**Gianni Celli, Fabrizio Pilo, Giuditta Pisano \*, Simona Ruggeri and Gian Giuseppe Soma**

Department of Electric and Electronical Engineering, University of Cagliari, 09123 Cagliari, Italy; gcelli@unica.it (G.C.); farizio.pilo@unica.it (F.P.); simona.ruggeri@unica.it (S.R.); giangiuseppe.soma@unica.it (G.G.S.)

**\*** Correspondence: giuditta.pisano@unica.it; Tel.: +39-070-675-5868

Received: 31 October 2020; Accepted: 31 December 2020; Published: 5 January 2021

**Abstract:** The European strategic long-term vision underlined the importance of a smarter and flexible system for achieving net-zero greenhouse gas emissions by 2050. Distributed energy resources (DERs) could provide the required flexibility products. Distribution system operators (DSOs) cooperating with TSO (transmission system operators) are committed to procuring these flexibility products through market-based procedures. Among all DERs, battery energy storage systems (BESS) are a promising technology since they can be potentially exploited for a broad range of purposes. However, since their cost is still high, their size and location should be optimized with a view of maximizing the revenues for their owners. Intending to provide an instrument for the assessment of flexibility products to be shared between DSO and TSO to ensure a safe and secure operation of the system, the paper proposes a planning methodology based on the non-dominated sorting genetic algorithm-II (NSGA-II). Contrasting objectives, as the maximization of the BESS owners' revenue and the minimization of the DSO risk inherent in the use of the innovative solutions, can be considered by identifying trade-off solutions. The proposed model is validated by applying the methodology to a real Italian medium voltage (MV) distribution network.

**Keywords:** distribution network planning; energy storage system; multi-objective optimization; optimal location; risk assessment; flexibility; distributed energy resources; distribution system operators; local services; system services; arbitrage; frequency control
