With energy consumption going up and businesses increasingly reliant on electricity to run today’s operations, it is more important than ever that Un-interruptible Power Supply (UPS) topologies and designs are appropriately matched up to applications to secure the highest levels of power protection and business continuity.
Electricity can be derived from a number of sources these days and the call for renewable technology in response to environmental pressures means the list is getting longer. Coal, gas-fired or nuclear generation stations have been the primary methods so far but wind, wave or solar power are beginning to make their mark.
The phenomenon that generates electricity is termed Electro-magnetic Induction. Its output is an alternating current (ac) waveform. The process by which this happens utilizes magnets on a shaft which rotate near to windings (usually three) into which UPS electric current is induced; referred to as Electro-motive Force. The process is the same in each case; the only difference between each of the energy generation types is the fuel it uses to make the shaft spin.
All un-interruptible 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 sine-wave so this type of inverter’s output waveform is also a sine-wave. 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).
The constantly running inverter of an Online un-interruptible 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 transformer-less 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.
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.
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 data-center during a power outage.