UPS System Glossary | Resources

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Active redundancy

Parallel UPS configuration in which several UPS units with equal outputs are parallel connected and share the load. In the event one UPS unit fails, the other units pick up its share without any interruption in the supply of power to the load.

Alternate power source

Backup source used in the event of a mains failure. The connection time and the duration of the source depend on the type of source used.

Back-up time

Time during which the UPS can supply the rated load with nominal-quality power while utility power is down. This time depends on the battery and the efficiency of the UPS. Typical backup ranges from five minutes to several hours.

Battery on shelves

Battery cell installation system whereby the cells are placed on several vertically stacked shelves or racks made of insulating material.

Battery (recombination)

Battery with a gas recombination rate at least equal to 95%, i.e. no water need be added over battery life. So, usually called "maintenance free."

Battery (tier-mounted)

Battery cell installation system whereby the cells are placed on tiers made of insulating material.

Battery (vented)

The battery cells are equipped with a filling port for distilled, demineralized water used to top up the free electrolyte.

Battery cells

The interconnected battery elements that supply electrical power created by electrolytic reaction.

Battery circuit breaker

DC circuit breaker that protects the battery of a UPS.

BMS (Building Management System)

System used for control/monitoring building utilities and systems. It is generally composed of sensors, actuators and programmable controllers connected to a central computer or several computers, equipped with specific software.

Bypass

The act of taking the UPS offline and feeding the critical bus from utility power. This can be done either manually, for service, or automatically in the event of failure or overload.

Charger

Device associated with the rectifier and used to supply the battery with the electrical power (DC current) required to recharge and/or float charge the battery

Circuit breaker (battery)

DC circuit breaker that protects the battery of a UPS.

Conductance

The technical description of conductance is the real part of complex admittance. It is a measure of an electrical circuit current response that results when an AC voltage pulse of a known frequency and amplitude is applied. Siemens is the international unit of measure which is sometimes called "MHOS", which of course is Ohms spelled backward. Conductance is also sometimes referred to as "admittance" or "acceptance", because it relates the electrical (conductive) efficiency of a circuit. The conductance value is one way to measure the resistive characteristic of any battery or cell. As batteries age and fail, their internal resistance will typically increase because the plate surface can sulfate or shed active material, which adversely affects its ability to perform. This increase in resistance can be observed with declining conductance readings. It is a measure of that decline over time that can be used to calculate the capacity loss in cell or battery as it occurs. This is done without discharging the cell or battery, and the simple rule of thumb is; high conductance is GOOD, low conductance is typically BAD.

Cos phi

A measure of the phase shift between the current wave and the voltage wave observed at the terminals of a load supplied with AC power at a given frequency.

Cos phi1

A measure of the phase shift between the fundamental current wave and the fundamental voltage wave observed at the terminals of a non-linear load.

Crest factor (Fc)

Ratio between the peak current value to the rms current value.

Current (inrush)

Temporary current observed in a network when electrical devices are energized, generally due to the magnetic circuits of the devices. The effect is measured by the currents maximum peak value and the rms current value it generates.

Current harmonics

All alternating current which is not absolutely sinusoidal is made up of a fundamental and a certain number of current harmonics which are the cause of its deformation (distortion) when compared to the theoretical sine-wave. For each current harmonic of order n and an rms value In, there is a voltage harmonic with an rms value Un. If Zsn is the voltage source output impedance for the harmonic of the nth order, then: Un = Zsn x In

Current loop (20 mA)

Transmission system used on certain devices and offering better performance than the RS232C. It provides a high degree of immunity to interference and is easy to implement, but has not been standardized.

Distortion (individual)

Ratio between the rms value of an nth order harmonic and the rms value of the fundamental.

Distortion (total)

Ratio between the rms value of all harmonics of a non-sinusoidal alternating periodic value and that of the fundamental. This value may also be expressed as a function of the individual distortion of each harmonic Hn = Yn /Y1

Electromagnetic compatibility

Possibility of a device to operate normally when installed near other devices, given the disturbances emitted by each device and their mutual sensitivities.

Filter (phase-shift)

Filter used to reduce, if necessary, the overall distortion due to the current harmonics injected into the utility upstream of a UPS by its rectifier-charger. Filtering is superior to that of a traditional filter of the L or C type.

Floating voltage

DC voltage applied to the battery to maintain its charge level. This voltage depends on the type of battery, the number of cells and the manufacturer ís recommendations.

Fourier theorem

Theorem stating that any non-sinusoidal periodic function (frequency f) may be represented as a sum of terms (series) made up of: n a sinusoidal term with frequency f, called the fundamental frequency, n sinusoidal terms with frequencies that are whole multiples of the fundamental frequency, (harmonics), n a possible DC component where n is a whole number. n = 1 corresponds to the fundamental, n > 1 to the harmonic of the nth order.

Harmonic

Sinusoidal term of the Fourier series expansion of a periodic function. The harmonic (or harmonic component) of the nth order is characterized by: Yn is the rms value of the given harmonic component, w is the angular frequency of the fundamental, related to frequency by : w = 2¼f; phin is the phase angle of the given harmonic component at t = 0.

Harmonics (current and voltage)

All alternating current which is not absolutely sinusoidal is made up of a fundamental and a certain number of current harmonics which are the cause of its deformation (distortion) when compared to the theoretical sine-wave. For each current harmonic of order n and an rms value In, there is a voltage harmonic with an rms value Un. If Zsn is the voltage source output impedance for the harmonic of the nth order, then: Un = Zsn x In

High-frequency interference

High-frequency parasitic current that is either conducted (electrostatic origin) or radiated (electromagnetic origin) by a device.

Individual distortion

Ratio between the rms value of an nth order harmonic and the rms value of the fundamental.

Inrush current

Temporary current observed in a network when electrical devices are energized, generally due to the magnetic circuits of the devices. The effect is measured by the currents maximum peak value and the rms current value it generates.

Interference (high-frequency)

High-frequency parasitic current that is either conducted (electrostatic origin) or radiated (electromagnetic origin) by a device.

Inverter

UPS subassembly that recomposes a sine-wave output (regulated and without breaks) using the DC current supplied by the rectifier-charger or the battery. The primary elements of the inverter are the DC/AC converter, a regulation system and an output filter.

Inverter (off-line or stand-by)

UPS configuration in which the inverter is parallel-mounted to the load supply line and backs up the utility. This configuration offers a substantial cost reduction but is applicable only to low outputs, under 3 kVA, because it results in an interruption lasting up to 10 ms during transfer and does not filter inrush currents.

Inverter (on-line)

UPS configuration in which the inverter is in series mounted between the utility and the load. All power drawn by the load passes via the inverter. This is the only configuration used for high outputs.

Load (linear)

Load for which voltage form and current form are similar, or in phase. Voltage and current are related by Ohm's law U(t) = Z x I(t).

Load (non-linear)

Load (generally with a switched-mode power supply) generating major harmonic currents. Current wave form is out of phase with the voltage wave form. Ohm's law is not applicable. It can be used only with each harmonic.

Load power

Apparent power Pu that the UPS inverter supplies under given load conditions. It is less than or equal to the rated output Pn. The ratio Pu/Pn defines the % load of the inverter.

Maintenance Bypass (Wrap Around)

Manually operated series of circuit breakers creating a make before break (preferably) parallel path around the UPS and static switch. Once energized, all load power is supplied through the bypass, and the UPS can be completely de-energized allowing any service activity. Micro-outage (or micro-interruption) Total loss in the supply of power for 10 ms.

MTBF (Mean Time Between Failures)

Mathematical calculation of the duration of normal operation of a repairable device between failures. The product, expressed in hours, is an indication on the reliability of the device.

MTTF (Mean Time To Failure)

Mathematical calculation of the duration of normal operation of a non-reparable device, i.e. for which a MTBF is not possible. The product, expressed in hours, is an indication on the reliability of the device.

MTTR (Mean Time To Repair)

Mathematical calculation (or statistical average if available) of the time required to repair a device.

Noise level

Acoustical decibel level of a source of noise, measured according to the applicable ISO standard.

Non-linear load

Load (generally with a switched-mode power supply) generating major harmonic currents. Current wave form is out of phase with the voltage wave form. Ohm's law is not applicable. It can be used only with each harmonic.

Off-line inverter (or stand-by)

UPS configuration in which the inverter is parallel-mounted to the load supply line and backs up the utility. This configuration offers a substantial cost reduction but is applicable only to low outputs, under 3 kVA, because it results in an interruption lasting up to 10 ms during transfer and does not filter inrush currents.

On-line inverter

UPS configuration in which the inverter is in series mounted between the utility and the load. All power drawn by the load passes via the inverter. This is the only configuration used for high outputs.

Output (rated)

Apparent power Pn that the UPS can deliver under given load conditions (power factor = 0.8).

Overall distortion

Ratio between the rms value of all harmonics of a non-sinusoidal alternating periodic value and that of the fundamental. This value may also be expressed as a function of the individual distortion of each harmonic Hn = Yn /Y1

Percent load

Ratio between the power Pu drawn by the load and the rated output Pn of a UPS system (Pu/Pn). Sometimes referred to as the load factor.

Phase-shift filter

Filter used to reduce, if necessary, the overall distortion due to the current harmonics injected into the utility upstream of a UPS by its rectifier-charger. Filtering is superior to that of a traditional filter of the L or C type.

Power factor (l)

Ratio between the active power(true power) P supplied to a load and the apparent power S supplied to said load by an AC power supply.

Power source (alternate)

Backup source used in the event of a mains failure. The connection time and the duration of the source depend on the type of source used.

Power source (safety)

Power source for loads defined as critical by applicable safety regulations. This supply must not be affected by a mains failure and is generally separate from other supplies.

PWM (Pulse Width Modulation)

Inverter high-frequency chopping technique using a means of regulation enabling rapid modification of pulse widths over a single period, thus making it possible to maintain the inverter output within tolerances even for non-linear loads.

Rated output

Apparent power Pn that the UPS can deliver under given load conditions (power factor = 0.8).

Reactance (sub transient Uscx %, for generator)

Relative measurement (%) of the internal impedance of an AC generator during harmonic phenomena. This reactance, also called the longitudinal sub transient reactance of the generator, is sometimes identified as X"d. For most common generators, the value ranges between 15 and 20%. It can drop to 12% for optimized systems and to 6% for special devices.

Recombination battery

Battery with a gas recombination rate at least equal to 95%, i.e. no water need be added over battery life. So, usually called "maintenance free."

Rectifier/charger

UPS component that draws on the mains the power required to supply the inverter and to float charge or recharge the battery. The alternating input current is rectified and then distributed to the inverter and the battery.

Redundancy (active)

Parallel UPS configuration in which several UPS units with equal outputs are parallel connected and share the load. In the event one UPS unit fails, the other units pick up its share without any interruption in the supply of power to the load.

Redundancy (standby)

UPS configuration in which one or several UPS units operate on stand-by, with no load or only a partial load, and can immediately back up a faulty UPS unit by no-break transfer of the load, carried out by a static switch.

Reliability

Probability that a device will accomplish a required function under given conditions over a given period of time. Rms value of AC current with harmonics The rms value Yrms of a non-sinusoidal alternating current may be determined on the basis of the individual harmonic currents: where Y is the rms value of the fundamental.

RS232C (Recommended standard RS232C)

Standard defining the communication circuits between devices for synchronous and asynchronous transmissions on the following types of lines: two-wire, four-wire, point-to-point, lines and local links with short cables. Though the standard covers only transmissions over distances up to 15 meters, it is often possible to ensure correct transmission over greater distances using high-quality shielded cable in a reasonably satisfactory electrical environment. Most terminals and devices on the market can implement this transmission standard.

RS422A (Recommended standard RS422A)

Standard RS232C is sufficient for transmissions in a normal environment. For transmissions in a disturbed environment or over long distances, standard RS422A offers a differential operation option, with a balanced voltage, ensuring far superior performance. What is more, it can be used for multipoint links, with generally up to ten connection points (one sender and up to ten receivers).

RS485 (Recommended standard RS485)

This standard is similar to RS422A except that the number of possible links is greater and up to 32 senders may be interconnected to as many receivers. This system is particularly designed for local-area networks.

Safety installation

Installation supplying electrical equipment which may have a direct effect on the safety of users and must therefore remain energized even in the event of a mains failure. In general, characteristics concerning the power supply and conditions for transfer to the safety source for such electrical equipment are covered by applicable regulations.

Safety power source

Power source for loads defined as critical by applicable safety regulations. This supply must not be affected by a mains failure and is generally separate from other supplies.

Static bypass switch

Power-electronics device that can be used to switch from one source to another without interruption in the supply of power. In a UPS, transfer is from Mains 1 to Mains 2 and back. Transfer without interruption is possible due to the fact that there are no mechanical parts and the ultra-fast switching capabilities of the electronic components.

Sub transient reactance of generator (Uscx %)

Relative measurement (%) of the internal impedance of an AC generator during harmonic phenomena. This reactance, also called the longitudinal sub transient reactance of the generator, is sometimes identified as X"d. For most common generators, the value ranges between 15 and 20%. It can drop to 12% for optimized systems and to 6% for special devices.

Thermal Runaway

Thermal runaway occurs in a VRLA battery when the rate of internal heat generation exceeds the rate at which the heat can be dissipated into the environment. It is typically triggered by excessive charge voltage, or high ambient temperatures. As temperature rises, internal current rises, resulting in higher internal temperatures, and a self-sustaining run away cycle. Should this condition continue for an extended period, the VRLA battery temperature could increase until ultimately the cells will dry-out and the container, if plastic, may soften (100°C), rupture and melt (160°C).Thermal runaway is a dangerous condition that can cause damage to property, fire and harm to personnel.

Thevenin generator

For a given load, it is possible to consider the power supply as a voltage generator, referred to as a Thevenin generator, made up of a perfect voltage Uo generator, in series with an internal impedance Zs: n Uo is the voltage measured across the load terminals, given that the load is to be disconnected (load terminals forming an open circuit), n Zs is the equivalent impedance as seen from the load terminals (again considered an open circuit), obtained by short-circuiting the upstream voltage generator(s).

Tolerance in %

Limit for allowable variations for a given quantity, expressed as a percent of the rated value.

Transformer short-circuit voltage (Uscx %)

Relative measurement (%) of the internal impedance of a transformer. This short-circuit impedance is commonly called the short-circuit voltage because it is measured during a short-circuit test (shorted secondary winding subjected to a current set to In). For most common three-phase transformers, this value ranges between 3 and 6%.

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UL

Underwriters Laboratories (UL) is a non-governmental, non-profit certification organization in the United States in which not only government authorities are represented, but also consumer groups, "export" services, research, etc. Following certification, a product may bear the UL label.

UPS (Uninterruptible Power supply)

An electrical device providing an interface between the mains power supply and sensitive loads (computer systems, instrumentation, etc.). The UPS supplies sinusoidal AC power free of disturbances and within strict amplitude and frequency tolerances. It is generally made up of a rectifier/charger and an inverter together with a battery for backup power in the event of a mains failure.

UPS (parallel with redundancy)

A UPS made up of several parallel-connected UPS units with equal output ratings (P) and each equipped with its battery. If one unit fails, one or several of the others pick up the resulting excess load. If a UPS has a rated output n x P and is made up of n + k units, k is the level of redundancy for the entire set of n + k units.

UPS (parallel without redundancy)

A UPS made up of several (n) parallel-connected UPS units with equal output ratings (P) and each equipped with its battery, for large loads. The total output is equal to the number of units multiplied by their individual output (n x P). In this configuration, no UPS unit is redundant.

UPS (single)

A UPS made up of one single UPS unit (rectifier/charger, inverter and bypass) and a battery.

Vented battery

The battery cells are equipped with a filling port for distilled, demineralized water used to top up the free electrolyte.

Voltage (float)

DC voltage applied to the battery to maintain its charge level. This voltage depends on the type of battery, the number of cells and the manufacturers recommendations.

Voltage Harmonics

All alternating current which is not absolutely sinusoidal is made up of a fundamental and a certain number of current harmonics which are the cause of its deformation (distortion) when compared to the theoretical sine-wave. For each current harmonic of order n and an rms value In, there is a voltage harmonic with an rms value Un. If Zsn is the voltage source output impedance for the harmonic of the nth order, then: Un = Zsn x In

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. Provided utility power is flowing, it also replenishes and maintains energy storage.

A UPS protects equipment from damage in the event of a power failure. It is used in any situation where electrical equipment is sensitive to power loss or issues with power quality, for example, if a system experiences unsafe changes in voltage output. UPSes are typically used in settings pertaining to computer systems, data servers or industrial devices, or in settings with mission-critical equipment, such as medical and laboratory systems.

Energy can be stored in different ways. Rechargeable batteries are the most common. For simplicity, the examples and illustrations here are based on that technology. However, kinetic energy can also be stored in heavy, rotating flywheels or energy can be stored as fuel.

The more energy stored, the longer backup power can be maintained, with practical limitations that will be discussed later. The differences among UPS systems lie in the technology that enables them to do their jobs.

According to Verified Market Research, the UPS market is expected to grow to over $11.95 billion by .

How does a UPS work?

A battery-powered UPS typically contains the following components:

• Rectifier or charger. This converts alternating current (AC) to direct current (DC) and directs it to feed the inverter or to charge the battery.
• Inverter. This converts the electricity from DC into AC.
• Batteries. These are either lithium-ion, concerned about orderly shutdowns lead-acid (VRLA), flooded lead-acid or nickel-cadmium batteries.
• Static and maintenance bypass. This provides an alternative path for power to flow in the case of a UPS failure or overload.
• Control unit. This manages and coordinates the functions of a UPS, including monitoring system status, switching between power sources and regulating output voltage and frequency.

A UPS is typically plugged into an AC outlet and activates when it detects a power outage, voltage drop, surge or frequency variation. In the case of an outage, the UPS almost immediately switches to the battery -- fast enough so no electrical equipment is damaged. From there, the battery acts as a continual emergency power source until it uses all its power.

Depending on its size, a UPS can protect a single computer or a whole data center. Its stored power might last from a few minutes to several hours. The goal is to either last until generators are turned on or until protected devices are properly shut down.

What are the different types of UPSes?

The most common 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 AC, which is also required by most IT equipment (ITE).

Batteries, on the other hand, are 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 used on elevators, 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.

Figure 1. A schematic of a VFI system in normal operation.

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.

Figure 2. A schematic of a VFI system in operation when utility power fails.

UPS static and maintenance bypass. UPSes aren't uninterruptible. They're 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.

Figure 3. A schematic of a UPS in bypass mode.

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's 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, or 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's 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.

Figure 4. A schematic of a VFI UPS in economy mode.

Line interactive UPS. A true line interactive UPS, also known as voltage independent (VI), has the same output frequency 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 helps 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 5. A schematic of a line interactive UPS, showing how the battery and inverter help compensate for incoming voltage variations.

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 improvements in VFI UPSes have reduced that to 1% or less.

Figure 6. A schematic of a line interactive UPS when an incoming service fails.

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 aren't in the circuit until power fails. The output is filtered, but it isn't as stable as a true VI UPS.

Figure 7. A schematic of a standby UPS under normal operation.

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's some switching instability, but the delay is short enough for most computer power supplies to ride through.

When power is restored -- either through the utility or a generator -- the inverter is disconnected, line power is switched back on and the batteries are recharged by the rectifier, which is much smaller than in a VFI or VI UPS.

Figure 8. A schematic of a standby UPS when power fails.

Unfortunately, standby or VFD UPSes are sometimes advertised as line interactive. It's important to know 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.

Mechanical and non-battery UPS 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:

• Motor-generator (MG) sets combine a motor with a generator. The motor is equivalent to the rectifier in a VFI UPS, and the generator is equivalent to the inverter. Utility power drives a rectifier that runs a DC motor and charges the batteries. When power fails, the batteries keep the motor turning so the generator continues to deliver power to the load. MG sets are more often used to maintain power to other mechanical equipment, such as air conditioners, than to power actual ITE, although there was a time when they were quite prevalent with old mainframe computers.
• Rotary UPSes (DRUPS) are like MG sets except there are no batteries -- other than to start a generator -- and a built-in gas or diesel engine starts and maintains power when a utility failure occurs. A flywheel keeps the generator turning long enough for the generator to stabilize before a mechanical clutch attaches it to the generator. Again, these tend to be used more to maintain power to air conditioners than to power the ITE. They're often selected as a cost-effective alternative to separate generators when uninterrupted cooling is particularly critical to maintain.
• Flywheel UPSes have similarities to both MG sets and DRUPS but with a major difference. The generator is turned by an electric motor when utility power is available, but the system incorporates only a heavy flywheel to keep the generator turning, usually until an auxiliary generator resumes power. The flywheel rides on nearly friction-free air or magnetic bearings in a vacuum-sealed case that can maintain power for as long as 30 seconds. Combined units can extend the duration to several minutes without generating the heat common with other UPSes.

Considerations for selecting and using UPS systems

Important considerations for choosing a UPS system, include the following:

Power requirements. Organizations must first determine load capacities. This includes finding the total power load of all connected devices and the runtime required during an outage.

Type of UPS. An organization must also decide between standby, which is designed for less critical applications; line-interactive, which is designed for settings with slight power quality issues; and double-conversion UPSes, which is suitable for mission-critical applications.

Battery type and maintenance. Some batteries require regular maintenance or replacement. Lithium-ion batteries typically require the least amount of maintenance over time, followed by nickel-cadmium, VRLA, and flooded lead-acid batteries, respectively.

Modularity. Most modern battery-based UPSes are modular. They're comprised of multiple, smaller UPS and battery units that can be combined as required to provide capacity, redundancy or both. The frame must be large enough for long-term expectations for anticipation of long-term growth.

Actual modules can be purchased and installed as needed. For example, a 100 kilowatt (kW) UPS might have six 20 kW modules to provide N+1 redundancy. Battery backup 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 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 explains the results to the owner.

UPS power factor. Power factor (pf) is the difference between real power and apparent power. This is greatly misunderstood but 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 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's 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.

Batteries and battery duration

Batteries are an evolving technology, particularly because of their increased usage in electric vehicles. Batteries are heavy, so the floor structural strength should always be verified. Four types of batteries that are commonly used today include the following:

• Flooded lead acid or wet cells are the most expensive but have the longest service life -- generally 25 years or more. However, they require separate, fire-rated rooms with acid drains, hydrogen detection alarms, exhaust fans, eye wash stations, deluge showers and hazmat protective gear. They're also the heaviest, require regular maintenance, and are generally used in the largest and most sophisticated installations.
• VRLA, also known as sealed cells, use a paste electrolyte instead of liquid, and are contained in sealed packaging with small vents. They're charged more slowly than wet cells to avoid hydrogen emission, so they can be used in any space without special construction or protection. Warranties are usually 10 years, but actual service life is often only three to five years, depending on local power stability and how often the batteries are partially discharged and recharged. Longer-life VRLA batteries are available at an increased cost but must usually be specified. VRLAs are less heavy than wet cells.
• Lithium ion (Li-ion) are the newest batteries available and can be used without special rooms or construction in most jurisdictions. There might still be cities that consider them dangerous, but their chemistry and construction are totally different from those that have caught fire in ultra-compact electronics. Li-ion batteries are smaller and lighter than VRLAs, can be partially discharged and recharged without degradation and are expected to have a longer service life than VRLAs.
• Nickel-cadmium batteries are more resilient to extreme temperatures and discharges compared to lead-acid batteries, but they're heavier and have higher initial costs. They require moderate maintenance, including consistent checks and electrolyte adjustments. They typically last around 15-20 years.

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 to 60 minutes.

Eventually, the UPS will overheat and go into a 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 costs, 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 no input or output transformers are 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:

• Advantage. If input and output voltages are the same, there's no need for a full wraparound bypass that would include the transformers, which the maintenance bypass doesn't.
• Disadvantage. If input and output voltages must be different, not only are transformers required at either the input or output, but so is a full wraparound bypass that includes another transformer.
• Disadvantage. There's no output transformer to isolate the load from the UPS. Therefore, the electrical engineer must be cautious in designing the grounding system and addressing the mitigation of short circuits, which can destroy UPS output transistors. This is often done with external distribution transformers in large power distribution units.

Considerations for low power quality and generators

When dealing with low power quality and the use of generators, an organization should take several considerations into account to ensure the equipment's reliability and longevity.

Low power quality could indicate several different statuses, including voltage fluctuations and spikes, frequency variations and power outages. If voltage fluctuations and spikes are a main concern, then a UPS system should include voltage regulators and surge protectors. Likewise, if frequency variations are a concern, then the UPS system should include frequency regulation functions.

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 increase and decrease while trying to absorb the load.

UPS systems typically require continuous maintenance over time. Learn more about UPS maintenance and how it helps safeguard data center power.

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