Flexibility Aggregation Perimeter for Ancillary Services in Radial Distribution Systems

The flexibility provided by prosumers is recognized as a key element for the management of the electrical grids, in particular distribution grids, in the changing scenario of electricity generation and consumption. The flexibility useful to Distribution System Operators (DSOs) is strongly local, and it is necessary to identify the buses of the grid where the flexibility can be usefully provided for a service requested by the DSO. To this end, the concept of flexibility perimeter is adopted; it is defined as a set of buses such that flexibility can be provided by any set of prosumers within it while the grid operation remains secure. The flexibility perimeter is a relevant concept for flexibility markets; in particular, for aggregators, as the flexibility provided by the perimeter prosumers can be offered as a single product to participants to electricity markets that require grid services. Identifying the flexibility aggregation perimeters for congestion relief services in a radial grid is straightforward; for flexibility services intended to solve voltage profile problems, the identification of perimeters is not easy, even in radial grids. The paper proposes a method for identifying and evaluating the flexibility perimeters for grid services in radial distribution grids, based on the concept of Load Area; it can help decision-makers to choose the size of the flexibility perimeter as a compromise between technical efficiency and market efficiency. For voltage support services, the size of the perimeter influences the technical soundness and the market efficiency of the aggregation in opposite ways; influence is assessed quantitatively. The results of two case studies show the features of the method.