## 1 Introduction
The most significant advantage of this solution is that even the smallest DRES owners can become active participants in the operation of the power system. Additionally, the decentralized approach-when based on DRES- enhances significantly the security in energy supply. Realizing that the DRES, particularly the converter-interfaced ones, can have functionalities that could support the power grid, some recently issued grid codes and standards ask for the provision of various services by the DRES in the form of system support functions [10; 11; 12]. Examples of such services are: the provision of reactive power for voltage regulation purposes, the FRT capability, the injection of specific currents during a grid fault and the limitation of the rate of change of the electrical power of DRES (ramp rate limitation). It is noted that, by considering those services as system-support functions, their provision becomes mandatory in order for the DRES to get connection permission. Thereby, it is evident that no motivation is granted to the DRES for offering such services, despite the fact that some of these services imply a direct or indirect cost to the DRES owner; for instance the provision of reactive power may result in active power curtailment if the RES converter is not oversized. However, the TSOs of some weak transmission systems, e.g., Puerto Rico [13] and Ireland [13; 14; 15], have issued new requirements for the DRES directly connected to the transmission system. These requirements have the form of system support functions after realizing the consequences of the increased DRES penetration and the subsequent decommission of conventional SGs in those weak grids. The recent advances in power electronics have enabled the converter-interfaced DRES to exhibit new functionalities that are useful to the distribution grid and the transmission system: (i) they may provide virtual inertial response, just like the conventional SGs, in an energy-neutral way if they are equipped with fast energy storage systems (FSS), like supercapacitors. In such cases, the inertial response can be offered even when the primary energy sources (sun or wind) are unavailable; (ii) they may be controlled to operate with a certain headroom, i.e., as FCRs. In fact, the slope of their P-f droop and the frequency deadband can be easily adjustable through setpoints sent remotely; (iii) by changing their control mode into voltage-controlled sources, the DRES can exhibit grid-forming capabilities, enabling, in this way, their participation in the black-starting procedure of distribution grids and even in their islanded operation; (iv) they can be controlled as active harmonic filters in order to mitigate voltage harmonic pollution within the distribution systems. Electrochemical energy storage systems (ESS), like Li-ion batteries, which are also converter-interfaced, can also provide all the aforementioned functionalities apart from inertial response, as this stresses the batteries in a rather unacceptable manner. Some differences also exist between the variable-output DRES -like photovoltaics (PVs) or wind energy conversion systems (WECS), which are converter-interfaced and the dispatchable DRES--like hydro or biomass--which are directly coupled to the grid via SGs. The latter can provide inherently inertial response due to the involved rotating masses, however, their actual inertia is hardly known. They can also act as FCR, however their governors are much slower than the respective reaction of the power-electronic converters. Furthermore, their reactive power capability is narrower compared to the converter-interfaced DRES due to limitations in rotor design or due to power-angle stability reasons. It is evident that the DRES can provide a number of useful services that can support either the distribution grid or transmission system if they are aggregated.