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Uninterruptible Power Supply

(UPS): Theory of Operation

An uninterruptible power supply, also uninterruptible power source, UPS or battery/flywheel backup, is an electrical apparatus that provides emergency power to a load when the input power source, typically the utility mains, fails. A UPS differs from an auxiliary or emergency power system or standby generator in that it will provide instantaneous or near-instantaneous protection from input power interruptions by means of one or more attached batteries and associated electronic circuitry for low power users, and or by means of diesel generators and flywheels for high power users. The on-battery runtime of most uninterruptible power sources is relatively short—5–15 minutes being typical for smaller units—but sufficient to allow time to bring an auxiliary power source on line, or to properly shut down the protected equipment.

While not limited to protecting any particular type of equipment, a UPS is typically used to protect computers, data centers, telecommunication equipment or other electrical equipment where an unexpected power disruption could cause injuries, fatalities, serious business disruption and/or data loss. UPS units range in size from units designed to protect a single computer without a video monitor (around 200 VA rating) to large units powering entire data centers, buildings, or even cities.

 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:

1. Power failure: defined as a total loss of input voltage.
2. Surge: defined as a momentary or sustained increase in the mains voltage.
3. Sag: defined as a momentary or sustained reduction in input voltage.
4. Spikes, defined as a brief high voltage excursion.
5. Noise, defined as a high frequency transient or oscillation, usually injected into the line by nearby equipment.
6. Frequency instability: defined as temporary changes in the mains frequency.
7. Harmonic distortion: defined as a departure from the ideal sinusoidal waveform expected on the line.

UPS units are divided into categories based on which of the above problems they address, and some manufacturers categorize their products in accordance with the number of power related problems they address.

The general categories of modern UPS systems are on-line, line-interactive or standby.  An on-line UPS uses a "double conversion" method of accepting AC input, rectifying to DC for passing through the rechargeable battery (or battery strings), then inverting back to 120V/240V AC for powering the protected equipment. A line-interactive UPS maintains the inverter in line and redirects the battery's DC current path from the normal charging mode to supplying current when power is lost. In a standby ("off-line") system the load is powered directly by the input power and the backup power circuitry is only invoked when the utility power fails. Most UPS below 1 kVA are of the line-interactive or standby variety which are usually less expensive.
For large power units, dynamic uninterruptible power supplies are sometimes used. A synchronous motor/alternator is connected on the mains via a choke. Energy is stored in a flywheel. When the mains power fails, an Eddy-current regulation maintains the power on the load. DUPS are sometimes combined or integrated with a diesel-generator, forming a diesel rotary uninterruptible power supply, or DRUPS.
A fuel cell UPS has been developed in recent years using hydrogen and a fuel cell as a power source, potentially providing long run times in a small space.
Offline / standby

Offline / standby UPS. Typical protection time: 0–20 minutes. Capacity expansion: Usually not available
The Offline / Standby UPS (SPS) offers only the most basic features, providing surge protection and battery backup. With this type of UPS, a user's equipment is normally connected directly to incoming utility power with the same voltage transient clamping devices used in a common surge protected plug strip connected across the power line. When the incoming utility voltage falls below a predetermined level the SPS turns on its internal DC-AC inverter circuitry, which is powered from an internal storage battery. The SPS then mechanically switches the connected equipment on to its DC-AC inverter output. The switchover 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. Generally speaking, dependent on the size of UPS connected load and the sensitivity of the connected equipment to voltage variation, the UPS will be designed and/or offered (specification wise) to cover certain ranges of equipment, i.e. Personal Computer, without any obvious dip or brownout to that device.

Line-interactive

Line-Interactive UPS. This illustration shows an isolated transformer, not an autotransformer, and does not show the way that the charger and inverter are connected to the secondary side of the same transformer. 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-voltageautotransformer. This is a special type of electrical transformer that can add or subtract powered coils of wire, thereby increasing or decreasing the magnetic field and the output voltage of the transformer.
This type of UPS is able to tolerate continuous undervoltage brownouts and overvoltage surges without consuming the limited reserve battery power. It instead compensates by 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 UPSs 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). Further, 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. Note that 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, and expense of the UPS. It is common for the autotransformer to only cover a range 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 watt device at 120 volts, the UPS will draw 8.32 amps. If a brownout occurs and the voltage drops to 100 volts, the UPS will draw 10 amps to compensate. This also works in reverse, so that in an overvoltage condition, the UPS will need fewer amps of current.

Double-conversion / online

The online UPS is ideal for environments where electrical isolation is necessary or for equipment that is very sensitive to power fluctuations. Although once previously reserved for very large installations of 10 kW or more, advances in technology have permitted it to now be available as a common consumer device, supplying 500 watts or less. The online UPS is generally more expensive but may be necessary when the power environment is "noisy" such as in industrial settings, for larger equipment loads like data centers, or when operation from an extended-run backup generator is necessary.
The basic technology of the online UPS is the same as in a standby or Line-Interactive UPS. However it typically costs much more, due to it having a much greater current AC-to-DC battery-charger/rectifier, and with the rectifier and inverter designed to run continuously with improved cooling systems. It is called aDouble-Conversion UPS due to the rectifier directly driving the inverter, even when powered from normal AC current.
In an online UPS, the batteries are always connected to the inverter, so that no power transfer switches are necessary. When power loss occurs, the rectifier simply drops out of the circuit and the batteries keep the power steady and unchanged. When power is restored, the rectifier resumes carrying most of the load and begins charging the batteries, though the charging current may be limited to prevent the high-power rectifier from overheating the batteries and boiling off the electrolyte.
The main advantage to the on-line UPS is its ability to provide an electrical firewall between the incoming utility power and sensitive electronic equipment. While the standby and Line-Interactive UPS merely filter the input utility power, the Double-Conversion UPS provides a layer of insulation from power quality problems. It allows control of output voltage and frequency regardless of input voltage and frequency.
Hybrid topology / Double conversion on demand

Recently there have been hybrid topology UPSs hitting the marketplace. These hybrid designs do not have an official designation, although one name used by HP and Eaton 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

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, and often the powered device does not need a power supply. Rather than converting AC to DC to charge batteries, then DC to AC to power the external device, and then back to DC inside the powered device, some equipment accepts DC power directly and allows one or more conversion steps to be eliminated. This equipment is more commonly known as a rectifier.
Many systems used in telecommunications use 48 V DC power, because it is not considered a high-voltageby most electrical codes and is exempt from many safety regulations, such as being installed in conduit and junction boxes. DC has typically been the dominant power source for telecommunications, and AC has typically been the dominant source for computers and servers.
There has been much experimentation with 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 must be greater than an equivalent 120 V or 240 V circuit, and greater current requires larger conductors and/or more energy to be 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.
Most switched-mode power supply (SMPS) power supplies for PCs can handle 325 V DC (230 V mains voltage × √2) directly, because the first thing they do to the AC input is rectify it. This does cause unbalanced heating in the input rectifier stage as the full load passes through only half of it, but that is not generally a significant problem. (Power supplies with a 115/230 V switch operate as a voltage doubler when in the 115 V position, which does require AC power, but the voltage doubler configuration also uses only half the rectifier, so it is certain to be able to handle the unbalance when operated from DC in the 230 V position.)