Intelligent Three Tie Contactor Switch Unit based Fault Detection and Isolation in AC Microgrids

Fault current levels vary in microgrids due to the plug-and-play nature of distributed energy resources (DERs) having different operating modes. This results in maloperation of time-graded overcurrent relay-based protection schemes in an ac microgrid. A higher penetration of DERs or microgrid islanding reduces the fault current contribution from the upstream section of feeder. This condition disrupts the coordination between protective devices operating with pre-defined I-t curves. To address such protection mis-coordination issues, a novel protection scheme that integrates an intelligent three tie contactor unit (ICU) at each DER node is proposed. The ICUs facilitate detecting the operating mode and network topology through their in-built peer-to-peer communication capability. Strategically, each ICU estimates the fault current level in its protection zone and autonomously updates the I-t curves of associated reclosers/breakers. The ICUs limit the synchronous generator-based DERs infeed at the zone of fault and facilitate the inverter-based DERs to ride-through while operating in volt-reactive power mode. It would ensure effectiveness of the existing time-graded overcurrent-based protection coordination for selectively isolating the fault. After the selective fault isolation, the local DERs re-synchronize and operate in parallel with the rest of microgrid system.