What is a bus?
Substation buses act as potential connection points within a substation and there may be multiple voltages of buses at one substation. When we evaluate headroom capacity, we do it for each individual bus at a substation which is why capacity is reported by bus, not by substation.
Why is it called a bus?
Substations have conductors that distribute electricity within the substation. Those conductors are called busbars. These busbars within a substation may be sectionalized for increased reliability, operational flexibility, or to accommodate different voltage levels in substations containing transformers. These busbar sections are known as buses, representing various connection points within the substation.