Uninterruptible power supply

An uninterruptible power supply (UPS), uninterruptible power source or sometimes called a battery backup is a device which maintains a continuous supply of electric power to connected equipment by supplying power from a separate source when utility power is not available.

A UPS is inserted between the source of power (typically commercial utility power) and the load it is protecting. When a power failure or abnormality occurs, the UPS will effectively switch from utility power to its own power source almost instantaneously.

While not limited to any particular type of equipment, a UPS is typically used to protect computers, telecommunication equipment or other electrical equipment where an unexpected power disruption could cause injuries, fatalities, serious business disruption or data loss. UPS units come in sizes ranging from units which will back up a single computer without monitor (around 200 VA) to units which will power entire data centers or buildings (several megawatts). Larger UPS units typically work in conjunction with generators.

Historically, UPSes were very expensive and were most likely to be used on expensive computer systems and in areas where the power supply is interrupted frequently. However, UPS units are now more affordable, and have become an essential piece of equipment for data centers and business computers, but are also used for personal computers, entertainment systems and more.

In certain countries, where the electrical grid is under strain providers struggle to ensure supply during times of peak demand (such as Summer where air-conditioning usage increases). In order to prevent blackouts, electrical utilities will sometimes use a process called rolling blackouts or load shedding, which involves cutting the power to large groups of customers for short periods of time. Several major blackouts occurred in 2003, most notably the 2003 North America blackout in the north-eastern US and eastern Canada and the 2003 Italy blackout, both of which affected over 50 million people, and brought attention to the need for UPS power backup units.

A UPS is not to be confused with a standby generator, which does not provide protection from a momentary power interruption and may result in an interruption when it is switched into service, whether manually or automatically. However, such generators are typically placed before the UPS to provide cover for lengthy outages.

Common power problems

There are nine common power problems that UPS units are used to correct. They are as follows:

Power failure - Total loss of utility power
Power sag - Short term under-voltage
Power surge (spike) - Quick burst of over-voltage
Under-voltage (brownout) - Low line voltages for an extended period of time
Over-voltage - Increased voltages for an extended period of time
Line noise - distortions superimposed on the power waveform.
Frequency - variation of the power waveform.
Switching transient – under-voltage or over-voltage for up to a few nanoseconds.
Harmonic Distortion - multiples of power frequency superimposed on the power waveform.

UPS units are divided into categories based on which of the above problems their UPS units address.

UPS designs

The general categories of modern UPS systems are on-line or off-line, the latter often referred to as standby. An on-line UPS always powers the load from its own internal energy supply, the battery in the case of a static battery UPS, which is in turn continuously charged by the input power. In a standby system the load is powered by the input power directly and the backup power circuitry is only invoked when the utility power fails. Most UPS below 1 kVA are of the standby variety which are cheaper, though inferior to on-line systems which have no delay between a power failure and backup power being supplied.

Fuel cell UPS have also been developed in recent years using hydrogen and a fuel cell as a power source potentially providing long runtimes in a small space.

Rotary

Rotary uninterruptible power supply equipment uses a motor-generator system to create a perfect sine wave output. These units can be configured as (1) a motor driving a mechanically connected generator, or (2) a combined synchronous/synchronous motor/generator wound in alternating slots of the field and stator. The motor side of the unit in case #2 can be driven directly by an AC power source or by a 6-step double-conversion motor drive. Case #1 uses an integrated flywheel as a short-term energy source instead of batteries to allow time for external, electrically coupled gensets to start and be brought online. Case #2 can use batteries or a free-standing electrically coupled flywheel as the short-term energy source. Sometimes, in case #1, the flywheel itself is used to start the generator in a mechanically coupled diesel configuration.

Rotary UPS equipment is far more tolerant of lightning strikes than static equipment. It can provide up to 17x fault clearing capabilities without going to bypass. These units provide superior current inrush handling for inductive loads such as motor startup or compressor loads as well as medical MRI and cath lab equipment.

The life cycle of these units is usually far greater than that of their static siblings, up to 30 years or more.

Standby (offline)

With this design, the UPS simply passes utility power through to the load until either a power failure, sag or spike occurs, at which point, the UPS switches the load onto battery power and disconnects the utility power until it returns to an acceptable level. In this design, the UPS unit only charges the battery when it is running on utility power. This design is the most cost effective and typically makes use of a square wave or modified square wave inverter. These units are typically found in units 600 VA and below and designed for home use. This design solves problems 1 – 3, however the disadvantage of this is that any of the power problems numbered 4 - 9 will cause the UPS to switch to battery, and may cause it to completely drain the battery and shut off even though line voltage is still present true.

Line-interactive

Line interactive UPS units are designed so that the inverter is always connected to the output of the UPS. When line power is present, the inverter operates in reverse to charge the battery. When utility power fails, the UPS reverses the power flow from the inverter and provides power to the load. This design provides better filtering than a standby unit because the inverter is always connected to the load.

Line interactive units typically will incorporate an automatic voltage regulator. AVR allows the UPS to effectively step-up or step-down the incoming line voltage without switching to battery power. This allows the UPS to correct most long term over-voltages or under-voltages without draining the batteries. Another advantage is that it reduces the number of transfers to battery which extends the lifetime of the batteries.

Line-interactive UPS units are the most common design for units in the 0.5 kVA to 5 kVA range. They are typically used in small server environments.

Delta conversion online

Delta conversion is a new concept online technology. Unlike offline technology, no "switch on" time is required. Like other True-Online technology, a continuous separation of load and primary power is offered except the frequency (#7). With Delta Conversion, frequency is synchronized with main input. The reason for this technology (which all True-Online UPS have) is to synchronize the frequency with the main input (normal operation mode). This is necessary when UPS transfers from normal mode to bypass mode. It will request a frequency synchronize between input/output for a successful transfer. In fact, other True-Online UPS will supply separation frequency only when bypass mode is disabled. With the UPS, bypass mode is very important. Like brakes or air bags in a car, it is a "fail safe"; it will help to supply continuous power in case of UPS fault, overload, etc. Within the same range, Delta coversion is cheaper than other True-Online and is efficient up to 97%.

Dual conversion online

Dual conversion uninterruptible power supplies operate by converting incoming utility AC power to DC and then convert the DC back to AC power connected to the load. This is also called "double conversion" or "dual conversion." The batteries are directly connected to the DC level, which provides an excellent filter for removing line noise. Effectively, this design isolates the load from the incoming power and regenerates the sine wave. This yields many benefits. First, this design will protect against all 9 of the common power problems. It allows the UPS to use almost any incoming power, including generators. Second, this design allows the UPS to change incoming voltages and even frequencies easily. Third, because the load is always powered by the inverter, when power fails, there is no transfer time while the UPS switches from line power to battery power. While for most computer applications the switching time is not a problem, some industrial equipment can be harmed (air conditioner compressors for example), making this a better solution. These units are not quite as efficient as line interactive or standby units, however efficiencies do reach as high as 94%.

Online units are typically used in environments with sensitive equipment or environments where a generator is used to provide backup power to the UPS. Almost all UPS units 5 kVA and above are online, although they can be found in capacities as small as 1000 VA.

Because the AC output must be constantly generated by the UPS inverter, any failure of this inverter could potentially cause an interruption to the connected load. This is the very thing that the UPS is designed to avoid in the first place. As a measure of reliability, nearly all double conversion UPS units have a sophisticated monitoring system on the output that senses when the voltage or current goes out of specification. On larger UPS units, a solid state based bypass is then activated to shunt incoming AC directly to the attached load without interruption. This can be due to UPS output inverter failure, input rectifier failure (and eventual battery discharge) or other internal UPS failure. By detecting and shunting raw incoming AC directly to the load, these failures can be avoided but power filtering is elimnated or reduced. Another mode that requires bypass is fault clearing mode. A fault on the connected load, such as a short circuit in a power distribution panel or computer server power supply, may require more current than the UPS inverter can produce, in order for a fuse to blow, or a breaker to trip. During this mode of operation, bypass current is supplied directly to the output until the fault condition is resolved, usually in a matter of milliseconds. Without bypass modes, all of the other attached loads could lose power if even one experiences a fault. In addition to a high speed electronic bypass, most large (greater than 10KVA) UPS units have one or more layers of switches and/or breakers connected to the input and output to allow the entire unit to be bypassed, shut down, and isolated for maintenance without the connected loads being affected.

The ability for a UPS to bypass itself during abnormal conditions drastically increases the reliability of its output. Double online UPSes have their frequency and phase synchronized with its input (normal operation mode). It's very necessary when a UPS transfers from normal mode to bypass mode not to disturb the power in any way, including not abruptly changing phase or frequency. This can disturb timing circuits (extra zero crosses in the sine wave) or cause jerks in a motors output.

Larger UPSes are expensive but are often a better value. Fewer larger UPSes tend to be more reliable than many smaller units (that don't contain bypass circuits). These units may be marketed as Power conditioners. In data centers, multiple sets of UPS units may run in parallel providing dual sources of conditioned power to static switches that then send power to server loads. In such a system, a complete UPS failure can occur without the loads connected to the switches being affected.

Ferro-resonant

Ferro-resonant units operate in the same way as a standby UPS unit with the exception that a ferro-resonant transformer is used to filter the output. This transformer is designed to hold energy long enough to cover the time between switching from line power to battery power and effectively eliminates the transfer time. Because the transformer typically gives off a lot of heat, these units are typically large, bulky, and inefficient.

While this used to be the dominant type of UPS, they are no longer used for common applications. Power factor correcting equipment found in newer computer systems interacts with the transformer, causing potentially damaging oscillations, and the transformer itself can create distortions which yield power less acceptable than poor quality line AC. These units are still used in some industrial settings, but have mostly disappeared from use with general computer equipment.

DC systems

Many systems used in telecommunications use DC power (often 48 V). Rather than converting AC to DC to charge batteries, then DC to AC and then convert it back to DC again, some equipment accepts 48 V DC power directly. By simply converting AC power to DC power and adding batteries to the DC side, one or more conversion steps can be saved. There has been much experimentation with DC power for computer servers, in the hope of reducing the likelihood of failure and the cost of equipment. Because there is more current to transfer the same amount of energy at the lower DC voltage, larger conductors are needed, and more energy is lost as heat. On the surface, eliminating a conversion step may seem more reliable, but the ability of online double conversion AC systems to entirely remove themselves from operation and transfer to bypass mode during certain UPS failures and maintenance allows for the connected servers to continue to function on unconditioned AC power while the UPS is repaired. DC-based power systems do not have this luxury, as it requires that all equipment has special DC power inputs that cannot utilize AC voltages in the event of a main DC rectifier or power distribution failure. DC has typically been the dominant power source for telecommunications, and AC has typically been the dominant source for computers and servers. Higher voltage DC (370 volts), however, may find an eventual use in data center applications.

Internal UPS

Internal UPS are a group of uninterruptible power supplies (UPS) designed to be placed inside computer chasses. There are two types of Internal UPS. First type is miniaturized regular UPS that are made small enough to fit into a 5.25” CD-ROM slot bay of a regular computer chassis. The other type is re-engineered switching power supplies that utilize dual power sources of AC and/or DC as power inputs and have an AC-DC built-in switching management control units.

The first type often requires extra connection wires between the internal UPS and computer's power supply. Some internal UPS of this group output high voltage (110 V - 220 V) direct current (DC) and some output nine-step table wave AC. Neither design is safe or energy efficient. As of 2006, there are only couple companies still selling this type of internal UPS in Asia and some part of Europe

The second group of internal UPS replaces the regular switching power supplies. There are three main design mechanisms:

Optic-coupling that imitates AC during AC outages. This mechanism was first introduced by American Advanced Power of USA and Magnum Power of UK in 1997, as well as Apollo Power of Taiwan in 1998. This design provides a low-cost solution but its efficiency is low and it has a very low overall wattage ( <300 W) limit.
An analog-circuitry-controlled AC-DC switching mechanism. This design also provides a low-cost solution. However, because of the bulky component circuit board, little space is available for increasing wattage output. Plus, the final products are very sensitive to factors such as local heat and causing frequent operational errors. Nevertheless, because of its low cost, it is still popular in China. Most Asian internal UPS manufacturers belong to this category.
A CPU controlled AC-DC switching mechanism. This design was first introduced by American Advanced Power Inc. of USA and Amsdeli of Canada. It provides error-free switching control and a complicated communication protocol between the power supply and computer.

Using a UPS

Choosing a UPS

Besides choosing a UPS design, there are 2 key ratings to be aware of when choosing a UPS unit. The first is the volt amp (VA) and current (watt) ratings. Both the ratings represent the maximum amount of load that the UPS can support and the connected load typically should not exceed 80% of either. Special considerations must be made when connecting certain equipment such as printers or any type of motorized load. The second factor in deciding which unit to purchase is the amount of runtime the unit will be able to provide when the power fails. This number will vary with the load amount that is plugged into the UPS. For example, a unit may run a single computer for 30 minutes, but with 2 computers it may only last half that time or less. Larger units typically can provide more runtime for the same load than smaller units, however that is not always the case. Some UPS units are designed to provide extended runtime or have the ability to have external battery packs connected.

Another consideration is the anticipated usage. If the UPS is only intended to provide enough power to gracefully shut down the computers, serial or USB ports on the UPS and support software are essential. If the purpose of the UPS is to provide power until a standby generator kicks in (typically under a minute), the UPS input capabilities should be matched to the generator outputs. Specifically, most standby generators made for home use (15KW or less) and most portable generators lack microprocessor voltage-and-frequency control and may not create a smooth sine wave. This can result in voltage and frequency fluctuating by 5% or more. While most UPS systems handle voltage fluctations gracefully, most do not handle frequency fluctuations well. A UPS with a wide "frequency window" is essential is such cases. However, this can double the cost of the unit.

If the UPS needs to be quiet when running from battery, or will power AC motors (found in as air conditioners and fans), you need a UPS that outputs a smooth sine wave. For all other uses, a Pulse-width modulation (PWM) waveform is preferred. The PWM waveform converts more efficiently into DC power, the DC power produced is smoother, and guarantees control of the Root mean square (RMS) output voltage. UPS systems with square wave, or "simulated", "approximated" or "stepped" sine wave output do not give smooth sine waves. Instead, their characteristics are similar to PWM systems, except they lack RMS output voltage control.

Features to look for:

Output frequency regulation within 0.5% (prevents connected equipment from over heating)
Electromagnetic interference (EMI) AC noise suppression (noise filtering).
Reasonable cost for replacement batteries.
If energy efficiency is important avoid "Standby On-Line Hybrid", "Standby-Ferro", and "Double Conversion On-Line" UPS systems.
If the UPS outputs a sine wave, a high quality unit will feature a voltage regulating transformer.
If the UPS outputs a square wave, a high quality unit will use Pulse-width modulation (PWM)

Replacing batteries

In order to provide the desired protection, UPS units must be properly maintained. Sealed lead/acid batteries have a useful lifetime of 3-5 years. In determining when to replace batteries, it is important to remember that the batteries can be completely bad after 3-5 years and lose their ability to hold a charge gradually over that time. If a UPS started with 1 hour of runtime for the connected load, after 1 year, it may only provide 45 minutes and after 2 years, it may only provide 20 minutes. Some UPS units have user replaceable batteries, but some require a qualified technician or electrician to replace the batteries. Battery failure can also be caused by temperatures exceeding 77 degree F.

Disposing of UPS batteries

UPS units contain sealed lead-acid batteries and electronics which can be detrimental to the environment. In the United States, it is illegal to dispose of lead-acid batteries in a landfill, and they must be properly recycled. Sealed lead-acid batteries are recycled in the same manner as car batteries, so any auto shop that accepts used car batteries for recycling will also accept sealed lead acid batteries.

Alarms

Along with other building systems, UPS units can be equipped with alarms that will notify an off-site location such as a Central station (alarm monitoring center) if there is a failure or malfunction.

References

Part of this article was originally taken from a public-domain entry in Federal Standard 1037C
Part of this article was copied with permission from Uninterruptible Solutions

External links