Uninterruptible power supply An uninterruptible power supply, or UPS, is a device or system that maintains a continuous supply of electric power to certain essential equipment that must not be shut down or deprived of electrical power unexpectedly due to the failure of the normal supply to which it is connected.


The equipment is a back up source designed for auto change over in a few cycles or back up floating. This is inserted between a primary power source, such as normal power supply in Electric power generating stations or from a commercial supply in other industries, and the primary power input of equipment to be protected, for the purpose of eliminating the damages or effects of a temporary power outage and transient anomalies.

They are generally associated with some auxiliaries of electric power generating stations, telecommunications equipment, computer systems, marine equipment. Other facilities requiring such back up supply are airport landing systems and air traffic control systems where even brief commercial power interruptions could cause injuries or fatalities, serious business disruption or data loss. They are also associated with hospitals, nursing homes and similar industries providing medical facilities for humans and animals.

Historically a UPS was likely to be used in areas where the power supply is interrupted frequently (such as third world countries and some rural areas in first world countries). However that view has been changing in recent years as the number of instances of blackouts continues to increase. In North America in particular, the electrical grid is under increasing strain particularly during heavy demand periods such as summer when air conditioning use is at its highest. In order to prevent blackouts, electrical utilities will, from time to time, use a process called load shedding. This reduces the amount of power being sent to the consumers but does not eliminate it entirely. This drop in voltage is also sometimes called a voltage sag or a brownout. A UPS will also protect equipment upon the occurrence of a brownout by using its internal batteries to correct the drop in voltage. The single biggest event that brought attention to the need for UPS power backup units was the big power blackout of 2003 in the north-eastern US and eastern Canada.

UPS design[]

Most uninterruptible power supply designs for telecommunications equipment use a transformer along with one or more rectifiers to convert the incoming commercial AC power into a low voltage DC supply, typically in the range of 12 to 50 volts. One or more rechargeable batteries are connected in parallel with the rectifiers to maintain the voltage should the power fail. Various arrangements exist to ensure that the batteries, which are on a continuous trickle charge, can be maintained at an appropriate voltage and state of charge, as well as be given a boost charge should the charge state become too low. Because a battery backed DC output is used, this type of uninterruptible power supply is only suitable in specialised telecommunications applications where the equipment does not require a commercial AC power feed.

Older uninterruptible power supply designs that supply commercial quality AC power to equipment contain a motor-generator system with a large flywheel that keeps the generator rotating and producing electric power while an auxiliary motor is started at the moment of power interruption. Sometimes the flywheel itself is used to start the motor. These systems can typically cover a 30 second interruption until the auxiliary motor starts.

Modern uninterruptible power supply systems used with commercially available computer equipment consist of: a static (electronic) rectifier, a static (electronic) inverter, a static switch and an energy storage system. Primary power feeds the rectifier, which converts the power from AC to DC. The DC produced by the rectifier is connected to the inverter and to a storage system consisting of batteries or in some cases a flywheel based energy storage system. The inverter is connected to the electronic equipment of interest (load). When the incoming power line is not available or unusable the rectifier shuts off and the storage system gives up its energy to the inverter. The larger the stored energy system or the lower the level of power used by equipment connected to the inverter, the longer the UPS can provide power to the connected equipment. The static switch can be used to provide power to the load when the rectifier and inverter are off as is the case during maintenance or when the connected equipment requires more power than the inverter can provide.

UPS systems that channel the power needed by the connect equipment through the rectifier and inverter at all times are known as On-Line dual conversion UPS systems. Alternatives to this type of configuration exist.

DC output[]

Some systems particularly in telecommunications use DC (often 48V) rather than AC for the output from the backup power system. This saves a conversion step and pretty much eliminates issues such as harmonics and power factor from the load side. However it also requires all load equipment to have special power supplies and means that special wiring practices are needed.

The nine power problems[]

There are nine standard power problems that a UPS may encounter. They are as follows:

  1. Power failure.
  2. Power sag (undervoltage for up to a few seconds).
  3. Power surge (overvoltage for up to a few seconds).
  4. Brownout (long term undervoltage for minutes or days).
  5. Long term overvoltage for minutes or days.
  6. Line noise superimposed on the power waveform.
  7. Frequency variation of the power waveform.
  8. Switching transient (undervoltage or overvoltage for up to a few nanoseconds).
  9. Harmonic multiples of power frequency superimposed on the power waveform.

Some manufacturers categorise their UPS's as a level 3, 5, or 9, if it can handle the first 3, 5, or 9 power problems respectively. Obviously the degrees of protection vary from manufacturer to manufacturer.

It is generally considered, especially in larger installations, that the incoming commercial power should never be directly connected to the load (computer) equipment. Several types of UPS systems are available to ensure this does not happen. In one arrangement the inverter is run in hot standby, synchronised with the AC power but not powering the load allowing the rectifier(s), inverter(s) or battery to be removed from service for maintenance or in the event of a fault. This configuration is considered an Off-Line type. Output sizes from under 1 kilowatt to several kilowatts are commercially available. While most UPS equipment will only operate for about 10 minutes after an outage occurs, some telecommunications systems are designed to operate for over 24 hours without power.

  • NOTE: Do not confuse a UPS with a standby generator, which does not provide protection from a momentary power interruption, or which may result in a momentary power interruption when it is switched into service, whether manually or automatically. However, such a generator may be placed before the UPS to provide cover for lengthy outages.

Power correction technologies[]


Standby uninterruptible power supplies run offline (meaning that the battery is not engaged until a power outage occurs), offering level 1 protection against power failures only. These are the cheapest variety of uninterruptible power supplies and are intended only for the home user. (offline UPS)


In the Line Interactive UPS design, the battery-to-AC power converter (inverter) is always connected to the output of the UPS. Operating the inverter in reverse during times when the input AC power is normal provides battery charging.

When the input power fails, the transfer switch opens and the power flows from the battery to the UPS output. With the inverter always on and connected to the output, this design provides additional filtering and yields reduced switching transients when compared with the Standby UPS topology.

In addition, the Line Interactive design usually incorporates a tap-changing transformer. This adds voltage regulation by adjusting transformer taps as the input voltage varies. Voltage regulation is an important feature when low voltage conditions exist, otherwise the UPS would detect a power failure and switch to battery powered mode. Eventually the battery might become discharged and fail to power the load. Additionally, more frequent usage of the battery could reduce battery life or cause premature battery failure.

The ability to correct low or high line voltage conditions make this the dominant type of UPS in the 0.5-5kVA power range.

Delta conversion online[]

Delta conversion is a type of Line interactive technology. In this configuration the primary power source is blended with power from the inverter. As the primary power varies away from its normal value the inverter comes to life to make up the difference. Unlike Off line technology no switch "ON" time is required. Unlike On-line technology no continuous separation of load and primary power is offered. Delta Conversion provides protection from all power anomalies except #7. Delta conversion is efficient, with system efficiency of up to 97% under nominal conditions when the inverter needs to do no work to correct deficiencies in the primary power. As the inverter does more work to correct deficiencies in the primary power the efficiency drops. At practical levels the efficiency of this technology can be less than that of On-line systems.

Double power conversion online[]

Double conversion online uninterruptible power supplies convert AC power to DC and then convert the DC back to AC to power the connected equipment. The batteries are directly connected to the DC level. This effectively filters out line noise and all other anomalies from the AC power for level 9 protection. An additional benefit of this technology is in the continuity: in all 9 problem conditions, the system remains in the same operating mode. Relative to other UPS topologies there are efficiency losses due to the double conversion of all of the power the load requires. Technological improvements have resulted in efficiencies of 94%, which gives this technology some advantages over other types that offer fewer modes of protection for the sake of 1 or 2% more in efficiency in some modes.

See also[]

Part of this article was originally taken from a public-domain entry in Federal Standard 1037C

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