Ground Fault Detector for DC Microgrids Using Sequentially-Switched Grounding Connections

With the increasing demand of renewable energy and recent research in smart Direct Current (DC) microgrids, electric systems with high presence of power electronics are gaining momentum. One piece of evidence is the emergence of ungrounded low voltage DC (LVDC) microgrids and their energy management, which are crucial for achieving high efficiency systems. However, they are difficult to protect against electrical faults, especially against Ground Faults (GF), due to the commutation frequencies and the duality of AC and DC currents that cause a lack of selectivity in conventional protection relays. In this paper a GF detection method is proposed for addressing this issue. The method discerns between the affected zone (AC or DC) by sequentially switching a grounding resistor among the different neutrals and DC midpoints of the system. The method is based on the frequency domain analysis of the voltage waveform measured on this grounding resistor. Afterwards, in the case of AC fault, the phase-to-neutral voltages in the affected zone are measured and compared with the voltage across the grounding resistor to identify the faulty phase. In the case of DC faults, the polarity of this last voltage also allows the identification of the faulty pole. To validate the method, numerous simulations and experimental tests have been performed obtaining satisfactory results.