## 1 Introduction
However, in order for these services to be treated as AS (and not simply as system support functions) they should be properly measured and quantified in a unified and commonly agreed (by all stakeholders) methodology. In addition, the costs associated with the provision of these services and the financial benefits should be identified, so that relevant viable business models can be developed. Furthermore, when dealing with AS offered to the transmission system by aggregating their origin from individual DRES within the distribution system, methods to M & Q them should be established for all nodes where the service is offered. This is essential for the proper remuneration of each DRES according to its contribution, while taking into account technical constraints imposed by the distribution grid. It should be mentioned that currently there exist no established AS market in the distribution system. Models of such AS markets can be found only in the technical literature (e.g., H2020 Project SmartNet [16]). However, these models focus on market structures without paying attention on the way the AS should be measured and quantified in order to become tradable quantities. Currently, the term \"ancillary service\" is frequently used for DRES in the technical literature, so as to describe their various functionalities, e.g., reactive power for voltage regulation purposes, virtual inertia, primary frequency control, harmonic mitigation or even power smoothing. Many of the aforementioned AS have been recently introduced in the literature and are still under investigation; hence, the absence of particular metrics for their quantification is a common problem. This applies at metrics regarding both the overall service provision at particular points of the network (e.g., at the transmission-distribution interface), as well as the contribution of each AS provider, as mentioned in [17; 18]. Therefore, it is essential to establish new quantification methods that will reflect the actual provision of every AS at the points of interest, with relatively high accuracy. The latter is important to enable AS that are offered at the distribution system to participate in future AS markets. This paper is an attempt to define a number of AS that can be offered by DRES connected in the distribution system. By definition it is meant: (i) the way an AS should be M & Q and, (ii) an initial, thus not exhaustive, identification of the financial costs incurred for providing this AS and the benefits that result from its use. Further elaboration with respect to detailed evaluation of the financial costs and benefits is required but this falls in the frame of the development of viable bossiness modelsaround each AS which is beyond the scope of this paper. Some of the AS are useful for the operation of the distribution grid, thereby they are procured by the respective DSO. Other AS, particularly those related to the stability of the power system, are aggregated and offered to the TSO at the point of interconnection (POI) of the distribution grid with the transmission grid. In such cases, aggregators may be involved for trading these AS, but the DSO should always be present in order to assure that the technical constraints within the distribution grid are respected. To the best of the authors' knowledge, there are no relevant references regarding the quantification of various AS offered by DRES connected in the distribution system. This is particularly stressed out in [18], where the lack of proper measurement and quantification is identified as one of the technical barriers that prevent the introduction of AS in distribution networks. The same is true with respect to the financial costs and benefits associated with each AS. For these reasons, the approach presented in this paper is all new. On the other hand, new standards that specify a number of system support functions for DRES give clues on the way these functions should be measured.