## 1 Introduction  
Ancillary services (AS) are defined by transmission system operators (TSOs) as services necessary for the reliable operation of the power system, [1] or needed to ensure security of supply, [2, 3]. According to [4], which is a common report by the European Network of TSOs of Electricity (ENTSO-E) and several European Union (EU)/distribution system operators (DSO) entities (E.DSO, Eurelectric, etc.) AS are defined as services provided to DSOs and TSOs to keep the operation of the grid within acceptable limits for security of supply and are delivered mainly by third parties (with respect to the control of the source) or by the TSOs and DSOs themselves. The currently available AS, in all European synchronous areas, have mainly as a target to keep frequency and voltage within specific safe bands and restore their values to the normal range after an imbalance occurrence. Therefore, these AS mainly refer to frequency response, voltage control through reactive power support and black-start capability, [5] and can be classified as follows [4]: (i) frequency AS (mainly for balancing); (ii) services for congestion management; (iii) non-frequency AS, such as voltage control and grid restoration. Frequency-related AS are further distinguished as frequency containment reserves (FCRs), frequency restoration reserves (FRRs) and replacement reserves (RRs). Currently, these AS exist only at the transmission system level and are usually remunerated (by the TSO or the respective AS market) following various approaches such as, [6; 7]: (i) a regulated price: in this approach a fixed price is set by the regulator or the TSO; (ii) a pay as bid price: the AS provider receives the price of the accepted offer; (iii) a common clearing price: the successful sellers are paid the price of the most expensive accepted or the least expensive rejected offer. The AS remuneration includes several components that reflect the different costs of the providing entity, e.g., the fixed allowance and the availability price refer to the cost of the seller to make a specific amount of AS available, costs regarding the actual exploitation of the product and the one that may rise each time that the provider is called upon (in a specific period of time), respectively, i.e., the opportunity cost. For this reason, there are AS markets operating in parallel with the operating energy market. Currently, these AS are mainly provided by synchronous generators (SGs) driven by conventional fuel-driven power plants (PPs). It is noted that the aforementioned AS belong to the steady state or quasi-steady state condition of the transmission system. Therefore, their measurement and the subsequent quantification is easily made by conventional measuring devices that need to measure active or reactive power and energy. It should be mentioned that currently the inertial response, which belongs to the dynamic and transient area of operation, is considered an inherent property of SGs and therefore, is neither measured nor remunerated. The increasing DRES penetration is expected to displace in the future the conventional PPs. This is actually an EU target that currently drives many research activities [8]. However, decommitment of conventional PPs will lead to the reduction of AS which are essential for the power system stability and safe operation. To mitigate this problem, three options are currently investigated: (i) the use of conventional PPs as providers of AS, for instance for provision of inertial response or reactive power to the TSO or as spinning reserves [9], (ii) use of utility-scale storage systems operated by third parties, i.e., other than the TSOs and DSOs, (iii) aggregation of those AS (and additional ones) from DRES connected in the distribution system level. Obviously, the last option represents the most decentralized approach and indirectly transforms the distribution systems into virtual power plants (VPPs) as seen from the transmission system.