A control strategy for inverter-interfaced microgrids under symmetrical and asymmetrical faults

The increase in the renewable energy penetration level imposes the microgrid concept, which can be consisted of several inverter-interfaced distributed resources (DERs) and loads, operating in dual state; either connected with the utility grid or isolated in island mode. The power sharing among the connected DERs is carried out by the combination of the droop characteristics of each DER according to the active and reactive power demand of the loads. When a fault occurs within the microgrid operating in island mode, it is very difficult to be detected due to the lack of large current production capacity. The fault situation can be further complicated, if the fault takes place between two phases or between a single phase and the earth. This paper proposes a fault detection method for the symmetrical and asymmetrical faults, which is independent of any further communication means. The three-phase faults can be detected through the impedance variation of the islanded microgrid, while the asymmetrical ones by the negative sequence components of the output voltage of each DER. A significant contribution is the voltage recovery after the fault clearance with a seamless transient effect. After the fault clearance, the microgrid will continue feeding its loads, through the implementation of a positive- and negative-sequence control strategy. The effectiveness of the proposed control strategy is evaluated through a set of simulation tests, conducted in PSIM software environment.