Low-impedance bus differential — Security and reliability in complex bus arrangements

The concentration of electric power at system buses adds to both the need for high-speed tripping in case of faults and the need to avoid false tripping in case of external faults. This combined need for both superior dependability and security has led to many enhancements and system modifications in bus protection. Bus configurations with multiple zones are particularly difficult to provide with dependability and security to meet system needs. Zone switching, bypass breakers, and transfer buses all contribute to protection system complexity. Addressing this complexity in a protective relay requires both measuring elements and logic that can operate with the speed necessary for system stability. This paper details how security and dependability are addressed in a low-impedance relay. The characteristics of zone switching, current transformer location during breaker bypass, and transition transients are addressed. Reliability and security of low-impedance systems are compared to distributed and high-impedance protection schemes. Conclusions on speed requirements and impacts on protection are presented.