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A large data-center-scale UPS being installed by electriciansAn uninterruptible power supply (UPS) or uninterruptible power source is a type of continual power system that provides automated backup electric power to a load when the input power source or mains power fails. A UPS differs from a traditional auxiliary/emergency power system or standby generator in that it will provide near-instantaneous protection from input power interruptions by switching to energy stored in battery packs, supercapacitors or flywheels. The on-battery run-times of most UPSs are relatively short (only a few minutes) but sufficient to "buy time" for initiating a standby power source or properly shutting down the protected equipment. Almost all UPSs also contain integrated surge protection to shield the output appliances from voltage spikes.
A UPS is typically used to protect hardware such as 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 range in size from ones designed to protect a single computer without a video monitor (around 200 volt-ampere rating) to large units powering entire data centers or buildings. The world's largest UPS, the 46-megawatt Battery Energy Storage System (BESS), in Fairbanks, Alaska, powers the entire city and nearby rural communities during outages.[1]
Common power problems
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The primary role of any UPS is to provide short-term power when the input power source fails. However, most UPS units are also capable in varying degrees of correcting common utility power problems:
Some manufacturers of UPS units categorize their products in accordance with the number of power-related problems they address.[2]
A UPS unit may also introduce problems with electric power quality. To prevent this, a UPS should be selected not only by capacity, but also by the quality of power that is required by the equipment that is being supplied.
Technologies
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The three general categories of modern UPS systems are on-line, line-interactive and standby:[3][4]
Most UPS below one kilovolt-ampere (1 kVA) are of the line-interactive or standby variety which are usually less expensive.
For large power units, dynamic uninterruptible power supplies (DUPS) 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 as long as the flywheel's energy is not exhausted. DUPS are sometimes combined or integrated with a diesel generator that is turned on after a brief delay, forming a diesel rotary uninterruptible power supply (DRUPS).
A fuel cell UPS was developed by the company Hydrogenics using hydrogen and a fuel cell as a power source, potentially providing long run times in a small space.[5]
Offline/standby UPS: The green line illustrates the flow of electric power. Typical protection time: 5&#;20 minutes. Capacity expansion: Usually not available.The offline/standby UPS offers only the most basic features, providing surge protection and battery backup. The protected equipment is normally connected directly to incoming utility power. When the incoming voltage falls below or rises above a predetermined level the UPS turns on its internal DC-AC inverter circuitry, which is powered from an internal storage battery. The UPS then mechanically switches the connected equipment on to its DC-AC inverter output. The switch-over time can be as long as 25 milliseconds depending on the amount of time it takes the standby UPS to detect the lost utility voltage. The UPS will be designed to power certain equipment, such as a personal computer, without any objectionable dip or brownout to that device.
Line-interactive UPS: The green line illustrates the flow of electric power. 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 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 may also be performed by a buck&#;boost transformer which is distinct from an autotransformer, since the former may be wired to provide galvanic isolation.
This type of UPS is able to tolerate continuous under voltage brownouts and overvoltage surges without consuming the limited reserve battery power. It instead compensates by automatically selecting different power taps on the autotransformer. Depending on the design, changing the autotransformer tap can cause a very brief output power disruption,[6] which may cause UPSs equipped with a power-loss alarm to "chirp" for a moment.
This has become popular even in the cheapest UPSes because it takes advantage of components already included. The main 50/60 Hz transformer used to convert between line voltage and battery voltage needs to provide two slightly different turns ratios: One to convert the battery output voltage (typically a multiple of 12 V) to line voltage, and a second one to convert the line voltage to a slightly higher battery charging voltage (such as a multiple of 14 V). The difference between the two voltages is because charging a battery requires a delta voltage (up to 13&#;14 V for charging a 12 V battery). Furthermore, it is easier to do the switching on the line-voltage side of the transformer because of the lower currents on that side.
To gain the buck/boost feature, all that is required is two separate switches so that the AC input can be connected to one of the two primary taps, while the load is connected to the other, thus using the main transformer's primary windings as an autotransformer. The battery can still be charged while "bucking" an overvoltage, but while "boosting" an undervoltage, the transformer output is too low to charge the batteries.
Autotransformers can be engineered to cover a wide range of varying input voltages, but this requires more taps and increases complexity, as well as the expense of the UPS. It is common for the autotransformer to cover a range only from about 90 V to 140 V for 120 V 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 -W device at 120 V, the UPS will draw 8.33 A. If a brownout occurs and the voltage drops to 100 V, the UPS will draw 10 A to compensate. This also works in reverse, so that in an overvoltage condition, the UPS will need less 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 damaging the batteries. The main advantage of an online UPS is its ability to provide an "electrical firewall" between the incoming utility power and sensitive electronic equipment.
The online UPS is ideal for environments where electrical isolation is necessary or for equipment that is very sensitive to power fluctuations.[7] Although it was at one time reserved for very large installations of 10 kW or more, advances in technology have now permitted it to be available as a common consumer device, supplying 500 W or less. The online UPS may be necessary when the power environment is "noisy", when utility power sags, outages and other anomalies are frequent, when protection of sensitive IT equipment loads is required, 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.
Online UPS typically has a static transfer switch (STS) for increasing reliability.
Other designs
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Hybrid topology/double conversion on demand
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These hybrid rotary UPS[8] designs do not have official designations, although one name used by UTL is "double conversion on demand".[9] 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 off-line/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.[9] 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.
Ferroresonant
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Ferroresonant units operate in the same way as a standby UPS unit; however, they are online with the exception that a ferroresonant 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 ferroresonant UPSs are 82&#;88% efficient (AC/DC-AC) and offer excellent isolation.
The transformer has three windings, one for ordinary mains power, the second for rectified battery power, and the third for output AC power to the load.
This once was the dominant type of UPS and is limited to around the 150 kVA range. These units are still mainly used in some industrial settings (oil and gas, petrochemical, chemical, utility, and heavy industry markets) due to the robust nature of the UPS. Many ferroresonant UPSs utilizing controlled ferro technology may interact with power-factor-correcting equipment. This will result in fluctuating output voltage of the UPS, but may be corrected by reducing the load levels, or adding other linear type loads.[further explanation needed]
DC power
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A UPS designed for powering DC equipment is very similar to an online UPS, except that it does not need an output inverter. Also, if the UPS's battery voltage is matched with the voltage the device needs, the device's power supply will not be needed either. Since one or more power conversion steps are eliminated, this increases efficiency and run time.
Many systems used in telecommunications use an extra-low voltage "common battery" 48 V DC power, because it has less restrictive 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 48 V 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 would be higher than an equivalent 115 V or 230 V circuit; greater current requires larger conductors, or more energy lost as heat.
High voltage DC (380 V) 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.[10]
For lower power devices that run on 5 V, some portable battery banks can work as a UPS.
Rotary
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A rotary UPS uses the inertia of a high-mass spinning flywheel (flywheel energy storage) 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 on line since it spins continuously under normal conditions. However, unlike a battery-based UPS, flywheel-based UPS systems typically provide 10 to 20 seconds of protection before the flywheel has slowed and power output stops.[11] It is traditionally used in conjunction with standby 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 W of protection, to justify the expense and benefit from the advantages rotary UPS systems bring. A larger flywheel or multiple flywheels operating in parallel will increase the reserve running time or capacity.
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,[12] 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 to 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, such as ball bearing replacement. In larger systems redundancy of the system ensures the availability of processes during this maintenance. Battery-based designs do not require downtime if the batteries can be hot-swapped, which is usually the case for larger units. Newer rotary units use technologies such as magnetic bearings and air-evacuated enclosures to increase standby efficiency and reduce maintenance to very low levels.
Typically, the high-mass flywheel is used in conjunction with a motor-generator system. These units can be configured as:
In case No. 3 the motor generator can be synchronous/synchronous or induction/synchronous. The motor side of the unit in case Nos. 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 No. 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 Nos. 2 and 3 can use batteries or a free-standing electrically coupled flywheel as the short-term energy source.
Form factors
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Smaller UPS systems come in several different forms and sizes. However, the two most common forms are tower and rack-mount.[13]
Tower models stand upright on the ground or on a desk or shelf, and are typically used in network workstations or desktop computer applications. Rack-mount models can be mounted in standard 19-inch rack enclosures and can require anywhere from 1U to 12U (rack units). They are typically used in server and networking applications. Some devices feature user interfaces that rotate 90°, allowing the devices to be mounted vertically on the ground or horizontally as would be found in a rack.
Applications
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N + 1
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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.[14]
Multiple redundancy
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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 different circuit (i.e. to a different circuit breaker).
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 1 + 1 or 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.[15][16]
Outdoor use
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When a UPS system is placed outdoors, it should have some specific features that guarantee that it can tolerate weather without any effects 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.[17]
Outdoor UPS systems can either 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.
Internal view of a solar inverter. Note the many large capacitors (blue cylinders), used to store energy briefly and improve the output waveform.A solar inverter, or PV inverter, or solar converter, converts the variable direct current (DC) output of a photovoltaic (PV) solar panel into a utility frequency alternating current (AC) that can be fed into a commercial electrical grid or used by a local, off-grid electrical network. It is a critical BOS&#;component in a photovoltaic system, allowing the use of ordinary AC-powered equipment. Solar inverters have special functions adapted for use with photovoltaic arrays, including maximum power point tracking and anti-islanding protection.
Harmonic distortion
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UPS output waveform (yellow) compared to normal 120 VAC 60 Hz power waveform (violet)The output of some electronic UPSes can have a significant departure from an ideal sinusoidal waveform. This is especially true of inexpensive consumer-grade single-phase units designed for home and office use. These often utilize simple switching AC power supplies and the output resembles a square wave rich in harmonics. These harmonics can cause interference with other electronic devices including radio communication and some devices (e.g. inductive loads such as AC motors) may perform with reduced efficiency or not at all. More sophisticated (and expensive) UPS units can produce nearly pure sinusoidal AC power.
Power factor
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A problem in the combination of a double-conversion UPS and a 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 can cause the voltage from the AC mains or a generator to also become non-sinusoidal. The voltage distortion then can cause problems in all electrical equipment connected to that power source, including the UPS itself. It will also cause more power to be lost in the wiring supplying power to the UPS due to the spikes in current flow. This level of "noise" is measured as a percentage of "total harmonic distortion of the current" (THDI). Classic UPS rectifiers have a THDI level of around 25%&#;30%. To reduce voltage distortion, this requires heavier mains wiring or generators more than twice as large as the UPS.
There are several solutions to reduce the THDI in a double-conversion UPS:
Classic solutions such as passive filters reduce THDI to 5%&#;10% at full load. They are reliable, but big and only work at full load, and present their own problems when used in tandem with generators.
An alternative solution is an active filter. Through the use of such a device, THDI can drop to 5% over the full power range. The newest technology in double-conversion UPS units is a rectifier that does not use classic rectifier components (thyristors and diodes) but uses high-frequency components instead. A double-conversion UPS with an insulated-gate bipolar transistor rectifier and inductor can have a THDI as small as 2%. This completely eliminates the need to oversize the generator (and transformers), without additional filters, investment cost, losses, or space.
Communication
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Power management (PM) requires:
The basic computer-to-UPS control methods are intended for one-to-one signaling from a single source to a single target. For example, a single UPS may connect to a single computer to provide status information about the UPS, and allow the computer to control the UPS. Similarly, the USB protocol is also intended to connect a single computer to multiple peripheral devices.
In some situations it is useful for a single large UPS to be able to communicate with several protected devices. For traditional serial or USB control, a signal replication device may be used, which for example allows one UPS to connect to five computers using serial or USB connections.[19] However, the splitting is typically only one direction from UPS to the devices to provide status information. Return control signals may only be permitted from one of the protected systems to the UPS.[20]
As Ethernet has increased in common use since the s, control signals are now commonly sent between a single UPS and multiple computers using standard Ethernet data communication methods such as TCP/IP.[21] The status and control information is typically encrypted so that, for example, an outside hacker can not gain control of the UPS and command it to shut down.[22]
Distribution of UPS status and control data requires that all intermediary devices such as Ethernet switches or serial multiplexers be powered by one or more UPS systems, in order for the UPS alerts to reach the target systems during a power outage. To avoid the dependency on Ethernet infrastructure, the UPSs can be connected directly to main control server by using GSM/GPRS channel also. The SMS or GPRS data packets sent from UPSs trigger software to shut down the PCs to reduce the load.
Batteries
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Battery cabinetThere are three main types of UPS batteries: Valve Regulated Lead Acid (VRLA), Flooded Cell or VLA batteries, and Lithium Ion batteries. The run-time for a battery-operated UPS depends on the type and size of batteries and rate of discharge, and the efficiency of the inverter. The total capacity of a lead&#;acid battery is a function of the rate at which it is discharged, which is described as Peukert's law.
Manufacturers supply run-time rating in minutes for packaged UPS systems. Larger systems (such as for data centers) require detailed calculation of the load, inverter efficiency, and battery characteristics to ensure the required endurance is attained.[23]
Common battery characteristics and load testing
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When a lead&#;acid battery is charged or discharged, this initially affects only the reacting chemicals, which are at the interface between the electrodes and the electrolyte. With time, the charge stored in the chemicals at the interface, often called "interface charge", spreads by diffusion of these chemicals throughout the volume of the active material.
If a battery has been completely discharged (e.g. the car lights were left on overnight) and next is given a fast charge for only a few minutes, then during the short charging time it develops only a charge near the interface. The battery voltage may rise to be close to the charger voltage so that the charging current decreases significantly. After a few hours this interface charge will not spread to the volume of the electrode and electrolyte, leading to an interface charge so low that it may be insufficient to start a car.[24]
Due to the interface charge, brief UPS self-test functions lasting only a few seconds may not accurately reflect the true runtime capacity of a UPS, and instead an extended recalibration or rundown test that deeply discharges the battery is needed.[25]
The deep discharge testing is itself damaging to batteries due to the chemicals in the discharged battery starting to crystallize into highly stable molecular shapes that will not re-dissolve when the battery is recharged, permanently reducing charge capacity. In lead-acid batteries this is known as sulfation, but deep-discharge damage also affects other types such as nickel-cadmium batteries and lithium batteries.[26] Therefore, it is commonly recommended that rundown tests be performed infrequently, such as every six months to a year.[27][28]
Testing of strings of batteries/cells
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Multi-kilowatt commercial UPS systems with large and easily accessible battery banks are capable of isolating and testing individual cells within a battery string, which consists of either combined-cell battery units (such as 12-V lead acid batteries) or individual chemical cells wired in series. Isolating a single cell and installing a jumper in place of it allows the one battery to be discharge-tested, while the rest of the battery string remains charged and available to provide protection.[29]
It is also possible to measure the electrical characteristics of individual cells in a battery string, using intermediate sensor wires that are installed at every cell-to-cell junction, and monitored both individually and collectively. Battery strings may also be wired as series-parallel, for example two sets of 20 cells. In such a situation it is also necessary to monitor current flow between parallel strings, as current may circulate between the strings to balance out the effects of weak cells, dead cells with high resistance, or shorted cells. For example, stronger strings can discharge through weaker strings until voltage imbalances are equalized, and this must be factored into the individual inter-cell measurements within each string.[30]
Series-parallel battery interactions
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Battery strings wired in series-parallel can develop unusual failure modes due to interactions between the multiple parallel strings. Defective batteries in one string can adversely affect the operation and lifespan of good or new batteries in other strings. These issues also apply to other situations where series-parallel strings are used, not just in UPS systems but also in electric vehicle applications.[31]
Consider a series-parallel battery arrangement with all good cells, and one becomes shorted or dead:
The only way to prevent these subtle series-parallel string interactions is by not using parallel strings at all and using separate charge controllers and inverters for individual series strings.
Series new/old battery interactions
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Even just a single string of batteries wired in series can have adverse interactions if new batteries are mixed with old batteries. Older batteries tend to have reduced storage capacity, and so will both discharge faster than new batteries and also charge to their maximum capacity more rapidly than new batteries.
As a mixed string of new and old batteries is depleted, the string voltage will drop, and when the old batteries are exhausted the new batteries still have charge available. The newer cells may continue to discharge through the rest of the string, but due to the low voltage this energy flow may not be useful, and may be wasted in the old cells as resistance heating.
For cells that are supposed to operate within a specific discharge window, new cells with more capacity may cause the old cells in the series string to continue to discharge beyond the safe bottom limit of the discharge window, damaging the old cells.
When recharged, the old cells recharge more rapidly, leading to a rapid rise of voltage to near the fully charged state, but before the new cells with more capacity have fully recharged. The charge controller detects the high voltage of a nearly fully charged string and reduces current flow. The new cells with more capacity now charge very slowly, so slowly that the chemicals may begin to crystallize before reaching the fully charged state, reducing new cell capacity over several charge/discharge cycles until their capacity more closely matches the old cells in the series string.
For such reasons, some industrial UPS management systems recommend periodic replacement of entire battery arrays potentially using hundreds of expensive batteries, due to these damaging interactions between new batteries and old batteries, within and across series and parallel strings.[32]
Standards
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See also
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References
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An uninterruptible power supply (UPS) is a device that allows a computer to keep running for at least a short time when incoming power is interrupted. As long as utility power is flowing, it also replenishes and maintains the energy storage. The more energy stored, the longer power can be maintained, with practical limitations that will be discussed later. The differences among UPS systems lies in the technology that enables them to do their jobs.
Energy can be stored in different ways. Rechargeable batteries are the most common. For simplicity the examples and illustrations in this article will be based on that technology. However, kinetic energy can also be stored in heavy, rotating flywheels or energy can be stored as fuel.
The most commonly used type of UPS is also the most effective, generally called a full-time or full double conversion UPS. For any UPS, incoming utility power is alternating current (AC), which is also what is required by most information technology equipment (ITE).
Batteries, on the other hand, are direct current (DC) devices, so all battery-type UPSes must convert -- or rectify -- the incoming AC power to DC to charge the batteries. The UPS must also still deliver AC to the ITE, so DC power must be converted back to AC through a device known as an inverter.
In a double conversion UPS, power flows continuously through the rectifier and then through the inverter to the ITE. The output voltage and frequency are completely isolated from, and independent of, the input voltage and frequency. They can even be completely different than the input, so this system is technically classified as voltage and frequency independent (VFI).
Voltage and frequency independent: Figure 1 below shows a VFI system in normal operation. Anomalies in the input power are dealt with in two ways. A surge suppression device (SPD) absorbs particularly bad voltage spikes. These can be caused by lightning strikes on power lines, large motors as used on elevators or medical electronics equipment; welders or numerous other sources. But even the smallest variations, including voltage sags or brownouts never make it through a VFI UPS to the output.
Batteries are excellent electrical shock absorbers, but they also maintain a steady and constant voltage to the inverter, which completely resynthesizes the voltage and current so that the power delivered to the ITE is clean and steady. Connecting air conditioners or other motors to the UPS serving the ITE could contaminate this clean output power, so it's not recommended.
Note the bypass circuit going around the UPS. We'll address that later.
The battery is always in the circuit in normal operation, delivering small amounts of power when needed, such as during brownouts, so there isn't the slightest interruption in output power.
When utility power fails, as shown in Figure 2 below, the battery continues to deliver stored energy to the inverter, which continues to deliver clean power to the ITE. When utility power is restored, power flows back through the rectifier, feeds the inverter and recharges the batteries.
UPS static and maintenance bypass: UPSes are not uninterruptible. They are electrical or mechanical devices, so they not only require routine maintenance, but also are subject to component failures. For these reasons, all UPS systems have a built-in bypass to route incoming power around the system and directly to the ITE when necessary.
The high-quality SPD is still in the circuit but is only slightly better than running your home electronics on a power strip with surge protection. It won't stop utility power interruptions or deal with voltage sags or brownouts. If the UPS fails, the bypass operates immediately as a static switch.
When a technician must work on the system, the bypass is operated manually to render the internal components safe. If utility power fails while the UPS is in bypass, power to the ITE is interrupted. Any installation with only one UPS has this vulnerability. Figure 3 below shows the UPS in bypass mode.
Note that major spikes have been removed, but the voltage drop continues through.
Economy mode operation: The first law of thermodynamics, conservation of energy, states that energy can be neither created nor destroyed. No electrical or mechanical device is 100% efficient, so every conversion incurs a loss, which escapes as heat.
UPS systems are far more efficient than a decade ago, and they maintain close to the same efficiency from low to high load. But there is still loss in both the rectifier and the inverter, which are eliminated when the UPS is in bypass mode. Many VFI UPSes now offer a sophisticated version of bypass known as economy mode (eco mode), as shown in Figure 4 below. An eco-mode UPS can return to full VFI operation when needed.
When rectifier and inverter losses are eliminated, power and cost are saved until power fails and full UPS operation is needed. Some users set the system for VFI operation during the day, and have it automatically switched to eco mode at night if those operations are considered less critical. Eco mode is generally highly dependable, but many users are leery of switching the modes back and forth. Further, new VFI UPS efficiencies are within 1% or less of what can be achieved in eco mode, so many users now consider this alternate operating mode unnecessary.
Note that eco mode UPSes incorporate high-quality filters, which also incur a small loss, and that there is usually a short instability when switching modes. Eco mode efficiency is statistical, but it can be 99% if power failures rarely occur and are short term.
Line interactive UPS: A true line interactive UPS, also known as voltage independent (VI), is called that because the output frequency is the same as the input. They look virtually identical to VFI UPSes in eco mode, except for the size of their rectifiers, and the inability to switch to VFI mode.
The smaller rectifier needs only to charge the batteries, which help absorb anomalies and boost power when there are voltage sags. The batteries take over completely when power fails. Figure 5 below shows how the battery and inverter help compensate for incoming voltage variations by running in parallel with the output.
Figure 6 below shows a line interactive UPS when incoming service fails. The battery takes over, just as it would in a double conversion UPS, but the bypass switches the utility out of the circuit. Because the ITE runs on utility power most of the time, the second conversion through the inverter is avoided until power fails, eliminating one of the efficiency loss components.
A decade ago, VI UPSes could have an efficiency advantage of 5% or more over VFI units, but the huge improvements in VFI UPSes has reduced that to 1% or less.
Standby UPS: Figure 7 below is usually called a standby UPS and is classified as voltage and frequency dependent (VFD). Like a VI UPS, power is delivered directly to the ITE, but the battery and inverter are not in the circuit until power fails. The output is filtered, but it isn't as stable as a true VI UPS.
As shown in Figure 8 below, when power fails, the utility is switched out of the circuit, and the battery and inverter are switched in. There is some switching instability, but the delay is short enough for most computer power supplies to ride through.
When power is restored -- either utility or generator -- the inverter is disconnected, line power is switched back in and the batteries are recharged by the rectifier, which is much smaller than in a VFI or VI UPS.
Unfortunately, standby or VFD UPSes are sometimes advertised as line interactive. It's important to be sure which type a UPS is. The internationally recognized VI and VFD identifications provide absolute distinctions, but aren't always used by manufacturers, particularly for smaller systems.
There are three major types of mechanical UPSes, two of which are also non-battery. All three are true VFI or double conversion systems, but the intermediate conversion is purely mechanical:
UPS power factor: Power factor (pf) is the difference between real power and apparent power. This is greatly misunderstood but very important for the buyer to know. Historically, most large UPSes had a pf of 0.8, which meant a 100 kilovolt-Ampere (kVA) UPS could only deliver 80 kilowatts (kW) of real power. Most of today's modern UPSes have pfs of 0.9 to 1.0, which means the real power in kW is much closer to, or even equal to, the apparent power in kVA.
Central vs. distributed UPS: Distributed UPS usually means small UPSes mounted in each equipment cabinet, although sometimes there is a UPS for each cabinet row. Small VFI UPSes exist, but many are VFD or VI designs, so it's important to know which technology is being purchased. Small, rack mounted UPSes often have pfs of only 0.7, so a UPS advertised as 1,000 kVA might only deliver 700 watts. These have their place, but usually in situations with only one or two equipment racks where a centralized, standalone UPS would be uneconomical.
Small, distributed UPSes aren't always maintained as well as larger systems, so failed batteries often go unrecognized until it's too late.
There are a couple important considerations for choosing a UPS system, including:
Modularity: Most modern battery-based UPSes are modular. They are comprised of multiple, smaller UPS and battery units that can be combined as required to provide capacity, redundancy or both. It's no longer necessary to over-buy in anticipation of long-term growth. The frame just has to be big enough for long-term expectations.
Actual modules can be purchased and installed as needed, and an additional module or two can be installed to provide redundancy. For example, a 100 kW UPS might have six 20 kW modules to provide N+1 redundancy. Battery capacity can be added modularly in the same way. Further, modules in most systems are hot swappable, so a failed module can be removed and returned to the factory and a replacement sent overnight for user installation without interrupting operations.
As noted above, Flywheel UPSes can also be modularly combined for increased size, run duration and/or redundancy. These, however, must be added and maintained by trained personnel.
Step function: When large loads are suddenly imposed on electrical equipment, power can become momentarily unstable, such as when power is restored to homes and the lights flicker or when large motors start and lights momentarily dim. This is of most concern when running 2N UPS redundancy, because failure of one UPS requires the second UPS to instantly absorb the total load.
It's also of concern in VFD UPSes where the full load is transferred to the inverter on power fail, and can be problematic in VI systems, or with systems operating in eco mode. In evaluating large UPS systems, it's important that the electrical engineer obtains transient load data from UPS vendors, compares it and explain the results to the owner.
Batteries are an evolving technology, particular because of their increased usage in electric vehicles. Batteries are heavy, so the floor structural strength should always be verified. Three types of batteries are in common use today:
Battery duration: UPS systems emit heat, regardless of battery type, so there's a limit to how long a UPS can operate without air conditioning. The actual limit depends on factors such as room size, other equipment and building heat load, but the generally accepted rule is 30 minutes to 60 minutes.
At some point the UPS will overheat and go into self-protective thermal shutdown. Therefore, without a generator to restart cooling, longer battery durations are a waste of space and money and greatly increase battery replacement cost, particularly when VRLA batteries are used. Failure of one battery requires replacement of the entire string or other cells will fail prematurely. If IT personnel are concerned about orderly shutdowns, that's better accomplished with a feature available on most large UPSes that sends a signal over the network to shut down ITE when battery life reaches a preset level.
With generators, UPSes are often configured with only a few minutes of battery. Quality generators should start and stabilize within seconds, but longer durations are sometimes requested to provide time in case generators fail to start. This should be unnecessary with redundant generators.
Battery strings: The most common UPS component to fail is a battery. Therefore, the best configuration uses at least two battery strings to provide the required duration.
Battery monitoring and maintenance: Many newer UPS systems incorporate third-party battery monitoring. If they don't, it should be specified as an added requirement. Batteries tend to fail when suddenly put under load, which is exactly when they're most needed. There are several types of monitors, with debate among manufacturers as to which is best, but any monitoring system will alert to weak or failed cells before disaster strikes. Wet cells require regular maintenance. Batteries should be replaced whenever monitoring indicates a weak cell.
Transformers and grounding: Note that there are no input or output transformers shown in the UPS illustrations. Transformers were once standard in electronic UPSes, but are now rarely seen, which accounts for a good deal of the efficiency improvement. Eliminating transformers has one other potential advantage and two potential disadvantages:
VI and VFD UPSes can be problematic with unstable power. Because there are usually one or more flickers before power is restored solidly, these UPSes incorporate logic that keeps them from reverting to normal operation until power is stabilized.
VI and VFD UPSes shouldn't be used in locations with unstable power because they also incorporate a lock-out feature that prevents them from returning to normal if they switch back and forth too often, requiring manual restoration. The same problem can occur if generators are switched to the load too quickly, and wax and wane while trying to absorb the load.
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