## 1 Introduction
Hence, the suggestions for the measurement of the proposed AS, particularly those associated with transient or dynamic DRES responses, are largely based on these standards. Since several AS are examined in this paper, detailed reviews and the relevant state-of-the-art with respect to their M & Q are presented in the rest of the manuscript in the respective Sections. The established AS in the transmission system level formed the basis for the definition of new AS offered by DRES connected in the distribution system. Detailed reviews of the main AS that already exist at the transmission system level with respect to their definition per country, the PPs that provide these AS and their remuneration can be found in [1, 6, 18]. The remainder of the paper is organized as follows: In Section 2 of the paper, the background and assumptions for the definition of several new AS that can be offered by converter-interfaced DRES connected to the distribution system are presented. The new AS addressed in this paper are: (1) inertial response; (2) primary frequency response; (3) active power smoothing (ramp-rate limitation); (4) exchange of reactive power for voltage regulation; (5) Fault-Ride-Through (FRT) and contribution to fault clearing; (6) voltage harmonic mitigation. In Section 3, for each of the aforementioned individual AS: (i) a brief overview of the state-of-the-art with respect to technical literature and Grid Codes is conducted; (ii) the theoretical and mathematical formulation of the new M & Q method is proposed and the challenges with respect to its practical implementation are identified; (iii) the associated costs and benefits are roughly estimated and analyzed in order to form the basis for the development of respective business models. Some technical barriers towards the development and future adoption of the suggested AS are briefly presented in Section 4, while Section 5 presents the conclusions. Background and Assumptions for the Introduction of New AS Offered by the DRES at Distribution System Level  
Currently, the frequency and non-frequency related AS are procured and remunerated by conventional PPs directly connected to the transmission system level. National TSOs define various AS in their own way, especially with respect to the frequency-related AS. Several differences exist also with respect to: (i) the type of source that provides the AS; (ii) the obligation (mandatory or optional AS provision); (iii) the fundamental values that are being remunerated; (iv) the remuneration and procurement schemes. However, the inertial response is still considered as a system support function, since it is provided by the inherent inertia of the conventional SGs, and, for this reason, it is not remunerated, [19]. In addition, recently published Standards and Guidelines [20] require the large-scale variable-output DRES that are directly connected to the transmission system, to offer frequency or voltage support as system support functions, which are also not remunerated, as in the previous case. However, in order for a distribution system to be able to provide certain AS, the DRES within it need to be intentionally controlled, leading in many cases to curtailment of active power (operation with a headroom) that would be otherwise injected into the grid, and in other cases to increased operational cost due to reduced efficiency and additional losses (e.g., when exchanging reactive power [21] or operating below a certain power level [22]). Therefore, it is necessary to accurately measure and quantify all these frequency and voltage support actions in order for them to be treated as AS. This section presents a new approach on how to M & Q these services. This applies to services offered to the distribution grid at the terminals of the DRES and to services offered, in aggregated form, to the transmission system at the POI with the distribution grid.