IN high voltage substations, an independent source of supply is necessary for actuating the protective gear, operation of trip relays, release of trip order to the circuit breakers, interlocking, annunciation, audible alarm, remote position indication and other similar purposes.
The supply is particularly important for trip circuits. A reliable source of supply is necessary which can perform the above functions at the time of system faults. The choice is to select one of the following two systems:
1. AC supply fro station auxiliary transformer.
2. DC supply from independent source that is from storage batteries.
If we use AC supply from the station transformer, it can go to very low voltage levels in case of faults very close to the station. Therefore this source is not reliable and can not be used for station protective relays.
An independent supply from DC storage batteries is ideal for protective relays and switchgear trip circuit. This supply is not affected by the system faults. Lead acid accumulators or Alkaline accumulators can be used for this purpose. Lead acid is most commonly used. Complete DC equipment for a substation may be divided into three parts that is, storage batteries, charging equipment and distribution board.
It is utmost important to monitor that dc supply circuits and trip circuits remain always healthy and dc supply is available at points designed in the scheme, for this purpose trip circuit supervision relays are used.
