Basics of Offline UPS Systems

UPS Introduction:
                         The “Uninterruptable Power Supply” is also called as Standby Power Supply, since they stand by whilst the main power supply is working. These power supplies usually consists of inverters operating from DC storage source (like battery). In many applications like major computer installations, process control in chemical plants, hospital intensive care units continuous power supply is highly essential. In these applications, temporary power failure can cause a great inconvenience and economic losses.  The UPS plays an important role in these situations.

The following figure shows the configuration of an offline UPS system. It is also known as Line-Preferred UPS or Passive Standby. The Offline UPS System is the simplest form of back up power systems.

Operation:

§  The Offline UPS systems normally operate off-line and the load is normally powered by the utility line.

§  When the utility power levels goes beyond acceptable limits, or fails, the load is transferred from the utility line to the UPS. 

§  The actual transfer time is very fast, but the detection time somehow long and therefore the offline UPS is not as reliable as an online UPS.

Advantages, Disadvantages of Offline UPS System:

§  The major advantage of the off-line UPS systems is lower cost, smaller size and weight and higher efficiency. 

§  It has higher efficiency because most of the time the UPS system is offline and the load is powered by the utility.

§  The disadvantage of the Offline UPS system is that switching to the inverter is required when the load is most vulnerable (ie, upon failure of the normal power source)

§  These kind of systems provide no line conditioning or voltage regulation and provide only limited surge and spike protection. During the sustained low voltage periods(brownouts) the Offline UPS can inaccurately detect a blackout and prematurely switch onto battery.

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