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Uninterruptible Power Supplies Design and Topology

With energy consumption going up and businesses increasingly reliant on electricity to run today’s operations, it is more important than ever that Uninterruptible Power Supply (UPS) topologies and designs are appropriately matched up to applications to secure the highest levels of power protection and business continuity.

An uninterruptible power supply sits between a critical load and its source of ac power, usually a mains supply. It provides two functions: a secure source of power for when the mains supply fails and a clean, stable and regulated supply when mains power is present.

It is this second, lesser known, of the two functions that is becoming increasingly important for today’s businesses, particularly those that rely heavily on computer and telecommunications technology. This type of equipment is more sensitive to fluctuating currents and voltages and other power problems (surges, sags, brownouts, transients and harmonics), which are typically associated with raw mains energy. Disturbances like this can cause costly breakdown of equipment and data loss or corruption.

There three primary types of static UPS are: Online, Offline and Line Interactive, which all differ in their ability to perform these critical functions and vary in the degree of security and level of power protection they provide.

Line Interactive and Offline UPS are limited in their design to smaller applications such as home and small office. By comparison, Online UPS provide superior electrical performance, reliability and resilience and are suitable for powering loads ranging from critical file servers and telcoms PBXs to entire industrial manufacturing sites and data centres.

UPS Design. Inverters. All uninterruptible power supplies incorporate an inverter, which digitally creates an ac waveform from a dc supply to power the connected load.

In Online UPS, the inverter is designed for continuous operation. Raw mains power is a true sinewave so this type of inverter’s output waveform is also a sinewave. This type of UPS is classified as Voltage and Frequency Independent (VFI). Its inverter operation is unaffected by changes in voltage or frequency supplied by mains power.

Line Interactive or Offline UPS contain inverters that are much less sophisticated and are only used in standby mode, ready to supply the load when mains power fails or fluctuates outside of its pre-set input voltage and frequency window. The least sophisticated inverter type is that which is used in a purely Offline UPS (the type used in home office applications).

The size of its inverter limits the UPS’s load capacity and is measured in terms of VA (inverter output voltage x Amps).

Rectifiers. The constantly running inverter of an Online uninterruptible power supply requires a continuous source of dc and this is supplied by a rectifier connected to mains power.

Rectifiers can generate a high level of harmonics, depending upon their design, method of operation and type of UPS. In addition to supplying the inverter, in a transformer-based Online UPS design, the rectifier also charges the battery set. In a transformerless design, a booster-converter sits between the rectifier and inverter to increase the dc voltage from the rectifier (or battery set) to the higher dc voltage level required by the inverter. The output of the booster-converter also charges the battery set.

In Line Interactive or Offline UPS, the battery charger is connected to mains power.

Battery Sets. A UPS battery set is sized in Watts. It provides a dc supply, on mains power supply failure, to the inverter. The length of time an inverter runs on batteries is known as ‘battery runtime’, ‘autonomy’ or ‘back-up time’.

A battery set may comprise a single battery string (or multiples) connected in parallel. A string consists of one or more battery blocks connected in series. Battery runtime is measured in Ah (Ampere-hour) and is dependent upon the rating of the battery set and size of the attached load. The lower the load, the longer battery runtime will be.

Filters. UPS contain EMI/RFI (Electro-magnetic Interference and Radio Frequency Interference) filters to protect them (and their loads) from harmful, mains-borne spikes and electrically induced noise. They do this by suppressing or ‘clamping’ peak voltages to acceptable levels.

These filters vary in complexity and usage within the three static uninterruptible power supply topologies. The filters can also help prevent electrical pollution, which can be generated by the high-frequency electronics within the UPS itself, from disrupting upstream or downstream equipment.

Bypasses. The primary reason Online uninterruptible power supplies are the preferred choice to protect critical loads is that they incorporate an automatic bypass, which provides it with resilience in case of system failure. This is known as ‘safe failure to mains’ and means that, if system failure occurs, the bypass will transfer the load to an alternative power source (mains power or an alternative power path such as maintenance bypass or centralised static switches that operate within parallel UPS configurations). Once the fault condition is cleared, the bypass will automatically transfer the load back to inverter output.

UPS Topologies. An Online or Double Conversion UPS maintains voltage and frequency variations within prescribed limits so output is independent of supply fluctuation. As well as conditioning mains power to negate problems, this is the only UPS solution that provides break-free supply to the plant or datacenter during a power outage.

A Line Interactive UPS uses built-in passive electronic regulation devices to stabilize and regulate voltage fluctuations. When mains supply power is present, the output frequency of the UPS tracks the input of the mains. Line Interactive and On-Line UPS use tower or ‘rackmount’ case formats and sit next to their respective loads, typically in a datacenter or computer room.

An Off-Line or Passive Standby UPS is a compact device designed for desktops or wall mounting, which uses its own output to track voltage and frequency variations in the mains supply power.

Accessories. UPS accessories include external maintenance bypasses, which allow maintenance to be carried out without disruption to the load, parallel kits, utilised to achieve higher levels of operating resilience and battery extension packs, which provide a simplified and convenient way to boost extended runtime.

Depending primarily upon load size and type, application and criticality, a complete UPS solution may consist of one or more of the three main topologies. Online, for example, is considered the ultimate power protection solution, whereas Line Interactive is an intermediate option. Offline offers basic power protection and is well suited to small and confined applications such as small offices or home.

About the Author

Robin Koffler is the General Manager for Riello UPS Ltd the UK subsidiary of Riello UPS (RPS S.p.A) a leading European manufacturer of Uninterruptible Power Supplies and a co-author of The Power Protection Guide(ISBN 978-0-9554428-0-3)- available from Amazon.com

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