Reliable protection systems for locally supplied medium-voltage distribution networks

Under emergency conditions, portions of medium-voltage (MV) networks can be locally supplied for a significantly long time period by moveable generating units connected to the low-voltage (LV) side of a standard distribution transformer, which in this case operates as a step-up transformer. It is, therefore, necessary to ensure a proper operation which must include the detection of any possible faulty condition. When the MV network portion is operated with isolated neutral (as it is usually the case), a reliability issue arises for earth faults detection by the protection system, due to the negligible capacitive fault current values. In this work, possible detection methods are identified through analytical and numerical simulations and two promising solutions are proposed, i.e. the intentional earthing of one phase conductor and the connection of a suitable neutral forming transformer with earthed neutral terminal, both operated at the MV side of the MV/LV transformer.