Page 463 - Handbook of Electrical Engineering

452 HANDBOOK OF ELECTRICAL ENGINEERING

• 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 charg-
ing 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.

17.2.1 UPS Battery Chargers

Battery charger technology for AC and DC UPSs can be simple as in the case of a domestic car
battery charger, or complex as in instrumentation or fire and gas battery chargers. Complex battery
chargers are designed to have:-

• Predetermined current and voltage versus time charging characteristics.
• Electronic protection against overloads and short circuits.
• Minimum supervision and maintenance.
• Occasionally a form of automatic duty-standby change over facility is required.

Modern chargers use fast acting and accurate electronic devices to control the desired output
characteristics. The rectifying device can be diodes or thyristors.
The rectifying device is usually in the form of a single phase for units up to about 25 kVA,
or a three-phase bridge-connected device for larger units. The rectifying device is fed by a single-
phase or three-phase transformer. The output from the rectifier is passed through a current detection
circuit (a resistance shunt or special magnetic device) and a smoothing reactor (or choke). Signals
are taken from the current detector and from the output terminals, are fed back to a control circuit
which produces the desired current and voltage characteristics. The control circuit also incorporates
overcurrent and overvoltage protection so that the battery and its load are not damaged during
abnormal conditions. Some loads cannot tolerate overvoltages, not even for a short time.