Protections to consider with Automatic Bus Transfer Scheme

Automatic Bus Transfer Scheme (ABTS) provides many benefits to electric power system control and automation operations. In a Main-Tie-Main (M-T-M) bus configuration consisting of two independent sources with a normally open tie breaker, the ABTS offers a quick way to automatically restore a bus that is affected by a loss of its own main source. Automatic retransfer can be done by opening the tie breaker and closing the affected main breaker upon the return of the source. With advanced technology in microprocessor relays, especially coupled with GOOSE application, implementing ABTS can achieve operational speed and reliability and reduced investment cost. However there are complicated system protection and coordination issues that could be overlooked; including: ; Circuit breaker trip and close coil conditions : Motor inrush from the affected bus, Transformer and/or main feeder overload from the healthy bus : Sync check for both buses after recovery of the affected main source : Tie breaker switch onto fault : Protection coordination with sub-feeder close-in fault : Breaker failures : Main bus protection without bus differential relay. This technical paper will present a live project that implements the Automatic Bus Transfer Scheme. Discussion focuses on applying relay functions and logic to resolve the above mentioned system protection and coordination issues.