## 1 Introduction
With [2], the Italian Regulator had opened the AS market participation to the DERs connected to the distribution networks by starting pilot projects that included DERs in service market participation. From the early stages of the pilot projects, the Italian TSO publicised the results, and they seemed to be profitable for the entire power system [3,4]. More recently, the focus has been turned to the use of flexibility by the DSO that can purchase flexible products from aggregators and producers [5,6] for fixing distribution network issues. The TSO/DSO interactions have been investigated in many recent papers [7, 8, 9, 10, 11], proposing new market schemes. Several approaches promote the DSO participation, introducing local market schemes that need to be coordinated with the traditional AS markets [12,13]. A review of coordination schemes between local and central markets is presented in [14], in which the most common information exchanges between operators are analysed. Different approaches for coordinated optimal power flow (OPF) calculations are reported in [15, 16, 17]. The mentioned methods cannot guarantee feasibility due to the limited applicability and the extensive communication requirements. In the distribution networks, the number of DER can be very large: for this reason, some aggregation approaches and flexibility resources modelling can become more attractive. For example, in [18,19], some models to aggregate the flexibility of distributed residential loads are presented. The participation of an aggregator in the energy market is proposed in [20], modelled through a bi-level mixed-integer linear programming (MILP) problem. The optimal management of consumers' flexibility for aggregators in the day-ahead market was addressed in [21] by formulating a MILP model. The authorsof [22] propose a set of bidding optimisation models that support the participation of EV aggregators in multiple market sessions, such as energy and tertiary reserves. All the mentioned works proposed do not consider the technical distribution network constraints in the optimisation models. They assume that the DSO must be capable of solving all the eventual network problems that may arise from the flexibility offered. On the contrary, only more recent approaches [23; 24; 25] aggregating the DERs flexibility preserve the secure and reliable operation of the distribution network. In particular, in [23], an efficient grid-congestion management power flow algorithm with flexible assets is proposed. The paper [24] formulates an algorithm capable of providing flexibility to DSOs. The approaches suggested in [25] assume that the DSO quantifies the flexibility needed to solve the grid problem following their operating costs as a separate problem, and the aggregator assists it. Finally, Refs. [26; 27; 28; 29; 30] solve an OPF problem to estimate flexibility. OPF-based approaches seem to accurately identify the flexibility curve and, therefore, will be adopted in the proposed paper. As well as the research studies, some operative solutions, already implemented in real applications, have been proposed around the EU Countries. There are examples dealing with flexible products for distribution with other interactions between TSOs and DSOs. In the United Kingdom, an independent software company has recently developed a market platform on which DSOs can purchase, via long-term bilateral contracts, local flexibility services. Up to now, this platform has enabled over 300 flexibility providers with a flexibility volume of over 10 GW, and over 1200 competitions have been advertised by the platform users [31]. In Germany, market operators and energy service providers are collaborating on the ENergyERA (ENERA) project to develop a platform that efficiently allocates the local flexibility sources based on price signals. The market platform will be available, after the demonstration phase, to the system operators and the flexibility providers of the project consortium [32].