The transformer inrush currents in large MV-cable installations

In large MV-cable installations important problems may occur during the first energization of transformers, since the consequent relevant inrush current may cause the tripping of the general protection, and, at the same time, the system’s total blackout. In order to thoroughly investigate this problem, reference was made to an important underground installation in central Italy, namely the LNGS laboratory, the largest for astroparticle experiments in the world. A special attention was paid to the phenomenon of inrush currents, which were examined also with reference to experimental test results, obtained from a sequence of energizations of an MV/LV power transformer. Moreover, a simulator was implemented in a Matlab/Simulink environment for an effective evaluation of the behavior of a whole electrical installation. The same simulator was used to analyze a case study regarding the LNGS laboratory. Finally, possible solutions aimed at devising a proper selectivity plan for the protection system are proposed and commented.