Bus Reactor used to keep the voltages at a safe level for all of the other equipment on the line and in the substation.

Bus reactors are not used for voltage control that would be a miss application of an apparatus.

Shunt reactors can be used to control bus voltage but that would a rare case if it was the only piece of equipment used to control the voltage on the bus, due to the fact that it is a fixed amount of corrective action.

Most common equipment for voltage control in a substation is a transformer with on-load tap changer. With heavy loading on the bus low voltage may still be an issue at which shunt capacitor bank should be installed.

Static Var Compensators are voltage control system comprising of banks of shunt reactor and capacitors to have the ability to compensate for high and low outage conditions.

Shunt reactors are commonly used in EHV transmission line where the shunt capacitance has a dominant effect when the line is energized but open at the remote end or when operating in a lightly loade condition. During these times shunt reactors are connected to the line at both ends.

The used of bus reactors is mostly to reduce downstream short circuit current on the bus or to balance bus reactance to balance current flow in a loop or ring. The use of duplex reactors have similar function.

reactors have many applications into power systems, can be used as current limiter device if connected in series but also can be used to mitigate the overvoltages due to lightning and switching of breakers if connected in shunt, as Raymond already said also are used to compensate in a long transmission lines.

Regards.

Thank you Raymond Lee & Enrique Gallegos for the comment