One of the types of UPS systems is called an On-line/double conversion UPS, this technology provides the highest levels of power protection from all mains-borne power problems and is often regarded as the most dependable type of technology.
The UPS converts the incoming mains AC power to DC power. The DC current provides power to the battery and the inverter, which converts the DC back to AC and provides a clean, regulated and conditioned AC output to the protected load. The battery is always in circuit and therefore when a mains failure occurs the battery carries on supplying power to the inverter which means there is no-break in the supply to the protected load and no switching is involved. A static bypass circuit is incorporated into the UPS which provides a failsafe mechanism, by providing AC mains to the load should the UPS be overloaded or in the unlikely event of an internal fault.
Online UPS systems are normally recommended when used with generators due to the UPS tolerance to voltage and frequency variations that can be associated with generators. An RS232 communications port for use with SNMP/server close down software along with Emergency Power Off (EPO) capability are normally a standard item on these systems. Power ratings are typically from 700VA to > 600kVA in single module format, with parallel systems and N+1 capability available from typically 5kVA upwards. Battery extension packs are available to extend the back-up time from a few minutes to a few hours.
An offline / standby UPS provides very basic protection against mains-borne problems. AC mains power passes straight through the UPS which has RFI filtering and limited spike protection provided.
In the event of a mains failure, or the AC mains voltage is outside the UPS operating window, the inverter starts up, a relay closes and allows the inverter to provide power to the load. Upon AC mains returning the mains output is switched back to mains and the inverter then shuts down. There are a few milli-seconds break during transfer to/from the inverter. The output when on inverter is invariably a square wave or stepped sine wave output.
Offline units typically have power ratings from 250VA to 2kVA. They typically have limited communications capability and whilst relatively cheap are normally recommended for only a single PC / workstation type application.
A Line-Interactive UPS provides more superior protection than an offline UPS. However, it still does not provide protection against all mains-borne problems unlike an on-line double conversion UPS.
These UPS's work in a similar way to an off-line UPS but have superior spike protection than an off-line system. They have a wider tolerance to voltage variations due to a voltage trimming transformer or similar. This increases the incoming AC mains voltage when it is low and decreases the incoming AC mains voltage when it is high and prevents the UPS switching to battery power and draining the batteries needlessly. During a mains failure, or if the incoming mains is outside operating limited the inverter starts up and provides power as described in an offline system. The output waveform on a line interactive UPS is normally a sine wave output.
Some manufacturers offer a range of battery packs which extend the back-up time from a few minutes to several hours. However, the battery charger in these UPS systems are limited and depending on how many battery packs are used; it may not be large enough to re-charge the batteries or the re-charge time could take many days. An RS232 communications port for use with SNMP / server close down software along with Emergency Power Off (EPO) capability are normally a standard item on these systems. Power ratings are typically from 400VA to 5kVA and can be offered in either a tower type or rack-mounted format and whilst not the recommended option, still provide cost-effective protection for server type applications.
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