Answer / jai prakash

generally we gave dc supply to the all protective equipment
like relays ,indicators , and for all controlling devices
through batteries in substation .

Battery Chargers are basically Automatically Stabilised D.
C. Power Sources meant to feed the connected load and
simultaneously trickle (float) charge the connected Battery,
thereby maintaining it (Battery) continuously healthy &
ready to feed the load in an emergency situation of A.C.
Supply failure. This arragement ensures an uninterrrupted
D.C. Supply to the connected load at all times - by the
Battery Charger in case of A.C. Power Supply availability or
by the Battery in case of failure of A.C. Power Supply.

Answer / muhammad mustaghas

1) The operation of monitoring devices such as relays and
the tripping mechanisms of breakers require an independent
power source, which does not vary with main source being
monitored.Batteries provide this power source for the
successful operation of switching and control devices in
power protection system.
It is therefore necessary to ensure that batteries and
charges are regularly inspected and maintained at the
highest possible level of efficiency at all times to enable
correct opeartion of relays at the correct time.
2)UPS means uninteruptable power supply which accept AC as
input and convert this AC into DC for the storage purpose
and when require AC supply then again it convert DC into AC
for its out put terminal.Basically UPS is used to stored
the energy in from of current to fullfill the requirement
for emergency supply but batteries only provide the DC
power supply to its load and can store and supply the
limited amount of power.

Answer / amit khamesra

In lieu of a more reliable power supply source, DC supply
is used in place of normal AC supply. If there is a
blackout, AC power will not be available n the electrical
protection devices will not operate. To avoid any such
situation, DC supply is used. DC supply is provided by
batteries and charger is used to keep the batteries healthy
by trickle charging the batteries in Float / Boost mode as
required. UPS is an uninteruptable power supply which uses
converter inverter set. AC input is first converted to DC
and again converted to AC. The battery bank installed for
UPS is connected in between converter inverter set so that
if AC supply to converter fails, battery comes on load and
supplies DC to inverter for maintaining the AC power supply
at output end.

Answer / tarun aggarwal ymca

In AC substations if AC power fails, for tripping of Switchgear an alternative supply is needed. This supply is DC supply from Batteries. AC supply can not be stored where as DC can be stored in batteries. Hence all circuit breakers are with DC voltage coils. In case of a line to line fault or line to earth fault the incoming / remote breaker needs to be tripped. At this point AC is not available and hence DC is used. 
While AC over voltage or Under voltage are abnormal conditions and hence AC supply is present hence AC coil can be used. 
For charging of battery, battery charger is used. Charger takes AC voltage, steps down and rectifies in to DC voltage and supplies charging current to battery. 




UPS (Uninterrupted Power Supply), consists of a rectifier, battery and inverter and its output is AC supply. During utility power supply fails, this UPS supplies AC voltage to instruments for monitoring important parameters. But this also depends on Battery. Hence efficiency of UPS is low, as well as reliability is low due to more components. Hence using battery is better for tripping functions in protection circuits. In case AC power fails for longer duration standby DG set is started for charging battery.

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