ABSTRACT
A bus may interconnect multiple input sources and loads; the loads may be connected at different voltages, other than the bus voltage through step-up or step-down transformers. A bus fault can shut down a large area; furthermore, the fault currents at bus are high due to interconnection of many sources and loads. Rarely, bus differential protection is applied to low-voltage bus systems. Generally, these are single bus systems, or sectionalized buses with bus tie breakers which can be open or closed. Electromechanical single-phase high-impedance relays have been popular in the industry for the last 50 years. A series resistance in the relay coil is introduced, which reduces the current through the coil to milliampere range. Microprocessor-based multifunction relays duplicating the performance of electromechanical high-impedance relays are available. Electromechanical low-impedance relays have been used in the past for protection of generators. A historical problem in bus protection systems has been unequal core saturation of the current transformers.
