Selective overcurrent protection for large MV installations

The coordination of protection systems may prove very difficult at large installations, especially when many circuits are present and several hierarchical levels must be satisfied, involving both protection devices and automatic control systems. In this context, protection selectivity plays an important role as concerns the continuity level of power supply and the general power quality of the whole electrical system. The issue was investigated at an important underground laboratory of the Italian National Institute for Nuclear Physics (INFN). A preliminary analysis was performed paying special attention to the phenomenon of inrush currents, which was investigated also through laboratory tests consisting of a sequence of energizations of a power transformer. In order to replicate the behavior of a large installation, a digital simulator was implemented. Moreover, in this paper a case study is reported where this same simulator was used to analyze a portion of the plant under investigation, assuming as reference a number of significant scenarios involving both normal operating conditions and fault contingencies. The results of the experimental tests, the simulation analysis and possible strategies for a selective protection plan are finally examined and compared, with an aim to supply suggestions to correctly protect very large, sensitive electrical installations.