What is UPS | Types of UPS system | Information About UNINTERRUPTIBLE POWER SUPPLY | Operation of On-Line UPS

UNINTERRUPTIBLE POWER SUPPLY

An UPS or Uninterruptible Power Supply is a device or system which provide an interrupted free supply of power to the AC load. Its used where even a temporary loss of power supply could have severe consequence. An UPS is a power conditioner which:

characteristics of UPS  System

i.                 Provides good quality power to the load at any condition of power supply.

ii.                When the main voltage is fluctuates it regulates the load voltage.

iii.              Provides complete isolation between the load and the mains.

iv.              Minimizes EMI problems and Suppresses the line transient (voltage spikes).

v.                Provides a constant voltage and frequency supply to the critical load.

 

The basic Block diagram of an UPS 

The basic operation of UPS

The block diagram show in the above, is the basic block diagram of UPS system. A rectifier converts a single-phase or three-phase AC voltage into DC voltage, which supplies power to the inverter and also supplies power to the battery bank to charge it (using charger when ac main is ON). When the AC mains is ON The inverter gets a DC input voltage from the rectifier, and from the battery bank when the AC mains is OFF. Inverter converts this DC voltage into AC voltage and applies it to the load through a suitable filter which eliminate all interrupted i.e., voltage spikes. If the PWM inverter is used then the filter can be eliminated. A static switch will connect or disconnect the battery from the input of the inverter depending on the status of AC mains.

Classification of UPS

Depending on the feature UPS system are classified as:

I.                On-Line or inverter preferred UPS system.

II.              Off-Line or line preferred UPS system.

III.            Line-interactive UPS system.

All of these UPS system configuration have been discussed in the below sections.

On-Line UPS (Inverter-preferred)

 

Block diagram of On-line UPS

Operation of On-Line UPS system

Shows the block diagram of the On-Line UPS systems. On this mode of operation the load always connected to the inverter though the UPS static switch. The UPS static switch is a normally ‘ON’ switch. It turns off only when the UPS system is fails. In that case the ‘main static off ‘switch and used only when UPS is to be bypassed. The various operation modes of on-line ups system are:

i.                 Mode I: when the AC main is ON, the rectifier will supply the power to the inverter as well as battery bank. Therefore, it acts as a rectifier cum charger. The inverter output connected to the load via ups static switch. Battery will be charged in this mode.

ii.               Mode II: If the supply power fails suddenly, the rectifier will be zero and hence the battery-bank now supplies power to the inverter without any interruption and delay. There will not be any changed in inverter as well as the load.

After restoration of the line supply, the charger supplies the inverter and recharge the battery automatically, first in constant current mode then in constant potential mode. various rates of battery recharge may be set depending upon the application. The inverter has to designed carefully because it supplies the load continuously. There should not be any frequent failure of the inverter system,

iii.             Mode III: in case if the UPS fails, the normally OFF mains static switch is turned ON which automatically transfers the ac line to the load in is than ¼ cycle period without any phase discontinuity. This is not only maintain the power to load but also actuates the failure alarm to dram attention of the attendant.

This type of ups is more popular because it can provide full isolation of critical load from ac line and also provide power conditioning. Also mode of operation does not change during failure to power. Its changeover time is very low and there is no interruption during transfer from line to battery and vice versa.

This type of ups system protects the critical load against spikes, voltage variations, frequency, surges and brownouts which re not available in the offline ups system.

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