An uninterrupted power supply (UPS), also known as a battery back-up, provides emergency power and, depending on the topology, line regulation as well to connected equipment by supplying power from a separate source when utility power is not available. It differs from an auxiliary or emergency power system or standby generator, which does not provide instant protection from a momentary power interruption. A UPS, however, can be used to provide uninterrupted power to equipment, typically for 5-15 minutes until an auxiliary power supply can be turned on or utility power is restored.
While not limited to safeguarding any particular type of equipment, a UPS is typically used to protect computers, data centers, telecommunication equipment or other electrical equipment where an unexpected power disruption could cause injuries, fatalities, serious business disruption or data loss. UPS units come in sizes ranging from units which will back up a single computer without monitor (around 200 VA) to units which will power entire data centers or buildings (several megawatts).
Common power problems
There are various common power problems that UPS units are used to correct. They are as follows (with a typical example of damage that might be caused):
Power failure — Total loss of utility power, with possibility of severe transient conditions upon failure and/or restoration: Causes electrical equipment to stop working (transients may cause permanent damage).
Voltage sag — Transient (short term) under-voltage: Causes flickering of lights.
Voltage spike — Transient (short term) over-voltage i.e. spike or peak: Causes wear or acute damage to electronic equipment.
Under-voltage (brownout) — Low line voltage for an extended period of time: Causes overheating in motors.
Over-voltage — Increased voltage for an extended period of time: Causes light bulbs to burnout.
Line noise — Distortions superimposed on the power waveform: Causes electromagnetic interference.
Frequency variation — Deviation from the nominal frequency (50 or 60 Hz): Causes motors to increase or decrease speed and line-driven clocks and timing devices to gain or lose time.
Switching transient — Instantaneous undervoltage (notch) in the range of milliseconds to seconds: May cause erratic behavior in some equipment, memory loss, data error, data loss and component stress.
Harmonic distortion — Multiples of power frequency superimposed on the power waveform: Causes excess heating in wiring and fuses.
UPS units are divided into categories based on which of the above problems they address. Manufacturers categorize their products in accordance with the number of power related problems they address.
Technologies
The general categories of modern UPS systems are on-line, line-interactive, and standby. An on-line UPS uses a "double conversion" method of accepting AC input, rectifying to DC for passing through the battery (or battery strings), then inverting back to AC for powering the protected equipment. A line-interactive UPS maintains the inverter in line and redirects the battery's DC current path from the normal charging mode to supplying current when power is lost. In a standby ("off-line") system the load is powered directly by the input power and the backup power circuitry is only invoked when the utility power fails. Most UPS below 1 kVA are of the line-interactive or standby variety which are usually less expensive.
For large power units, Dynamic Uninterruptible Power Supply are sometimes used. A synchronous motor/alternator is connected on the mains via a choke. Energy is stored in a flywheel. When the mains power fails, an Eddy-current regulation maintains the power on the load. DUPS are sometimes combined or integrated with a diesel-genset, forming a diesel rotary uninterruptible power supply, or DRUPS.
Fuel cell UPS have been developed in recent years using hydrogen and a fuel cell as a power source, potentially providing long run times in a small space. A fuel cell replaces the batteries used in other UPS designs.
没有评论:
发表评论