220V DC system:

The 220V DC system supplies direct current as source of operating power for control, signaling, relays, tripping and closing of switchgears, emergency motors of most important auxiliary systems
Under normal conditions of station generation, the storage battery units are kept floating in DC bus bars by means of the chargers (also known as float chargers)
The charger of each Battery unit,which is a rectifier with AC input, is normally made to take all DC requirements of the Power station without allowing the battery to discharge. This is achieved by maintaining the DC output voltage of battery charger a few volts higher than the voltage of battery.
With this, the charger besides meeting all the DC requirements of the power station, supplies a few hundred milliamps of direct current to the battery to compensate the loss in the capacity of the battery due to action between the plates of the cell.
The complete AC power system failure in a power station is known as emergency situation. DC battery units are designed to supply station DC loads for an emergency period of one hour
In case of AC mains failure the full battery of 115 cells will supply the load ie 230 volts. If the emergency lasts for one hour with an appropriate load of 450 Amps, then battery will supply the load for one hour when its end voltage will drop down to 1.75 volts per cell ie 201 volts.

Float and Boost Charging of Batteries:

Float charging:

Float charging is used where the battery rarely gets discharged
A typical application where float charging can be used would consist of the float charger, battery and the load in parallel
During normal operation, the load draws the power from the charger. When the supply to the charger is interrupted, thebattery steps in.
Float charging of a battery involves charging the battery at a reduced voltage
This reduced voltage reduces the possibility of overcharging
The Float charger ensures that the battery is always in the charged condition and is therefore considered "floating"
The Float charger starts by applying a charging voltage to the battery. As the battery gets charged, its charging current reduces gradually. The float charger senses the reduction in charging current and reduces the charging voltage
If the battery gets drained, the float charger will again increase the charging voltage and process continues. Float chargers can be connected indefinitely to the batteries.

Boost charging:

Boost charging involves a high current for short period of time to charge the battery. It is generally if the battery has been discharged heavily. Boost charge enables the quick charging of depleted batteries.
For instance, a two volt lead acid battery which has been discharged will initially be boost charged with a charging voltage of around 2.35-2.4 volts. However, as the battery voltage rises, the charger will switch over to the float charge mode with a float voltage of 2.25 volts.
Most battery chargers come equipped with provisions for both boost and float charging.

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