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 unexpectedly. The equipment is inserted between a primary power source, such as a commercial utility, and the primary power input of equipment to be protected, for the purpose of eliminating the effects of a temporary power outage and transient anomalies. They are generally associated with telecommunications equipment, computer systems, and other facilities such as 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.
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 started.
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 a 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.
The nine power problems
One UPS manufacturer defines nine power problems that a UPS may encounter. They are as follows:
- Power failure.
- Power sag (undervoltage for up to a few seconds).
- Power surge (overvoltage for up to a few seconds).
- Brownout (long term undervoltage for minutes or days).
- Long term overvoltage for minutes or days.
- Line noise superimposed on the power waveform.
- Frequency variation of the power waveform.
- Switching transient (undervoltage or overvoltage for up to a few nanoseconds).
- Harmonic multiples of power frequency superimposed on the power waveform.
A UPS is rated as a level 3, 5, or 9, if it can handle the first 3, 5, or 9 power problems respectively. It is generally considered, especially in larger installations, that the incoming commercial power should never be directly to the load (computer) equipment. Several type of UPS systems are availble to ensure this does not happen. In one arrangement the inverter 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 a 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.
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