Bus Protection Considerations for Various Bus Types

Choosing a bus protection scheme requires several key considerations. The complexity of bus protection varies considerably depending on such factors as the bus layout, allowed bus switching scenarios, availability of suitable current transformers (CTs), and maintenance issues. Buses with statically assigned zone currents are well suited for the use of high-impedance bus relaying (if suitable CTs are available) and do not require disconnect status inputs. For substations with terminals capable of switching between different buses, it is necessary to dynamically assign input currents to the bus zones of protection based on the disconnect auxiliary contact status. To avoid misoperations resulting from disconnect auxiliary contact failures, such substation configurations can require the use of a second trip criterion, such as a check zone. Low-impedance numerical relays are especially well suited for protecting more complex bus configurations with dynamic zone selection because they enable zone reconfiguration without any need for corresponding changes to physical CT wiring. While bus differential relaying provides protection for all bus arrangements, fast bus tripping and directional elements can provide adequate protection for some arrangements. Surge arresters and CTs (depending on their placement, saturation, and ratio) generally influence bus protection scheme selection and performance. This paper examines several common bus configurations, presents appropriate protection schemes for each configuration, and analyzes the protection scheme complexity, advantages, and disadvantages.