In general, the way of neutral connection to earth in MV networks is chosen on the basis of legal and technical issues, related to safety and operation of electrical networks; therefore different solutions can be adopted. The Italian choice, up to now, has been to operate the MV networks isolated in almost all the cases. This solution was adopted forty years ago since it assured simple operation together with earth fault currents reduced to acceptable values, even if some drawbacks like greater stress for insulation and intermittent arc phenomena were expected. Recently, the issue of neutral to earth connection has been reconsidered, taking into account the raising requirements for power quality and the environmental constraints (cables instead of overhead lines). Following that, Enel Distribution has begun the experimentation of connecting neutral to earth through impedance in MV networks (Petersen coil). This experimentation involved different aspect related to · the connection schemes · the choice of fixed or mobile coil · the protection system adaptation to the new network · the automation to manage different connection to earth during the experimentation · the monitoring of the operation of three pilot plants.
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