## 5 Conclusions  
This paper presents a new approach for endowing ST-connected hybrid AC/DC distribution grids to provide active power-frequency regulation services to the upstream AC power system. The ability to provide this regulation service is of utmost relevance in future scenarios characterized by the massive integration of DER in distribution grids, where hybrid AC/DC distribution grids may play an important role. The proposed control strategy relies in a set of droop-based controllers to be embedded in the ST power conversion stages that are responsible for modulating its electrical variables (DC grids voltage, LV AC grid voltage and frequency) as well as the local ESS active power. Moreover, droop-based controllers are also associated to controllable DER available in the hybrid distribution network, making them responsive to ST electric variables. The load-voltage sensitivity of AC and DC connected loads is also exploited as a possible contribution within the scope of the active power-frequency regulation. The effectiveness of the proposed approach is illustrated in different operating scenarios of a hybrid AC/DC network connected to an ST. The numerical simulation results, obtained for different operating conditions, provide support to the possibility of further exploiting the proposed solution in ST connected hybrid AC/DC distribution grids regarding the need of providing active power-frequency regulation services to the upstream power grid. The number of controllable DER resources and non-controllable resources considered in the test case under discussion in this paper suggests that the proposed approach can be extended to any number of controllable DER resources and non-controllable resources available in a hybrid AC/DC distribution network. Further, the proposed approach can be potentially extended to more than one Smart Transformer connected to the main power system, and to provide power-frequency regulation in hybrid distribution grids operating autonomously in islanded mode. Although not properly addressed in this paper, these subjects constitute research topics to be covered in the future due to its relevance. Moreover, according to [34], the use of the DAM approach is adequate to develop computational models suited for real-time platforms, but such an evaluation did not take place in the scope of this work, constituting though a aspect to be exploited in future research works. [PERSON] carried out the main research task and wrote the full manuscript. [PERSON] proposed the original idea and contributed to the revision of the obtained results and of the whole manuscript. [PERSON] contributed with a valuable evaluation of the scenarios to be considered and contributed to the revision of the whole manuscript. All authors have read and agreed to the published version of the manuscript. This work is financed by the ERDF--European Regional Development Fund through the Operational Programme for Competitiveness and Internationalisation--COMPETE 2020 Programme and by National Funds through the Portuguese funding agency, FCT--Fundacao para a Ciencia e a Tecnologia,  
Figure 23: Active power-frequency support provided by the ST to the main power system, obtained from non-controllable loads (dotted area), controllable energy storage (dot-dash area), controllable EV charging (dashed area) and controllable PV generation (white area). Positive values represent power consumption. within project ESGRIDS--Desenvolvimento Sustentavel da Rede Eletrica Inteligente, Ref. SAICTPAC/0004/2015-POCI-01-0145-FEDER-016434. **Conflicts of Interest:** The authors declare no conflict of interest. **Nomenclature**