Uninterruptible power supply

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A small free-standing UPS. The unit in the photo has IEC connector inputs and outputs A small free-standing UPS. The unit in the photo has IEC connector inputs and outputs
A small free-standing UPS. The unit in the photo has IEC connector inputs and outputs
A large datacenter-scale UPS being installed by electricians

An uninterruptible 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, utility power restored, or equipment safely shut down.

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).


[edit] 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):

  1. 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).
  2. Voltage sag — Transient (short term) under-voltage: Causes flickering of lights.
  3. Voltage spike — Transient (short term) over-voltage i.e. spike or peak: Causes wear or acute damage to electronic equipment.
  4. Under-voltage (brownout) — Low line voltage for an extended period of time: Causes overheating in motors.
  5. Over-voltage — Increased voltage for an extended period of time: Causes light bulbs to burnout.
  6. Line noise — Distortions superimposed on the power waveform: Causes electromagnetic interference.
  7. 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.
  8. 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.
  9. 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.

[edit] 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[clarification needed], 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.

[edit] Offline / standby

Offline / standby UPS. Typical protection time: 0 - 20 minutes. Capacity expansion: Usually not available

The Offline / Standby UPS (SPS) offers only the most basic features, providing surge protection and battery backup. Usually the Standby UPS offers no battery capacity monitoring or self-test capability, making it the least reliable type of UPS since it could fail at any moment without warning. These are also the least expensive, selling for as little as US$40. The SPS may be worse than using nothing at all, because it gives the user a false sense of security of being assured protection that may not work when needed the most.

With this type of UPS, a user's equipment is normally connected directly to incoming utility power with the same voltage transient clamping devices used in a common surge protected plug strip connected across the power line. When the incoming utility voltage falls below a predetermined level the SPS turns on its internal DC-AC inverter circuitry, which is powered from an internal storage battery. The SPS then mechanically switches the connected equipment on to its DC-AC inverter output. The switch over time is stated by most manufacturers as being less than 4 milliseconds, but typically can be as long as 25 milliseconds depending on the amount of time it takes the Standby UPS to detect the lost utility voltage.

[edit] Line-interactive

Line-Interactive UPS. Typical protection time: 5 - 30 minutes. Capacity expansion: Several hours

The Line-Interactive UPS is similar in operation to a Standby UPS, but with the addition of a multi-tap variable-voltage autotransformer. This is a special type of electrical transformer that can add or subtract powered coils of wire, thereby increasing or decreasing the magnetic field and the output voltage of the transformer.

This type of UPS is able to tolerate continuous undervoltage brownouts and overvoltage surges without consuming the limited reserve battery power. It instead compensates by auto-selecting different power taps on the autotransformer. Changing the autotransformer tap can cause a very brief output power disruption, so the UPS may chirp for a moment, as it briefly switches to battery before changing the selected power tap.

Autotransformers can be engineered to cover a wide range of varying input voltages, but this also increases the number of taps and the size, weight, complexity, and expense of the UPS. It is common for the autotransformer to only cover a range from about 90v to 140v for 120v power, and then switch to battery if the voltage goes much higher or lower than that range.

In low-voltage conditions the UPS will use more current than normal so it may need a higher current circuit than a normal device. For example to power a 1000 watt device at 120 volts, the UPS will draw 8.32 amps. If a brownout occurs and the voltage drops to 100 volts, the UPS will draw 10 amps to compensate. This also works in reverse, so that in an overvoltage condition, the UPS will need fewer amps of current.

[edit] Double-conversion / online

Typical protection time:
5 - 30 minutes
Capacity expansion:
Several hours

The Online UPS is ideal for environments where electrical isolation is necessary or for equipment that is very sensitive to power fluctuations. Although once previously reserved for very large installations of 10kW or more, advances in technology have permitted it to now be available as a common consumer device, supplying 500 watts or less. The Online UPS is generally more expensive but may be necessary when the power environment is "noisy" such as in industrial settings, for larger equipment loads like data centers, or when operation from an extended-run backup generator is necessary.

The basic technology of the online UPS is the same as in a Standby or Line-Interactive UPS. However it typically costs much more, due to it having a much greater current AC-to-DC battery-charger/rectifier, and with the rectifier and inverter designed to run continuously with improved cooling systems. It is called a Double-Conversion UPS due to the rectifier directly driving the inverter, even when powered from normal AC current.

In an Online UPS, the batteries are always connected to the inverter, so that no power transfer switches are necessary. When power loss occurs, the rectifier simply drops out of the circuit and the batteries keep the power steady and unchanged. When power is restored, the rectifier resumes carrying most of the load and begins charging the batteries, though the charging current may be limited to prevent the high-power rectifier from overheating the batteries and boiling off the electrolyte.

The main advantage to the on-line UPS is its ability to provide an electrical firewall between the incoming utility power and sensitive electronic equipment. While the Standby and Line-Interactive UPS merely filters the input utility power, the Double-Conversion UPS provides a layer of insulation from power quality problems. It allows control of output voltage and frequency regardless of input voltage and frequency.

[edit] Hybrid Topology / Double Conversion on Demand

Recently there have been hybrid topology UPSs hitting the marketplace. These hybrid designs do not have an official designation, although one named used by HP and Eaton is Double Conversion on Demand.[1] This style of UPS is targeted towards high efficiency applications while still maintaining the features and protection level offered by double conversion.

A hybrid (double conversion on demand) UPS operates as an offline/standby UPS when power conditions are within a certain preset window. This allows the UPS to achieve very high efficiency ratings. When the power conditions fluctuate outside of the predefined windows, the UPS switches to online/double conversion operation.[2] In double conversion mode the UPS can adjust for voltage variations without having to use battery power, can filter out line noise and control frequency. Examples of this hybrid/double conversion on demand UPS design are the HP R8000, HP R12000, HP RP12000/3 and the Eaton BladeUPS.

[edit] Ferro-resonant

Typical protection time:
5 - 15 Minutes
Capacity expansion:
Several Hours

Ferro-resonant units operate in the same way as a standby UPS unit with the exception that a ferro-resonant transformer is used to filter the output. This transformer is designed to hold energy long enough to cover the time between switching from line power to battery power and effectively eliminates the transfer time. Many ferro-resonant UPSs are 90-93% efficient and offer excellent isolation.

This used to be the dominant type of UPS and is limited to around the 15KVA range. These units are still mainly used in some industrial settings due to the robust nature of the UPS. Many ferro-resonant UPSs utilizing controlled ferro technology may not interact with power-factor-correcting equipment.

[edit] DC power

Typical protection time:
Several hours
Capacity expansion:

A UPS designed for powering DC equipment is very similar to an online UPS, except that it does not need an output inverter, and often the powered device does not need a power supply. Rather than converting AC to DC to charge batteries, then DC to AC to power the external device, and then back to DC inside the powered device, some equipment accepts DC power directly and allows one or more conversion steps to be eliminated. This equipment is more commonly known as a rectifier.

Many systems used in telecommunications use 48 volt DC power, because it is not considered a high-voltage by most electrical codes and is exempt from many safety regulations, such as being installed in conduit and junction boxes. DC has typically been the dominant power source for telecommunications, and AC has typically been the dominant source for computers and servers.

There has been much experimentation with 48v DC power for computer servers, in the hope of reducing the likelihood of failure and the cost of equipment. However, to supply the same amount of power, the current must be greater than an equivalent 120v or 240v circuit, and greater current requires larger conductors and/or more energy to be lost as heat.

High voltage DC (380 volts) is finding use in some data center applications, and allows for small power conductors, but is subject to the more complex electrical code rules for safe containment of high voltages.[3]

[edit] Rotary

Typical protection time:
20 - 60 seconds
Capacity expansion:
Several seconds

A Rotary UPS uses the inertia of a high-mass spinning flywheel to provide short-term ride-through in the event of power loss. The flywheel also acts as a buffer against power spikes and sags, since such short-term power events are not able to appreciably affect the rotational speed of the high-mass flywheel. It is also one of the oldest designs, predating vacuum tubes and integrated circuits.

It can be considered to be online since it spins continuously under normal conditions. However, unlike an electronic double-conversion UPS, it is only capable of providing reserve power for a few seconds before the flywheel has slowed and the protection fails. It is traditionally used in conjunction with standby diesel generators, providing backup power only for the brief period of time the engine needs to start running and stabilize its output.

The Rotary UPS is generally reserved for applications needing more than 10,000 watts of protection, to justify the expense of an extremely large and heavy power system that can only be transported by forklift or crane. A larger flywheel or multiple flywheels operating in parallel will increase the reserve running time, but at greatly increasing cost due to the size and weight of the precision-balanced flywheels.

Because the flywheels are a mechanical power source, it is not necessary to use an electric motor or generator as an intermediary between it and a diesel engine designed to provide emergency power. By using a transmission gearbox, the rotational inertia of the flywheel can be used to directly start up a diesel engine, and once running, the diesel engine can be used to directly spin the flywheel. Multiple flywheels can likewise be connected in parallel through mechanical countershafts, without the need for separate motors and generators for each flywheel.

They are normally designed to provide very high current output compared to a purely electronic UPS, and are better able to provide inrush current for inductive loads such as motor startup or compressor loads, as well as medical MRI and cath lab equipment. It is also able to tolerate short-circuit conditions up 17 times larger than an electronic UPS, permitting one device to blow a fuse and fail while other devices still continue to be powered from the Rotary UPS.

Its life cycle is usually far greater than a purely electronic UPS, up to 30 years or more. But they do require periodic downtime for mechanical maintenance (ball bearing replacement), while solid-state designs, using batteries, do not require downtime if the batteries can be hot-swapped, which is usually the case for larger units.

Typically, the high-mass flywheel is used in conjunction with a motor-generator system. These units can be configured as:[4]

  1. A motor driving a mechanically connected generator,
  2. A combined synchronous motor and generator wound in alternating slots of a single rotor and stator,
  3. A Hybrid Rotary UPS, designed similar to an Online UPS, except that it uses the flywheel in place of batteries. The rectifier drives a motor to spin the flywheel, while a generator uses the flywheel to power the inverter.

In case #3 the motor generator can be synchronous/synchronous or induction/synchronous. The motor side of the unit in case #2 and #3 can be driven directly by an AC power source (typically when in inverter bypass), a 6-step double-conversion motor drive, or a 6 pulse inverter. Case #1 uses an integrated flywheel as a short-term energy source instead of batteries to allow time for external, electrically coupled gensets to start and be brought online. Case #2 and #3 can use batteries or a free-standing electrically coupled flywheel as the short-term energy source.

[edit] Capacitors

UPSs can be equipped with maintenance-free capacitors to extend service life [5].

[edit] Applications

[edit] N+1

In large business environments where reliability is of great importance, a single huge UPS can also be a single point of failure that can disrupt many other systems. To provide greater reliability, multiple smaller UPS modules and batteries can be integrated together to provide redundant power protection equivalent to one very large UPS. "N+1" means that If the load can be supplied by N modules, the installation will contain N+1 modules. In this way, failure of one module will not impact system operation

[edit] Multiple redundancy

Many computer servers offer the option of redundant power supplies, so that in the event of one power supply failing, one or more other power supplies are able to power the load. This is a critical point - each power supply must be able to power the entire server by itself.

Redundancy is further enhanced by plugging each power supply into a circuit (i.e. to a different circuit breaker).

While it is common practice by uninformed people to plug each of these individual power supplies into one single UPS, redundant protection can be extended further yet by connecting each power supply to its own UPS. This provides double protection from both a power supply failure and a UPS failure, so that continued operation is assured. This configuration is also referred to as 2N redundancy. If the budget does not allow for two identical UPS units then it is common practice to plug one power supply into mains power and the other into the UPS.

[edit] Outdoor use

When a UPS system is placed outdoors, it should have some specific features that guarantee that it can tolerate weather with a 'minimal to none' effect on performance. Factors such as temperature, humidity, rain, and snow among others should be considered by the manufacturer when designing an outdoor UPS system. Operating temperature ranges for outdoor UPS systems could be around −40 °C to +55 °C.

Outdoor UPS systems can be pole, ground (pedestal), or host mounted. Outdoor environment could mean extreme cold, in which case the outdoor UPS system should include a battery heater mat, or extreme heat, in which case the outdoor UPS system should include a fan system or an air conditioning system.

[edit] Internal systems

UPS systems can be designed to be placed inside a computer chassis. There are two types of Internal UPS. The first type is a miniaturized regular UPS that is made small enough to fit into a 5.25″ CD-ROM slot bay of a regular computer chassis. The other type are re-engineered switching power supplies that utilize dual power sources of AC and/or DC as power inputs and have an AC/DC built-in switching management control units.

All these connections are connected to a common point called 'Earth'.

[edit] Cautions with generators

Some locations (hospitals, police stations, fire stations, etc.) have standby generators. The voltage and frequency of the power produced depends on the engine speed, and the speed is controlled by a system called a governor. Some are mechanical, some are electronic. The job of the governor is to keep the voltage and frequency constant as the load changes. In a large hospital, for example, the startup surge of an elevator can cause short "blips"in the frequency of the generator. In the USA the AM, FM and TV broadcast stations have generators, and since AM transmitters change load with the audio level, the generator is constantly trying to correct the output voltage and frequency as the load changes. And generators are rarely replaced. It's not uncommon to see 40 or 50 year old generators in regular use. And note that 85% of the broadcast transmitter sites are unmanned.

Many UPS units are incompatible with generators. The designers have written the microprocessor code to where they need exactly 50.0Hz (or 60.0Hz) power. If it is not, they don't "see" the incoming power at all.

Here is a typical scenario where this behavior is critically bad: Things are normal, the generator is off and the broadcast station is running on mains power. When the station loses power the UPS switches to its batteries and keeps its load running. The generator starts up, and a minute or so later the transfer switch moves the load (the station equipment) from the dead mains input to the generator output. The problem is that the load is constantly changing, and the generator frequency is constantly drifting plus and minus 1/2 cycle or so around 50.0Hz (or 60.0Hz). The UPS decides that the power is "bad" and its load remains on the UPS batteries. When the UPS batteries run out the loads that are plugged into the UPS either get switched to the mains input or just die, depending on the UPS design. This is despite the fact that there is plenty of slightly out-of-spec input power from the generator that the UPS could use to make DC (which has no frequency at all) and both charge the batteries and run the UPS inverter.

To complete the picture a timer starts when mains power returns, and 15 or maybe 20 minutes later (after the incoming power is stable) the transfer switch connects the station load back to mains power.

Only then does the UPS see power that meets its its overly tight specification and apply power to the load, and begin to recharge its batteries.

So the station has two outages: one of a minute or so due to the start-up and stabilize time of the generator, and a second one lasting a few tens of milliseconds when the transfer switch connects the station back to the mains power and shuts down the generator. The equipment that is plugged into the UPS does not see the first one, but instead has a major outage that starts when batteries die, and last as long as it takes to get mains power back. So why have a UPS at all?

UPS manufacturers that are known to have this design problem include APC, TrippLite, and Best (as shipped from the factory). The Best "Fortress" line (and maybe others) can have its tolerance window expanded.

The problems related to input frequency should only affect UPS not designed as "double conversion" (APC, BEST). These UPS have to generate an output frequency identical to the input frequency. If the input frequency is out of tolerance, or not properly recognized, UPS must transfer to battery. UPS designed with the "double conversion" topology could adapt to any input frequency and most of them do. They will create an output frequency according to internal clock.

A problem in the combination of "double conversion" UPS and generator is the voltage distortion created by the UPS. The input of a double conversion UPS is essentially a big rectifier. The current drawn by the UPS is non-sinusoidal. This causes the voltage from the generator also to become non-sinusoidal. The voltage distortion then can cause problem in all electrical equipment connected to the generator, including the UPS itself! This level of "noise" is measured in a percentage of "Total Harmonic Distortion of the current" (THD(i)). Classic UPS rectifiers have a THD(i) level around 25-30%. To prevent voltage distortion, this requires generators more than twice as big as the UPS.

There are several solutions to reduce the THD(i) in double conversion UPS: Classic solutions such as passive filters reduce (THD(i) to 5-10% at full load. They are reliable, but big and only work at full load and have their own problems with generators. Newer solution is an active filter. THD(i) can go down to 5% over full power range. Active filters are smaller, but they are expensive, consume energy and add components, thus risk of failures. The newest technology in double conversion UPS is a rectifier that doesn't use classic rectifier components (Thyristors and Diodes) but high frequency components (IGBT's). A double conversion UPS with an IGBT rectifier can have a THD(i) as small as 2%. This eliminates completely the need to oversize the generator (and transformers), without additional filters, investment cost, losses, space.

[edit] See also

[edit] Notes

[edit] References

  • Part of this article was based on a public-domain entry in Plant Engineering, February 2007 - UPS of the front line.
  • Part of this article was based on a public-domain entry in R&D Magazine - April 2008 - Selecting the Right UPS for Laboratory Applications.
  • EN 62040-1-1:2006 Uninterruptible power systems (UPS) — Part 1-1: General and safety requirements for UPS used in operator access areas
  • EN 62040-1-2:2003 Uninterruptible power systems (UPS) - Part 1-2: General and safety requirements for UPS used in restricted access locations
  • EN 62040-2:2006 Uninterruptible power systems (UPS)- Part 2: Electromagnetic compatibility (EMC) requirements
  • EN 62040-3:2001 Uninterruptible power systems (UPS) - Part 3: Method of specifying the performance and test requirements

[edit] External links

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