Performance issues with directional comparison blocking schemes

Distance and directional overcurrent protection can provide 100 percent coverage of transmission or subtransmission feeders through the use of teleprotection (communications-assisted tripping) schemes. We briefly review in this paper various types of teleprotection schemes and then present principles to successfully apply a directional comparison blocking scheme (DCB). We amplify these principles by examining the reasons why incorrect tripping occurred on feeders using DCB schemes in five field cases. Further, we alert readers to the conundrum of using DCB schemes in emerging electricity markets-dependability of protection versus less security of supply. We propose that protection engineers can apply the advantages of digital communications systems, functionality of numerical relays, and increased electricity network infrastructure to intelligently design line protection systems with high dependability and high security.