![500kVA-UPS](https://upload.wikimedia.org/wikipedia/commons/5/5e/500kVA-UPS.jpg)
An uninterruptible power supply or uninterruptible power source (UPS) is an electrical apparatus that provides emergency power to a load when the input power source or
mains power
Mains electricity or utility power, power grid, domestic power, and wall power, or in some parts of Canada as hydro, is a general-purpose alternating-current (AC) electric power supply. It is the form of electrical power that is delivered to ...
fails. A UPS differs from an auxiliary or
emergency power system
An emergency power system is an independent source of electrical power that supports important electrical systems on loss of normal power supply. A standby power system may include a standby generator, batteries and other apparatus. Emergency p ...
or
standby generator
Standby generators
A standby generator is a back-up electrical system that operates automatically.Robert B. Hickey ''Electrical Construction Databook'', McGraw Hill, 2002 , Chapter 14 Within seconds of a utility outage an automatic transfer swi ...
in that it will provide near-instantaneous protection from input power interruptions, by supplying energy stored in batteries,
supercapacitor
A supercapacitor (SC), also called an ultracapacitor, is a high-capacity capacitor, with a capacitance value much higher than other capacitors but with lower voltage limits. It bridges the gap between electrolytic capacitors and rechargeable ba ...
s, or
flywheel
A flywheel is a mechanical device which uses the conservation of angular momentum to store rotational energy; a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, ass ...
s. The on-battery run-time of most uninterruptible power sources is relatively short (only a few minutes) but sufficient to start a standby power source or properly shut down the protected equipment. It is a type of
continual power system.
A UPS is typically used to protect hardware such as
computer
A computer is a machine that can be programmed to Execution (computing), carry out sequences of arithmetic or logical operations (computation) automatically. Modern digital electronic computers can perform generic sets of operations known as C ...
s,
data center
A data center (American English) or data centre (British English)See spelling differences. is a building, a dedicated space within a building, or a group of buildings used to house computer systems and associated components, such as telecommunic ...
s,
telecommunication
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
equipment or other electrical equipment where an unexpected power disruption could cause injuries, fatalities, serious business disruption or
data loss Data loss is an error condition in information systems in which information is destroyed by failures (like failed spindle motors or head crashes on hard drives) or neglect (like mishandling, careless handling or storage under unsuitable conditions) ...
. UPS units range in size from ones designed to protect a single computer without a video monitor (around 200
volt-ampere
A volt-ampere ( SI symbol: V⋅A or V A, simplified as VA) is the unit for the apparent power in an electrical circuit. The apparent power equals the product of root mean square voltage (in volts) and root mean square current (in 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
Fairbanks is a home rule city and the borough seat of the Fairbanks North Star Borough in the U.S. state of Alaska. Fairbanks is the largest city in the Interior region of Alaska and the second largest in the state. The 2020 Census put the po ...
, powers the entire city and nearby rural communities during outages.
Common power problems
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:
#
Voltage spike
In electrical engineering, spikes are fast, short duration electrical transients in voltage (voltage spikes), current (current spikes), or transferred energy (energy spikes) in an electrical circuit.
Fast, short duration electrical transients ...
or sustained
overvoltage
When the voltage in a circuit or part of it is raised above its upper design limit, this is known as overvoltage. The conditions may be hazardous. Depending on its duration, the overvoltage event can be transient—a voltage spike—or perm ...
# Momentary or sustained
reduction in input voltage
#
Voltage sag
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
# Noise, defined as a
high frequency transient or
oscillation
Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
, usually injected into the line by nearby equipment
# Instability of the
mains frequency
The utility frequency, (power) line frequency (American English) or mains frequency (British English) is the nominal frequency of the oscillations of alternating current (AC) in a wide area synchronous grid transmitted from a power station to th ...
#
Harmonic distortion
In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...
, defined as a departure from the ideal
sinusoidal
A sine wave, sinusoidal wave, or just sinusoid is a mathematical curve defined in terms of the '' sine'' trigonometric function, of which it is the graph. It is a type of continuous wave and also a smooth periodic function. It occurs often in m ...
waveform
In electronics, acoustics, and related fields, the waveform of a signal is the shape of its graph as a function of time, independent of its time and magnitude scales and of any displacement in time.David Crecraft, David Gorham, ''Electronic ...
expected on the line
Some manufacturers of UPS units categorize their products in accordance with the number of power-related problems they address.
A UPS unit may also introduce problems with
electric power quality Electric power quality is the degree to which the voltage, frequency, and waveform of a power supply system conform to established specifications. Good power quality can be defined as a steady supply voltage that stays within the prescribed range, ...
. 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
The three 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
A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an inver ...
to DC for passing through the
rechargeable battery
A rechargeable battery, storage battery, or secondary cell (formally a type of Accumulator (energy), energy accumulator), is a type of electrical battery which can be charged, discharged into a load, and recharged many times, as opposed to ...
(or battery strings), then inverting back to 120 V/230 V 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 one kilo
volt-ampere
A volt-ampere ( SI symbol: V⋅A or V A, simplified as VA) is the unit for the apparent power in an electrical circuit. The apparent power equals the product of root mean square voltage (in volts) and root mean square current (in 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
A flywheel is a mechanical device which uses the conservation of angular momentum to store rotational energy; a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, ass ...
. 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
Most forms of uninterruptible power supply (UPS) can be either powered by battery or flywheel energy. These are ready for immediate use at the instant that the mains electricity fails, but the relatively small and finite amount of stored energy ...
(DRUPS).
A
fuel cell
A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requ ...
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.
Offline/standby
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
The line-interactive UPS is similar in operation to a standby UPS, but with the addition of a multi-tap variable-voltage
autotransformer
An autotransformer is an electrical transformer with only one winding. The "auto" (Greek for "self") prefix refers to the single coil acting alone, not to any kind of automatic mechanism. In an autotransformer, portions of the same winding act as ...
. This is a special type of
transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the 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
A buck–boost transformer is a type of transformer used to make adjustments to the voltage applied to alternating current equipment. Buck–boost connections are used in several places such as uninterruptible power supply (UPS) units for com ...
'' which is distinct from an autotransformer, since the former may be wired to provide
galvanic isolation
Galvanic isolation is a principle of isolating functional sections of electrical systems to prevent current flow; no direct conduction path is permitted.John Huntington ''Show Networks and Control Systems: Formerly Control Systems for Live ...
.
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 automatically selecting different power taps on the autotransformer. Depending on the design, changing the autotransformer tap can cause a very brief output power disruption, 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 1000-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.
Online/double-conversion
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. 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
A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an Power ...
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
Hybrid topology/double conversion on demand
These hybrid rotary UPS
designs do not have official designations, although one name used by UTL is "double conversion on demand".
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.
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
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 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.
DC power
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
A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As a r ...
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
Extra-low voltage (ELV) is an electricity supply voltage and is a part of the Low voltage bandIEC 61140:2016 Chapter 4.2 in a range which carries a low risk of dangerous electrical shock. There are various standards that define extra-low voltage. ...
"
common battery
In telecommunication, a common battery is a single electrical power source used to energize more than one circuit, electronic component, equipment, or system.
A common battery is usually a string of electrolytic cells and is usually centrally ...
" 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.
For lower power devices that run on 5 V, some
portable battery banks can work as an UPS.
Rotary
A rotary UPS uses the inertia of a high-mass spinning
flywheel
A flywheel is a mechanical device which uses the conservation of angular momentum to store rotational energy; a form of kinetic energy proportional to the product of its moment of inertia and the square of its rotational speed. In particular, ass ...
(
flywheel energy storage
Flywheel energy storage (FES) works by accelerating a rotor (flywheel) to a very high speed and maintaining the energy in the system as rotational energy. When energy is extracted from the system, the flywheel's rotational speed is reduced as a c ...
) to provide short-term
ride-through
In electrical power engineering, fault ride through (FRT), sometimes under-voltage ride through (UVRT), or low voltage ride through (LVRT), is the capability of electric generators to stay connected in short periods of lower electric network volt ...
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. 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, 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
A catheterization laboratory, commonly referred to as a cath lab, is an examination room in a hospital or clinic with diagnostic imaging equipment used to visualize the arteries of the heart and the chambers of the heart and treat any stenosis ...
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
A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races.
The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. It achieves this ...
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 bearing
A magnetic bearing is a type of bearing that supports a load using magnetic levitation. Magnetic bearings support moving parts without physical contact. For instance, they are able to levitate a rotating shaft and permit relative motion with ve ...
s 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:
# A motor driving a mechanically connected generator,
# A combined
synchronous motor
A synchronous electric motor is an AC electric motor in which, at steady state,
the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles. Sync ...
and generator wound in alternating slots of a single rotor and stator,
# 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 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
Smaller UPS systems come in several different forms and sizes. However, the two most common forms are tower and rack-mount.
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 unit
A rack unit (abbreviated U or RU) is a unit of measure defined as . It is most frequently used as a measurement of the overall height of 19-inch and 23-inch rack frames, as well as the height of equipment that mounts in these frames, whereby th ...
s). 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
''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.
Multiple redundancy
Many computer servers offer the option of redundant
power supplies
A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As a res ...
, 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
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the risk ...
).
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 2''N'' 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
Mains electricity or utility power, power grid, domestic power, and wall power, or in some parts of Canada as hydro, is a general-purpose alternating-current (AC) electric power supply. It is the form of electrical power that is delivered to ...
and the other into the UPS.
Outdoor use
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
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depe ...
, rain, and snow among others should be considered by the manufacturer when designing an outdoor UPS system.
Operating temperature
An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...
ranges for outdoor UPS systems could be around −40 °C to +55
°C
The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The ...
.
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.
![Sunny Boy 3000](https://upload.wikimedia.org/wikipedia/commons/e/ee/Sunny_Boy_3000.jpg)
A solar inverter, or PV inverter, or solar converter, converts the variable
direct current
Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even ...
(DC) output of a
photovoltaic
Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commercially us ...
(PV)
solar panel
A solar cell panel, solar electric panel, photo-voltaic (PV) module, PV panel or solar panel is an assembly of photovoltaic solar cells mounted in a (usually rectangular) frame, and a neatly organised collection of PV panels is called a photo ...
into a
utility frequency
The utility frequency, (power) line frequency (American English) or mains frequency (British English) is the nominal frequency of the oscillations of alternating current (AC) in a wide area synchronous grid transmitted from a power station to th ...
alternating current
Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...
(AC) that can be fed into a commercial electrical
grid
Grid, The Grid, or GRID may refer to:
Common usage
* Cattle grid or stock grid, a type of obstacle is used to prevent livestock from crossing the road
* Grid reference, used to define a location on a map
Arts, entertainment, and media
* News g ...
or used by a local,
off-grid
Off-the-grid or off-grid is a characteristic of buildings and a lifestyle designed in an independent manner without reliance on one or more public utilities. The term "off-the-grid" traditionally refers to not being connected to the electrical g ...
electrical network. It is a critical
BOS
''Bos'' (from Latin '' bōs'': cow, ox, bull) is the genus of wild and domestic cattle. ''Bos'' is often divided into four subgenera: ''Bos'', ''Bibos'', ''Novibos'', and ''Poephagus'', but including these last three divisions within the gen ...
–component in a
photovoltaic system
A photovoltaic system, also PV system or solar power system, is an electric power system designed to supply usable solar power by means of photovoltaics. It consists of an arrangement of several components, including solar panels to absorb and co ...
, allowing the use of ordinary AC-powered equipment. Solar inverters have special functions adapted for use with photovoltaic arrays, including
maximum power point tracking
Maximum power point tracking (MPPT) or sometimes just power point tracking (PPT), is a technique used with variable power sources to maximize energy extraction as conditions vary. The technique is most commonly used with photovoltaic (PV) solar sy ...
and anti-
islanding
Islanding is the condition in which a distributed generator (DG) continues to power a location even though external electrical grid power is no longer present. Islanding can be dangerous to utility workers, who may not realize that a circuit is st ...
protection.
Harmonic distortion
![APC Back-UPS 550 Uninterruptible Power Supply Output Waveform](https://upload.wikimedia.org/wikipedia/commons/c/c4/APC_Back-UPS_550_Uninterruptible_Power_Supply_Output_Waveform.jpg)
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
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" (THD
''I''). Classic UPS rectifiers have a THD
''I'' 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 THD
''I'' in a 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 present their own problems when used in tandem with generators.
An alternative solution is an active filter. Through the use of such a device, THD
''I'' 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
An insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch, which, as it was developed, came to combine high efficiency and fast switching. It consists of four alternating lay ...
rectifier and inductor can have a THD
''I'' as small as 2%. This completely eliminates the need to oversize the generator (and transformers), without additional filters, investment cost, losses, or space.
Communication
Power management
Power management is a feature of some electrical appliances, especially copiers, computers, computer CPUs, computer GPUs and computer peripherals such as monitors and printers, that turns off the power or switches the system to a low-power stat ...
(PM) requires:
# The UPS to report its status to the computer it powers via a communications link such as a
serial port
In computing, a serial port is a serial communication interface through which information transfers in or out sequentially one bit at a time. This is in contrast to a parallel port, which communicates multiple bits simultaneously in parallel. ...
,
Ethernet
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
and
Simple Network Management Protocol
Simple Network Management Protocol (SNMP) is an Internet Standard protocol for collecting and organizing information about managed devices on IP networks and for modifying that information to change device behaviour. Devices that typically ...
, GSM/
GPRS
General Packet Radio Service (GPRS) is a packet oriented mobile data standard on the 2G and 3G cellular communication network's global system for mobile communications (GSM). GPRS was established by European Telecommunications Standards Insti ...
or
USB
Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply (interfacing) between computers, peripherals and other computers. A broad ...
# A subsystem in the
OS that processes the reports and generates notifications, PM events, or commands an ordered shut down.
[Raymond, Eric Steven]
UPS HOWTO, section 3.3
The Linux Documentation Project, 2003–2007. Some UPS manufacturers publish their communication protocols, but other manufacturers (such as
APC) use
proprietary protocol
In telecommunications, a proprietary protocol is a communications protocol owned by a single organization or individual.
Intellectual property rights and enforcement
Ownership by a single organization gives the owner the ability to place restricti ...
s.
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. 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.
As Ethernet has increased in common use since the 1990s, control signals are now commonly sent between a single UPS and multiple computers using standard Ethernet data communication methods such as
TCP/IP
The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suit ...
. 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.
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
A power outage (also called a powercut, a power out, a power failure, a power blackout, a power loss, or a blackout) is the loss of the electrical power network supply to an end user.
There are many causes of power failures in an electricit ...
. 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
Battery cabinet
There 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
The lead–acid battery is a type of rechargeable battery first invented in 1859 by French physicist Gaston Planté. It is the first type of rechargeable battery ever created. Compared to modern rechargeable batteries, lead–acid batteries have ...
is a function of the rate at which it is discharged, which is described as
Peukert's law
Peukert's law, presented by the German scientist in 1897, expresses approximately the change in capacity of rechargeable lead–acid batteries at different rates of discharge. As the rate of discharge increases, the battery's available capacity ...
.
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.
Common battery characteristics and load testing
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
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
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.
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.
The deep discharge testing is itself damaging to batteries due to the chemicals in the discharged battery starting to
crystallize
Crystallization is the process by which solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some ways by which crystals form are precipitating from a solution, freezing, or more rarely deposi ...
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
Sulfation is the chemical reaction that entails the addition of SO3 group. In principle, many sulfations would involve reactions of sulfur trioxide (SO3). In practice, most sulfations are effected less directly. Regardless of the mechanism, the ...
but also affects other types such as
nickel cadmium batteries
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to r ...
and
lithium batteries
Lithium battery may refer to:
* Lithium metal battery, a non-rechargeable battery with lithium as an anode
** Rechargeable lithium metal battery, a rechargeable counterpart to the lithium metal battery
* Lithium-ion battery, a rechargeable batte ...
. Therefore, it is commonly recommended that rundown tests be performed infrequently, such as every six months to a year.
Testing of strings of batteries/cells
Multi-
kilowatt
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wa ...
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.
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.
Series-parallel battery interactions
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
An electric vehicle (EV) is a vehicle that uses one or more electric motors for propulsion. It can be powered by a collector system, with electricity from extravehicular sources, or it can be powered autonomously by a battery (sometimes cha ...
applications.
Consider a series-parallel battery arrangement with all good cells, and one becomes shorted or dead:
* The failed cell will reduce the maximum developed voltage for the entire series string it is within.
* Other series strings wired in parallel with the degraded string will now discharge through the degraded string until their voltage matches the voltage of the degraded string, potentially
overcharging and leading to
electrolyte
An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...
boiling and outgassing from the remaining good cells in the degraded string. These parallel strings can now never be fully recharged, as the increased voltage will bleed off through the string containing the failed battery.
* Charging systems may attempt to gauge battery string capacity by measuring overall voltage. Due to the overall string voltage depletion due to the dead cells, the charging system may detect this as a state of discharge, and will continuously attempt to charge the series-parallel strings, which leads to continuous overcharging and damage to all the cells in the degraded series string containing the damaged battery.
* If
lead-acid batteries are used, all cells in the formerly good parallel strings will begin to sulfate due to the inability for them to be fully recharged, resulting in the storage capacity of these cells being permanently damaged, even if the damaged cell in the one degraded string is eventually discovered and replaced with a new one.
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
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.
Standards
* IEC 62040-1:2017 Uninterruptible power systems (UPS) – Part 1: General and safety requirements for UPS
* IEC 62040-2:2016 Uninterruptible power systems (UPS) – Part 2: Electromagnetic compatibility (EMC) requirements
* IEC 62040-3:2021 Uninterruptible power systems (UPS) – Part 3: Method of specifying the performance and test requirements
* IEC 62040-4:2013 Uninterruptible power systems (UPS) – Part 4: Environmental aspects – Requirements and reporting
See also
*
Battery room
A battery room is a room that houses batteries for backup or uninterruptible power systems. The rooms are found in telecommunication central offices, and provide standby power for computing equipment in datacenters. Batteries provide direct cu ...
*
Emergency power system
An emergency power system is an independent source of electrical power that supports important electrical systems on loss of normal power supply. A standby power system may include a standby generator, batteries and other apparatus. Emergency p ...
*
Fuel cell applications
*
IT baseline protection
The IT baseline protection (german: IT-Grundschutz) approach from the German Federal Office for Information Security (BSI) is a methodology to identify and implement computer security measures in an organization. The aim is the achievement of an ad ...
*
Power conditioner
A power conditioner (also known as a line conditioner or power line conditioner) is a device intended to improve the quality of the power that is delivered to electrical load equipment. The term most often refers to a device that acts in one or ...
*
Dynamic voltage restoration
*
Net metering system with energy storage
*
Surge protector
A 'surge protector'' (or spike suppressor, surge suppressor, surge diverter, surge protection device (SPD) or transient voltage surge suppressor (TVSS) is an appliance or device intended to protect electrical devices from voltage spikes in alt ...
*
Switched-mode power supply
A switched-mode power supply (switching-mode power supply, switch-mode power supply, switched power supply, SMPS, or switcher) is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently.
Lik ...
(SMPS)
*
Switched-mode power supply applications
*
Emergency light
An emergency light is a battery-backed lighting device that switches on automatically when a building experiences a power outage.
In the United States, emergency lights are standard in new commercial and high occupancy residential buildings, ...
References
External links
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* {{citation , url=http://novaelectric.com/information/application-notes/the-different-type-of-ups-systems-available , archive-url=https://web.archive.org/web/20150530053241/http://novaelectric.com/information/application-notes/the-different-type-of-ups-systems-available/, archive-date=2015-05-30, url-status=dead , title=The Different Type of UPS Systems Available , ref=none
Electric power systems components
Fault tolerance
Voltage stability