In recent years, an anomalous increase of faults in underground medium voltage (MV) cable lines has been recorded in Italy, especially during summer; the largest number of faults affected cable joints. The assessment of joint thermal stress, both in normal operation and during faults, is paramount. The study presented in this paper focuses on cable heating effects due to short circuit currents flowing through cable screens during ground faults (e.g., in case of cross country faults, CCFs, whose current values are comparable to line-to-line short circuit), considering the contact resistance (CR) between cable screens and copper stocking due to inaccurate joint manufacturing. A thermal model, already developed and discussed by the authors in previous papers, has been extended and applied in this study in order to assess the CR effects in cable and joint heating during failures. Parametric studies have been carried out on a typical cable-joint system, varying fault current and CR values, as well as considering protection schemes normally adopted by distribution system operators (DSOs) in Italian MV distribution grids. Results show that for CR values larger than few milliohms, fault currents due to CCFs are able to overheat the joint well beyond the maximum tolerable temperature of insulation, thus leading to cable failures when the shortest fault clearing times (i.e., 120 ms) are considered.

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In order to improve power system operation, Battery Energy Storage Systems (BESSs) have been installed in high voltage/medium voltage stations by Distribution System Operators (DSOs) around the world. Support for restoration of MV distribution networks after a blackout or HV interruption is among the possible new functionalities of BESSs. With the aim to improve quality of service, the present paper investigates whether a BESS, installed in the HV/MV substation, can improve the restoration process indicators of a distribution grid. As a case study, an actual active distribution network of e-distribuzione, the main Italian DSO, has been explored. The existing network is located in central Italy. It supplies two municipalities of approximately 10,000 inhabitants and includes renewable generation plants. Several configurations are considered, based on: the state of the grid at blackout time; the BESS state of charge; and the involvement of Dispersed Generation (DG) in the restoration process. Three restoration plans (RPs) have been defined, involving the BESS alone, or in coordination with DG. A MATLAB ® /Simulink ® program has been designed to simulate the restoration process in each configuration and restoration plan. The results show that the BESS improves restoration process quality indicators in different simulated configurations, allowing the operation in controlled island mode of parts of distribution grids, during interruptions or blackout conditions. The defined restoration plans set the priority and the sequence of controlled island operations of parts of the grid to ensure a safe and better restoration. In conclusion, the results demonstrate that a BESS can be a valuable element towards an improved restoration procedure.

The paper describes an innovative method to implement the protection system of a distribution network foreseeing a closed-ring operation plant based on a Smart Grid architecture. A closed-ring operation scheme requires an entirely new protection system in terms of fault detector devices, components, data communication infrastructure and central control system. The installation remote terminal units in particular MV/LV substations, fault detector devices and circuit breakers enable to select rapidly a restricted part of network where the fault occurred.