Uninterruptible Power Supply (UPS) Systems are used extensively in critical environments to support sensitive electrical equipment when there is a power loss or a significant change in the primary power source. Backup power is provided to the UPS by a string of batteries that can instantly support the load when it detects a loss or other interruption in the available power.
Once a power anomaly has been detected, the UPS control will transfer over to DC battery power and convert the output to AC via an inverter. When normal power is restored, the UPS will switch back to normal and use available power to recharge the batteries.
General Functions of a UPS
- Absorb relatively small power surges.
- Smooth out noisy power sources.
- Continuously provides power to equipment during line sags.
- Automatically shuts down equipment during long power outages.
- Monitoring and logging of the status of the power supply.
- Display the voltage/current draw of the equipment.
- Restart equipment after a long power outage.
- Display the voltage currently on the power line.
- Provide alarms on certain error conditions.
- Provide short-circuit protection.
UPS Topology
- Standby (Offline) – Remains offline until power drops to an unsustainable level.
- Line Interactive – Employs buck/boost to adjust voltage and preventing battery use as often.
- Double-Conversion (Online) – Always conditions power by double conversion (AC-DC-AC).
UPS Operating Modes
- Online (Active) Mode: Critical load is supplied by inverter via utility AC power. Charging current is provided for batteries.
- Energy Saver Mode: Utility power is supplied directly to critical load through a continuous static switch and transfers to online mode when an abnormal condition is detected.
- Variable Mode: UPS operates as a traditional double-conversion UPS with selective load shifting capability.
- Bypass Mode: Critical load is directly supported by utility power and is subject to interruptions. UPS can be serviced.
- Battery Mode: Backup batteries provide DC power to maintain inverter operation and support critical loads.
UPS Components
- Rectifier – The main function of the rectifier is to convert AC power into DC power.
- Inverter – The reverse operation of rectification is inversion, where DC voltage is converted back to AC using transistors to rapidly switch currents on and off, creating pulsed DC in opposite polarities.
- Static Bypass – When an overload or fault occurs within a UPS, the static bypass switch automatically connects load to the main power supply. The static bypass is a safeguard against internal failure and is capable of bypassing the rectifier, batteries, and inverter.
- Batteries – Batteries and battery systems are the most important component of any UPS system. UPS batteries are the engine, or the power source, if the utility power fails or some type of power anomaly is experienced. Batteries are the most common failure/problem within any UPS system.
Basic UPS Maintenance
- The area surrounding the UPS should be checked daily, ensure adequate clearance.
- The operating environment must be kept suitable.
- The air filters off a UPS should be checked monthly and changed as necessary.
- Inspections must be made by qualified individuals at regular intervals to determine if components wiring, and connections exhibit evidence of overheating.
- Test static transfer from inverter to bypass and back. Use normal load, if possible.
