DC UPS

A DC uninterruptible power supply is basically a battery bank and a charger. However, it differs from a simple battery and charger system that may be associated with starting diesel engines, or similar rugged functions, because the output voltage must be maintained within a close tolerance of the nominal DC voltage.

DC uninterruptible power supplies are used for:

• Closing and tripping of circuit breakers and contactors in switchboards.

• Switchboard indicating lamps.

• Radio communication equipment.

• Emergency generator control panels.

• Start-up and shut-down lubricating oil pumps and auxiliary systems for gas turbines, large pumps and compressors.

When specifying the battery and charger system the following points should be considered.

• Rated voltage and current.

• Rated ampere-hour capacity.

• Rate of discharge

• Type of cell i.e. lead-acid or nickel-cadmium

• Ventilated batteries. Some types of cells can be non-venting but this greatly influences the charging process.

• Type of charger e.g. rectifier or thyristor.

• Boost, float and trickle charging requirements.

• Duty and standby units, and their interlocking and control philosophy.

• Volt-drop considerations in the DC outgoing cables.

• Overload and short-circuit protection.

• Tolerance on the DC output voltage during all load and charging conditions.

• Ambient temperature and appropriate derating factors for the cells and the charger.

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