO ® corporation for programming support and provision of academic licenses for Xpress Optimization Suite to Institute of Electrical Power Engineering at Lodz University of Technology.

**Conflicts of Interest:** The authors declare no conflict of interest.
