Protective Reactance in Large Power Stations

The concentration of a large generating capacity on the busbars of a single power station demands protection against an excessive flow of current into an accidental fault. Reactance coils can be used in various ways to limit the instantaneous current flow. Reactance coils in the generator leads, while limiting the flow of current from the different generators, are not efficient for limiting the current flow into a fault. Bus reactances are less bulky and less expensive than generator reactances; are more efficient in controlling the current flow, and have the advantage of localizing the effect of a fault to one or more sections of the busbars. They have the disadvantage that, under normal conditions of operation, the current flow across them may cause an undesirable drop in voltage. The ideal system of bus reactances would afford maximum protection to service and equipment in emergencies but, under normal operating conditions, would give minimum voltage difference between various points in the busbars. The straight bus, the ring bus, and the ring and transfer bus approach this ideal condition in the order named. Reactance coils in each feeder circuit are extremely effective in limiting the flow of current into a short circuit on the feeder and in isolating the effects of this short circuit from the other parts of the system.