Circulating currents in closed loop structure, a new problematic in distribution networks

Loop operation of distribution networks is more and more common. The increasing penetration of distributed generation and higher power quality requirement are making the loop structure more interesting. We study the impact of the unbalanced phase impedances. Measurements and simulations show that it creates a mutual coupling between the positive sequence and the zero sequence system. Balanced load creates a zero sequence voltage between two feeders. Their connection of the both ends to form a loop results in circulation of a zero sequence current. In case of an earth fault this current will be added to the fault current and can negatively influence the protection devices. Healthy lines do not have active current in the zero sequence system because there is only capacitance connection with the earth but the circulating current change this hypothesis. We propose a study of this problematic current for simple and multi loop in distribution networks.