Currents, potentials towards ground and fault protection in DC microgrids

With growing diffusion of DC microgrids, ground faults in the DC section of networks will become more and more likely. This is mainly due to the expected increase in size of such networks. Ground faults may involve significantly high currents, which may not be limited by front-end converters (FECs). The work presented in this paper is focused on the analysis of normal and faulty operation of a DC microgrid interfaced to a low voltage (LV) AC network by means of an IGBT FEC. Two different configurations of the LV star point of the transformer have been considered: connected to and isolated from the midpoint of the DC bus. Normal operation has been analyzed and the presence of high frequency homopolar currents has been detected in the first configuration. Ground faults of the positive pole have also been analyzed with decreasing values of fault resistance. DC values of the fault current and of the DC bus voltage have been analyzed. Moreover, the effect of the presence of a storage device has been investigated.