An Uninterruptible Power Supply (UPS), also known as an Uninterruptible Power Source, Uninterruptible Power System, Continuous Power Supply (CPS) or a battery backup is a device which maintains a continuous supply of electric power to connected equipment by supplying power from a separate source when utility power is not available. There are two distinct types of UPS: off-line and line-interactive (also called on-line).
An off-line UPS remains idle until a power failure occurs, and then switches from utility power to its own power source, almost instantaneously. An on-line UPS continuously powers the protected load from its reserves (usually lead-acid batteries or stored kinetic energy), while simultaneously replenishing the reserves from the AC power.
The on-line type of UPS, in addition to providing protection against complete failure of the utility supply, provides protection against all common power problems, and for this reason it is also known as a power conditioner and a line conditioner.
While not limited to safeguarding any particular type of equipment, a UPS is typically used to protect computers, 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). Larger UPS units typically work in conjunction with generators.
Historically, UPSs were expensive and were most likely to be used on expensive computer systems and in areas where the power supply is interrupted frequently. As prices have fallen, UPS units have become an essential piece of equipment for data centers and business computers, and are also used for personal computers, entertainment systems and more.
In certain countries, where the electrical grid is under strain, providers struggle to ensure supply during peak demand (such as summer, when air-conditioning usage increases). To prevent unplanned blackouts, electrical utilities will sometimes use a process called rolling blackouts or load shedding, which involves cutting the power to large groups of customers for short periods of time. Several major blackouts occurred in 2003, most notably the 2003 North America blackout in the north-eastern US and eastern Canada and the 2003 Italy blackout, both of which affected over 50 million people, and brought attention to the need for UPS power backup units.
A UPS should not be confused with a standby generator, which does not provide protection from a momentary power interruption and may result in an interruption when it is switched into service, whether manually or automatically. Such generators are typically placed upstream of the UPS to provide cover for lengthy outages. Integrated systems that have UPS and standby-generator components are often referred to as emergency power systems.
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):
UPS units are divided into categories based on which of the above problems they address. Some manufacturers categorize their supplies as a level 3, 5, or 9, if they address the first 3, 5, or 9 power problems respectively.
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 redirecting 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.
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.
Rotary uninterruptible power supply equipment use a motor-generator system to create a perfect sine wave output. These units can be configured as (1) a motor driving a mechanically connected generator, (2) a combined synchronous/synchronous motor/generator wound in alternating slots of the field and stator, or (3) a Hybrid Rotary UPS utilizing a Rectifier and Inverter as found in traditional double conversion UPS with the addition of a motor being driven by the inverter and coupled to a generator.[1] 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. Sometimes, in case #1, a diesel engine can be run up to speed and then mechanically coupled to the generator, or the flywheel itself can be used to start the diesel engine (which is mechanically coupled as required to the flywheel and generator).
Rotary UPS equipment can provide up to 17x fault clearing capabilities (peak current to blow a fuse) without going to bypass. This means that the unit is able to provide "short circuit current" to actually blow a fuse or trip a protection switch, instead of "protecting itself" like a solid-state UPS will do.
These units provide superior current inrush handling for inductive loads such as motor startup or compressor loads as well as medical MRI and cath lab equipment.
The life cycle of these units is usually far greater than that of their static siblings, 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.
The Off-line Standby Power Supply (SBS) offers the bare bones power protection of basic surge protection and battery backup. Through this type of SBS a user's equipment is 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 SBS turns on its internal DC-AC inverter circuitry, which is powered from an internal storage battery. The SBS 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 SBS to detect the lost utility voltage.
Users selecting this type of an SBS must be aware that their computer equipment, as well as most electronic equipment is designed for use in their country. In United States, for example, SBS are designed to operate from a 120 volt, 60 Hertz (Hz), sinewave utility source. Most Off-line SBS products on the market today only provide a sinewave output when operating normally from the utility line. When they switch to their internal DC-AC inverter they may only provide a square wave, modified square wave or quasi-sinewave, not a pure sinewave. In many cases equipment may appear to operate normally on these waveforms, but over time may be damaged by them. When only minimal protection is needed, it is always best to select an SBS or UPS that states it has an inverter with a true sinewave output. Most off-line SBS units will not be capable of accepting additional battery packs for extended battery operation. To keep the cost down and prevent overheating, their inverters are designed to only operate as long as the internal battery capacity allows. Units of all three design types typically provide from 5 to 15 minutes of battery back-up time when loaded to their full output capacity. Slightly longer backup times can be achieved by overrating the SBS or UPS size.
The Line-interactive UPS offers the same bare bones surge protection and battery back-up as the standby, except that it has the added feature of minimal voltage regulation while operating from the utility source. This line-interactive design came about due to the SBS inability to provide an acceptable output voltage to the connected equipment during “brown-out” conditions. Interestingly, however, many standby models now have a voltage regulation feature. A “brown-out” happens when the utility voltage remains excessively low for a sustained period. Under these conditions the off-line SBS would go to battery operation and if the brown-out was sustained long enough, the SBS battery would become fully discharged, turn the power off to the connected equipment and not be able to be turned back on until the utility voltage returned to normal. To prevent this from happening a voltage regulating transformer was added, hence the term line-interactive was born. This feature really does help as low voltage utility conditions are common. The down side for this design, most of the units available have to switch to battery momentarily when making transformer voltage adjustments and this can be a bit annoying in a quiet home office on a bad power day.
Again when selecting a Line-interactive UPS it is always best to select a model with a true sinewave output. Several manufacturers have models available that will accept extended battery packs to provide additional battery runtime. This type of UPS typically costs more than the off-line type, but is worth the additional cost.
The On-line UPS is ideal for environments where electrical isolation is necessary or for equipment loads of greater than 10kW. It does typically cost more, but like all electronic equipment today the cost is coming down as the technology advances. The true 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 off-line and line-interactive designs merely filter the input utility power, the Double-conversion on-line UPS provides a layer of insulation from power quality problems. This is accomplished with the use of a rectifier (converting AC to DC), a DC bus, and an inverter (converting DC back to AC). This "double conversion" enables the output signal to be independent from the input, isolating the input from the output. It allows control of output voltage and frequency regardless of input voltage and frequency.
On-line UPS are generally more expensive but may be necessary when the power environment is "noisy" such as in industrial settings, in dynamic load environments, or for larger equipment loads like data centers.
In data centers, larger models are ideal and the design might include multiple sets of UPS units running in parallel providing dual sources of conditioned power to static switches that then send power to server loads. In such a system, a complete UPS failure can occur without the loads connected to the switches being affected. Models known as power arrays also serve the larger needs by grouping the UPS electronics and batteries into modules to provide component redundancy and allow component-swapping service and expansion.
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.
While this used to be the dominant type of UPS, they are no longer used for common applications. Power factor correcting equipment found in newer computer systems interacts with static ferro-resonant transformers, causing potentially damaging oscillations, and the transformer itself can create distortions which yield power less acceptable than poor quality line AC. These units are still used in some industrial settings, but have mostly disappeared from use with general computer equipment. Many ferro-resonant UPSs utilizing controlled ferro technology may not interact with power-factor-correcting equipment.
Many systems used in telecommunications use DC power (often 48 V). Rather than converting AC to DC to charge batteries, then DC to AC and then convert it back to DC again, some equipment accepts 48 V DC power directly. By simply converting AC power to DC power and adding batteries to the DC side, one or more conversion steps can be eliminated. There has been much experimentation with DC power for computer servers, in the hope of reducing the likelihood of failure and the cost of equipment. Because there is more current to transfer the same amount of energy at the lower DC voltage, larger conductors are needed, and more energy is lost as heat. Eliminating a conversion step may seem more reliable, but the ability of online double conversion AC systems to entirely remove themselves from operation and transfer to bypass mode during certain UPS failures and maintenance allows for the connected servers to continue to function on unconditioned AC power while the UPS is repaired. DC-based power systems do not have this luxury, as it requires that all equipment have special DC power inputs that cannot utilize AC voltages in the event of a main DC rectifier or power distribution failure. DC has typically been the dominant power source for telecommunications, and AC has typically been the dominant source for computers and servers. Higher voltage DC (380 volts) is finding use in some data center applications.[2]
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 have been considered by the manufacturer when designing an outdoor UPS system. Operating temperature ranges for outdoor UPS systems could be around -40ºC to +55ºC.
An outdoor UPS system is normally made of several components designed for this particular task:
A proper outdoor UPS system requires that all its components are designed for this environment. As seen from the features of the components above, an outdoor UPS system is not an indoor UPS inside an outdoor enclosure.
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.
Outdoor UPS systems are ideal for protection of WiFi/GSM/CDMA/satellite base stations, wireless communications/perimeter surveillance and security/gate control systems, LED traffic light/roadway display systems and remote terminal units (RTUs).
Internal UPS are a group of uninterruptible power supplies (UPS) designed to be placed inside computer chassis. There are two types of Internal UPS. First type is miniaturized regular UPS that are made small enough to fit into a 5.25” CD-ROM slot bay of a regular computer chassis. The other type is 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.
The first type often requires extra connection wires between the internal UPS and computer's power supply. Some internal UPS of this group output high voltage (110 V - 220 V) direct current (DC) and some output nine-step table wave AC. Neither design is safe or energy efficient. As of 2006, there are only couple companies still selling this type of internal UPS in Australia, Asia and some part of Europe
The second group of internal UPS replaces the regular switching power supplies. There are three main design mechanisms:
The first question to ask when choosing a UPS system should be: is this unit going to be placed inside a controlled environment -e.g. air conditioned, dust free-? If the answer to this question is yes, choose an indoor UPS. If the answer is no, choose an outdoor UPS. If an outdoor UPS is placed in a controlled environment, it is probably a waste of money (exceptions to this include powering small loads during an extended period of time, where outdoor UPS systems are sometimes the only available option as they have large battery chargers -normally around 10Amps-). If an indoor UPS is placed in a non-controlled environment, the useful life of this system will be considerably shortened, threatening the integrity and backup of the equipment the UPS is protecting.
Besides choosing a UPS design, there are 2 key ratings to be aware of when choosing a UPS unit. The first is the load rating, expressed as both volt amps (VA) and watts (W). Both the ratings represent the maximum amount of load that the UPS can support and the connected load typically should not exceed 80% of either. Special considerations must be made when connecting certain equipment such as printers or any type of motorized load due to their higher starting currents.
The second factor in deciding which unit to purchase is the amount of runtime the unit will be able to provide when the power fails. This number will vary with the load amount that is plugged into the UPS. For example, a unit may run a single computer for 30 minutes, but with 2 computers it will generally last less than half that time. Larger units typically can provide more runtime for the same load than smaller units, however that is not always the case. Some UPS units are designed to provide extended runtime or have the ability to have external battery packs connected.
Another consideration is the anticipated usage. If the UPS is only intended to provide enough power to gracefully shut down the computers, IP-based network management or serial/USB ports on the UPS are essential. This communication will also require loading some level of software on the computer or network. If the purpose of the UPS is to provide power until a standby generator kicks in (typically under a minute), the UPS input capabilities should be matched to the generator outputs. Specifically, most standby generators made for home use (15 kW or less) and most portable generators lack microprocessor voltage-and-frequency control and may not create a smooth sine wave. This can result in voltage and frequency fluctuating by 5% or more. While most UPS systems handle voltage fluctuations gracefully, most do not handle frequency fluctuations well. A UPS with a wide "frequency window" is essential in such cases. However, this can double the cost of the unit. Only a double conversion UPS can deliver a stable output frequency when powered by an unstable input frequency.
If the UPS needs to be quiet when running from battery, or will power AC motors (as found in air conditioners and fans) or other equipment requiring a clean sine wave (such as high-end computer power supplies), a UPS that outputs a smooth sine wave is needed. For PCs and other common electronics, a quasi-sine wave waveform[3] is acceptable.
Another consideration should be based on the type of load or connected equipment the UPS will support. If the UPS is connected to ultra-sensitive electronics (like lasers), a rotary solution will be more suitable with 100% line to load isolation. This would not only protect the equipment from a power outage, but will also protect the connected equipment from any anomaly that comes from the utility feed.
Features to look for:
In order to provide the desired protection, UPS units must be properly maintained. Sealed lead/acid batteries have a useful lifetime of 3–5 years. In determining when to replace batteries, it is important to remember that the batteries can be completely bad after 3–5 years and lose their ability to hold a charge gradually over that time.[4] If a UPS started with 1 hour of runtime for the connected load, after 1 year, it may only provide 45 minutes of backup time. Battery failure can also be caused by temperature. If the application requires the battery to operate properly at temperatures exceeding 25 °C, using gel batteries will allow the UPS to work in temperatures between -40 to +70 °C.
See main article: Electronic waste. Many UPS units contain sealed lead-acid batteries and electronics which can be detrimental to the environment. In the United States, it is illegal to dispose of lead-acid batteries in a landfill, and they must be properly recycled. Sealed lead-acid batteries are recycled in the same manner as car batteries, so any auto shop that accepts used car batteries for recycling will also accept sealed lead acid batteries.