• Preface
  Preface
  
• Contents
  Contents
  
• Introduction
  Introduction
  
• Electrical Basics
  Voltage, Current and RMS
  
  Impedance
  
  Power
  
• Types of UPS
  Types of UPS
  
  Online Double Conversion UPS Systems
  
  Line Interactive UPS Systems
  
  Offline UPS Systems
  
  Sinewave Systems
  
  Squarewave Systems
  
  UPS Operation Demo
  
  EN62040-3 UPS Classification
  
• Types of Power Problems
  Types of Power Problems
  
  Black Outs
  
  Brown Outs
  
  White Outs
  
  Transients and Surges
  
• More Electrics
  Power Factor
  

In a Line interactive UPS the inverter is not running at all times, unlike the online double conversion systems. Instead, when the mains fails (or goes out of tolerance) the UPS quickly switches from providing power to the load from the mains, to running from it's own inverter (powered by the batteries).

There is a time delay when the load is not powered. This is called the transfer time and is typically very short (about 5ms). The vast majority of electrical equipment will not notice this transition.

When the inverter starts up this will be either square wave or sinewave depending upon the quality of the UPS system.

The basic elements of a line interactive system can be seen in the following diagram. Notice the action when you fail the mains supply, or the UPS itself. Note that unlike online double conversion there is no bypass and therefore a UPS failure will drop the load.

What sets line interactive units apart from the standby (offline) systems is the fact that they also perform some voltage regulation. This allows them to boost the incoming mains if the power drops too low, or to reduce (buck) if the voltage becomes too high. This is done without reverting to battery. This provides an added power protection feature and is known as Automatic Voltage Regulation or AVR. This is not to be confused with high tolerance voltage regulation as typically the output voltage will be within around 10% for an input deviation of around 30%. Tight tolerance systems usually offer regulation of 1% or better.

Because of this, the UPS voltage output is independent of the voltage input and such systems are termed Voltage Independent (VI) Systems.

The advantages of line interactive systems are:

• Efficiency. As the UPS is not running on inverter, the system is inherently efficient. Typically around 98%
• Quiet. Again, as the inverter is not powered there is no heat generated during normal use, and therefore no need for forced cooling (fans) making the system suitable for office and home environments.
• AVR. This allows a wider input voltage without the need to drop to battery thereby improving battery life and maintaining power