Optimal Protection Coordination of Active Distribution Networks With Synchronverters

Abstract

Synchronverters are inverters imitating synchronous generators (SGs) to curb adverse impacts of traditional inverters on power networks. However, synchronverters may generate unlimited currents during faults. In addition, limiting the synchronverter's fault currents by hard limiters could malfunction directional overcurrent relays (DOCRs). Thus, this paper employs virtual-impedance fault current limiters (VI-FCLs) that preserve the synchronverter's voltage source model to guarantee a reliable operation of DOCRs. Also, an optimal protection coordination (OPC) scheme is developed to size the VI-FCLs and determine the DOCRs' settings in a decoupled manner. The proposed protection scheme determines the highest values of the VI-FCLs that correspond to bolted faults at the synchronverters' terminals. These conservative values are then relaxed by adaptively varying the VI-FCL based on the fault severity to enhance the DOCR sensitivity. Further, a short circuit calculation algorithm is formulated to incorporate the developed synchronverter with VI-FCL into an OPC program to determine the optimal settings of DOCRs that minimize the total operating time of all primary and backup relays. Case studies ensure the proposed protection scheme's effectiveness in reliably protecting a radial 9-bus Canadian distribution system and a meshed 14-bus system powered by synchronverters.