LinuxHPC.org/Cluster Builder 1.3 - Uninterruptible Power Supply

Uninterruptible Power Supply

An uninterruptible power supply (UPS), sometimes called an uninterruptible power source, 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 backup 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, UPS units 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 North America in particular, the electrical grid is under increasing strain particularly during heavy demand periods such as summer when air conditioning use is at its highest. In order to prevent blackouts, electrical utilities will sometimes use a process called load shedding, which involves cutting the power to large groups of customers for short periods of time. The single biggest event that brought attention to the need for UPS power backup units was the 2003 North America blackout in the north-eastern US and eastern Canada.

A UPS is not to be confused with a standby generator, which does not provide protection from a momentary power interruption, which may result in a momentary power 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

Older uninterruptible power supply designs that supply commercial quality AC power to equipment contain a motor-generator system with a large flywheel that keeps the generator rotating and producing electric power while an auxiliary motor is started at the moment of power interruption. Sometimes the flywheel itself is used to start the motor. These systems can typically cover a 30 second interruption until the auxiliary motor starts.

The general categories of modern UPS systems are on-line, off-line and standby. The traditional definition of an on-line UPS was one that continuously powered the load from a DC bus that was supplied by two sources: batteries, and DC rectified from the incoming AC. In a standby UPS, the load is powered by the source until the power fails and then it quickly switches on an AC-to-DC converter and battery source. Since 2004, the majority of units sold below 1 kVA are standby UPS.

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.

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.

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 type of line interactive technology. In this configuration the primary power source is blended with power from the inverter. As the primary power varies away from its normal value the inverter comes to life to make up the difference. Unlike Off line technology no switch "ON" time is required. Unlike On-line technology no continuous separation of load and primary power is offered. Delta Conversion provides protection from all power anomalies except #7. Delta conversion is efficient, with system efficiency of up to 97% under nominal conditions when the inverter needs to do no work to correct deficiencies in the primary power. As the inverter does more work to correct deficiencies in the primary power the efficiency drops. At practical levels the efficiency of this technology can be less than that of On-line systems.

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 to power the connected equipment. 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 easily change incoming voltages and even frequencies. 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, 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.

Larger UPS are expensive but are often better value. Fewer larger UPS tend to be more reliable than many smaller units.

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 straight 48 V DC power. By simply converting AC power to DC power and adding batteries to the DC side, one or more conversion steps can be saved. This method reduces the likelihood of failure and the cost of equipment, however it does require that all equipment has special DC power inputs.

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 VA and wattage ratings. Both the VA (volt amp) and wattage ratings represent the maximum amount of load that can be plugged into a UPS in different ways. The connected load typically should not exceed 80% of either UPS rating. 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 plugged in, it might only last 15 minutes. 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.

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.

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.

Some manufacturers

Active Power Company - Manufacturer of flywheels for use in UPS systems. http://www.activepower.com/
American Power Conversion - Manufacturer of UPS , Power management software, and power system accessories.
Belkin
Chloride Power - Manufacture of UPS, Static Transfer Switches, Power Conditioners, DC Power Systems, Industrial Systems power management software.
Clary Corporation
CyberPower Systems - Manufacturer of Standby UPS, Line Interactive UPS, Double Conversion UPS, DC UPS, Surge Protection, and Power Management Software.
Energy Technologies - Manufacturers UPS, power and frequency conditioning for military vehicle and defence areas.
Hitec Power Protection - Manufacturer of Rotating UPS and Power conditioning Systems.
Leviton Manufacturing - Leviton's Power Solutions products include standby, line-interactive and online UPS, power distribution units (PDU's) and surge protective devices (SPD's or TVSS).
Liebert - A Division of Emerson Network Power.
Masterguard - Masterguard originated within SIEMENS AG. They were acquired by Chloride Power group.
MGE UPS Systems - Manufacture of UPS , power distribution units, complete power supply solutions for VoIP and WiFi, static transfer switches, harmonic filters, surge suppressors.
Mitsubishi Electric Automation
Oneac - Manufacture of interactive, online UPS systems, power management software. They were acquired by the Chloride Power group. Their product lines include power conditioners, uninterruptible power supplies, DC power solutions and voice & data line protection devices.
OPTI-UPS - Manufacturers of standby, line-interactive, and online UPS systems.
Power Innovations International - Makers of online UPS systems.
Power Quality Inc Supplier of UPS, UPS batteries,Power Quality mitigation devices and Power Quality engineering services in the North America and Asia.
Powerware (Eaton Powerware) - Powerware solutions include the power quality products, UPS, DC power systems, software, services.
TrippLite - Power Protection & Connectivity Products
TSi Power Corporation - Helping to solve power protection problems, to help satisfy special equipment input power requirements, including precision voltage regulation.

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