Impact of Adaptive Virtual Impedance Control of DERs Used for Power Sharing on the Protective Schemes of an Islanded Microgrid

Abstract

One of the salient features of the microgrids is operation in an autonomous (islanded) mode. This feature is more pronounced in industrial power plants with heavy duty such as mining, metal, and oil industries; as cyclic and fluctuating loads are the dominant ones in these applications. Distributed Energy Resources (DERs) with small to medium size are predominant in microgrids. Hence, a sophisticated framework is required to supply variable loads by DERs with much smaller ratings. Leading and drafting DERs in a specified sequence is a prescribed solution in this regard, which could be achieved by using virtual inertia and/or virtual reactance. However, the required changes in the existing inertia/reactance have a major impact on the protection schemes applied for DERs and also the distribution network. Nuisance tripping and fail to trip are the consequences of such countermeasures to alleviate the premature fatigue of some of the DERs and related storage systems that are electrically nearer the load than the others. The proposed relaying concept is a communication-assisted scheme that prepares a permissive signal for conventional instantaneous/timedelayed overcurrent relays to inhibit the nuisance tripping in transient conditions; and assure correct tripping in fault cases dealing with the blindness of the relays upon insertion of extra reactance in the fault path.