What do batteries do in a substation?

They supply the DC auxiliary system, protection relays, control and tripping circuits, SCADA and RTU, communications, alarms and indication, so that protection can still operate and the substation can still be controlled when the AC supply is lost. This is the “tripping battery” or “station battery”, and it is the reason a substation can clear a fault during a total AC failure. Loss of the DC supply means loss of protection and control of the high-voltage assets, which is why these banks are among the most closely managed low-value assets on a network.