Fault clearing in a converter-dominated microgrid with traditional protection means

The increased penetration of converter-interfaced distributed generation in microgrids has emerged the serious problem of protection during faults, due to the lack of large current injection. This paper proposes a fault detection and clearing control method for three-phase symmetrical faults in a microgrid with looped topology. The fault detection method is based on measuring the microgrid impedance variation during the fault, through the injection of a slightly distorted current. When the fault is identified, the distributed energy sources (DERs) switch their control strategy from droop control to current source mode, in order to inject a current proportional to the measured microgrid impedance. In this way, the DER closer to the fault injects a relatively larger current. The fault clearing process is carried out with simple overcurrent relays, which have the same settings. The coordination of the protection means is implemented from the discrete current injection of the DERs. Furthermore, in order to ensure the current injection even in the case of lack of power from the primary renewable source, a supercapacitor energy storage system (SESS) is added on the DC-link. A significant contribution is the voltage recovery after the fault clearance with a seamless transient effect. The effectiveness of the proposed control strategy is evaluated through simulation tests, conducted in PSIM software environment.