What are the main types of UPS?
UPSs come in three major varieties, which are also known as topologies:
Single-conversion systems
In normal operation, these feed incoming utility AC power to IT equipment. If the AC input supply falls out of predefined limits, the UPS utilizes its inverter to draw current from the battery, and also disconnects the AC input supply to prevent back-feed from the inverter to the utility. The UPS stays on battery power until the AC input returns to normal tolerances or the battery runs out of power, whichever happens first. Two of the most popular single- conversion designs are standby and line-interactive.
- Standby UPSs allow IT equipment to run off utility power until the UPS detects a problem, at which point it switches to battery power. Some standby UPS designs incorporate transformers or other devices to provide limited power conditioning as well. Standby UPSs are typically the lowest cost of all UPS types, but they may sacrifice some performance specifications found in other UPS designs. One of the typical design traits is that the standby UPS may not create a sine wave output on battery, but may output a stepped-wave or modified sine wave, that looks more like a “square” wave. Small desktop PCs may be fine with this waveform, but larger servers, networking equipment and storage units, with more complex power supplies, may have issues and not operate properly, or at all, with this type of modified waveform.
- Line-interactive UPSs regulate input utility voltage up or down as necessary before allowing it to pass through to protected equipment. However, like standby UPSs, they use their battery to guard against frequency abnormalities.
Figure 1. Internal design of a line-interactive UPS.
Double-conversion systems
As the name suggests, these devices convert power twice. First, an input rectifier converts AC power into DC and feeds it to an output inverter. The output inverter then processes the power back to AC before sending it on to IT equipment. This double- conversion process isolates critical loads from raw utility power, ensuring that IT equipment receives only clean, reliable electricity.
In normal operation, a double-conversion UPS continually processes power twice. If the AC input supply falls out of predefined limits, however, the input rectifier shuts off and the output inverter continues to operate, drawing power from the battery instead. The UPS continues to utilize battery power until the AC input returns to normal tolerances or the battery runs out of power, whichever occurs sooner. In case of a severe overload of the inverter, or a failure of the rectifier or inverter, the static switch bypass path is turned on quickly, to use unconditioned AC source power to support the output loads.
Figure 2. Internal design of a double-conversion UPS.
Multi-mode systems
Multi-mode UPSs combine features of both single-and double- conversion technologies to dynamically strike an ideal balance between efficiency and protection. Under normal conditions, they provide maximum efficiency by running in a highly-efficient energy saver mode. When problems occur, however, they automatically sacrifice some efficiency to deliver maximum levels of protection by automatically and immediately transitioning back to double- conversion mode. When the power quality returns to acceptable levels, the UPS transitions back into energy saver mode.
The end result is that data centers can save tens of thousands a year on energy without compromising data center performance or reliability.
Figure 3. Internal design of a multi-mode UPS