An uninterruptible power supply (also known as an uninterruptible power source or UPS) is an electrical power supply apparatus designed to maintain a supply load when the primary power source (including but not limited to mains power) fails. In contrast to a standby generator or emergency power system, a UPS is designed to take over nearly instantaneously in the event of power interruption. The energy delivered by an operating UPS is stored in batteries, flywheels, or supercapacitors. Most uninterruptible power sources have a short run-time, typically measured in minutes. This period is provided so that a slower-starting standby power source can be activated or so that the equipment protected by the UPS can be shut down safely. A UPS is a form of a continuous power system.
Uninterruptible power supplies are most often used to protect electronic hardware, e.g. computers, telecommunications equipment, and data centres. UPS units are essential in any application where power disruptions could cause serious business disruptions, data loss, injuries, or fatalities. UPS units vary in size depending on the amount of equipment they are designed to protect. They range from single-computer models rated for roughly 200 volt-amperes to massive units designed to protect whole buildings or data centres. Every UPS is designed expressly to provide short-term power in the event of a failure in the primary power input.
Modern UPS systems fall into three main categories: on-line, line-interactive, and standby. A UPS designed with online technology is one that takes AC input, rectifies it to DC to charge batteries or battery strings, and then inverts power back to 120/230V AC for the use of the equipment it protects. This process is also called the “double conversion” method. In a line-interactive UPS, the inverter remains in line. The DC current path through the batteries is redirected out of charging mode into supplying power when input power is lost. In a standby system, also known as an “off-line” UPS, input power is passed directly through the system and the backup circuit engages only when the primary power input fails. Smaller UPS units (under 1kVA) typically use the standby or line-interactive designs because they are cheaper.
In some larger units, a UPS takes the form of a Dynamic Uninterruptible Power Supply or DUPS. In this system, an alternator/synchronous motor is linked via choke to the mains supply. This stores energy with a flywheel. Thanks to an eddy-current regulation, the system steps in to maintain power whenever mains power is interrupted. A DUPS unit can deliver power as long as energy remains in the flywheel. Many installations combine a DUPS with a diesel generator which begins supplying power to make up for the lost mains input. This combination is called a diesel rotary uninterruptible power supply, or DRUPS.
The latest technological advance to appear in recent years is the fuel cell UPS. This uses a hydrogen fuel cell as an alternative power source. A fuel cell UPS has the potential to deliver longer run times from a very compact unit.
In any large organization with high-reliability standards, a UPS can be considered a single point of failure with the potential for widespread system disruption. Reliability can be improved by integrating smaller UPS modules together to supply the same protection as a single larger unit. The “N+1” design principle states that such an integrated system should contain one more module than is strictly necessary to supply the necessary load. This way the system can function as intended even if one module fails completely.
Redundant power supplies are employed in many data centres and computer installations. The intent is to have an alternate power supply ready for use if one fails. For a system to be considered properly redundant, each power supply must be capable of fully meeting the system’s power needs on its own.
Many redundant systems are further protected by incorporating a dedicated UPS for each power supply. This reduces the likelihood of operational failure by protecting the system against both power supply and UPS failures.
UPS units installed in an outdoors environment need additional design features to stand up to adverse weather. Manufacturers need to consider a full range of climate issues, including temperature, rain, snow, humidity, and more.
Outdoor UPS units fall into three main categories, depending on their mounting: pole, ground/pedestal, or host-mounted. Depending on its location, an outdoor UPS system may need special features to deal with extreme cold or extreme heat. In the former case, a UPS unit may need a battery heater mat; in the latter, it may need active ventilation or air conditioning.