## 1 Introduction  
Renewable energy is expected to be the main source of electricity generation in the next few years [1]. This change is expected to rely on an increasing amount of generation units being connected to the distribution grid, widely designated as Distributed Generation (DG) [2]. Alongside, the transportation sector is expected to move forward towards electrification as well. Electric Vehicles (EVs) are expected to have an increasing share in the global system load [3], requiring proper control and management strategies for charging their batteries [4; 5] in order to mitigate adverse impacts that may result in the operation and control of the electric power system. The deployment of large-scale Renewable Energy Sources (RES) also has system-wide and localized impacts in terms of electric power system operation and control practices. Challenges regarding frequency regulation, voltage regulation and inertia provision are among the most important adverse impacts being addressed [6; 7; 8; 9; 10]. In particular, the power-frequency regulation adequacy in systems dominated by converter-interfaced generation systems is becoming a key concern. The wide nature of the aforementioned challenges has led to a push for the design of specific grid codes as well as advanced grid management solutions in order to mitigate the resulting impacts [11; 12; 13]. Within the domain of smart grids, pioneering concepts such as microgrids and more microgrids are widely regarded as key elements to address the need to increase the controllability and observability of the electric power system in the face of the expected large-scale deployment of RES and EV [14; 15; 16; 17; 18]. AC microgrids initially attracted attention for historical and technical reasons [14; 15; 19], with several implementations existing already worldwide [14]. However, important advancements in power electronics have attracted an increasing amount of attention to DC distribution microgrids. DC microgrids are better suited to integrate DC resources such as PV generation, EV charging stations and Energy Storage Systems (ESS) [20; 21]. In this scope, the Smart Transformer (ST) is being envisioned as a suitable solution to increase the controllability and observability of distribution networks [22; 23; 24]. The ST is based on the adaptation of the three-stage Solid-State Transformer (SST) concept for the provision of advanced control functionalities to the distribution grids, in detriment of reductions in weight and volume [22; 23]. The DC link connectivity offered by the ST enables the development and controllability of hybrid AC/DC distribution networks, thus offering the possibility to exploit the microgrid concept based on hybrid AC/DC networks [22; 23; 24]. A possible approach regarding the exploitation of the ST as an enabler for the autonomous operation of AC distribution grid sections under the multi-microgrid concept is addressed in [25]. Moreover, the ST creates opportunities in terms of local and direct integration of ESS with hybrid AC/DC networks, which represents a suitable solution to address the local RES variability and underlying local impacts, such as in voltage control [26]. However, among the opportunities offered by the ST in terms of controllability and observability of hybrid AC/DC distribution networks, important research gaps are still to be addressed. An important research gap identified in the literature is the absence of comprehensive studies addressing the provision of power-frequency regulation services to the upstream AC system by exploiting the flexibility available in hybrid AC/DC distribution grids connected to an ST. The focus of this paper is to address this research gap. Being a converter-interfaced apparatus, the ST provides inherent decoupling in terms of voltage and frequency between the upstream Medium-Voltage (MV) network and its hybrid AC/DC distribution grid. This feature offers additional control possibilities involving the resources located in the hybrid distribution grid fed by the ST.