## 7 Discussion and Conclusions  
The growing penetration of RES at each voltage level and the progressive use of electricity for applications normally covered by other energy vectors, namely natural gas, are evident trends. Thus, more flexibility in services from distribution systems will be necessary to balance generation and consumption with adequate security margins. Markets for services should change to allow new resources to participate, and the interaction and the communication between the TSO and DSO at the TSO/DSO interface will have to be modified accordingly. The interaction between the DSO and TSO can imply blocks or augmented costs caused by the impact of increasing/decreasing power generation/consumption on distribution systems to allow DER to offer services for balancing and reserve. The paper proposes a quantitative analysis of such interactions with particular reference to the impact on the operation of distribution systems (i.e., voltage regulation and power flows) of TSO requests for services. The final goal is quantifying the flexibility that the TSO can procure from the distribution system without a harmful impact on the distribution network operation. The paper investigates the expected interactions between the use of flexibility for power system balancing and security and the operation of distribution systems. The result of this methodology is the price/quantity curves for upward and downward offers from a given (modelled or real) distribution network. The extra costs possibly sustained for the flexibility products or the blocks imposed by the DSO for avoiding harmful impacts on the distribution network operators are also outcomes of the methodology. The main result of the study is that although the distributed energy resources have been connected under the hypertrophic conditions generated by the fit and forget approach, distribution systems are not robust enough to accept changes in DER production/consumption without any operational action. In particular, voltage regulation is the most common cause of block that inhibits TSO from using flexibility or that requires DSO to change reactive power provision by DERs (that means extra costs). This is an interesting result that can be considered general validity because it is obtained by applying the proposed methodology to a test network composed of representative portions of the distribution system and reproduces a realistic or synthetic model of a real network (e.g., the DG penetration level is corresponding to the real scenario and the delivered energy to the loads is relevant to the share of the real demand of the province on which is located the primary substation). In addition, while the extra costs depend on the hypothesised regulatory and market frameworks, the possible harmful impacts on the distribution network operation rely only on the defined location of the DERs along the network lines. Thus, for obtaining general validity results, many and different scenarios of DER location should be simulated to assess the expected maximum and minimum feasible flexibility. However, if one possible scenario, such as the one presented in the paper, produces the reported impact on the network operation, it is reasonable to expect that a harmful impact caused by the uncontrolled use of flexibility may exist, and, with the proposed procedure, it would be predicted. For answering the questions formulated at the beginning of this paper, we can observe:* It is necessary to carefully assess to what extent can a TSO exploit flexibility without causing issues at the distribution level. Such extent depends firstly on the number of the DERs intentioned to participate in the market, and secondly on the specific position of the resources in the network. The more DERs participate in the market, more quantity can be managed, but the increasing potential does not necessarily increase the same quantity in the feasible flexibility that can be moved without harmful impact on the network operation.