Line Interactive UPS Systems Tutorial

Line Interactive UPS Systems

UPS - An Introduction:
A UPS is a device that maintains a continuous electric power supply to the connected equipment(load) by supplying power from a separate source when the main power supply is not available.
Click here to know more about basics and classification of UPS
Static UPS systems can be classified into three main categories:

[1] Online UPS system
[2] Offline UPS system
[3] Line Interactive UPS system

This post will discuss about the Line Interactive Uninterruptible Power Supply (UPS) only.
The Online UPS is explained in separate post. Click here to know more about the Online UPS.

The following figure illustrates the Line Interactive UPS configuration.

  • It is considered as a midway between the online and the off-line configurations.
  • It consists of a single bidirectional converter that connects the batteries to the load. 
  • Line interactive UPS units typically rate between 0.5 and 5 kVA for small server systems.
  • The efficiency of the line interactive UPS is about 97% when there are no problems in the line.
  • Under normal operation, the main power supply provides the supply to the load, and the batteries can be charged through the bidirectional inverter,(ie, it acts as a DC/AC converter or charger).
  • It may also have active power filtering capabilities.
  • When a failure occurs in the main power supply, the static switch disconnects the load from the line and the bidirectional converter acts as an inverter, which supplies energy from the batteries.
  • The main advantages of the line interactive UPS are the simplicity and the lower cost in comparison to the Online & Offline UPS.
  • Line Interactive UPS typically incorporate an Automatic Voltage Regulator (AVR). The AVR allows the UPS to effectively step up or step down the incoming line voltage without switching to battery power.
  • So this type of UPS is able to correct most long-term overvoltages or undervoltages without draining the batteries.
  • Another advantage is that it reduces the number of transfers to battery. So the lifetime of the batteries are extended.
  • The disadvantage of the line interactive ups is that under normal operation it is not possible to regulate output voltage and frequency.

Features, Advantages of Line Interactive UPS System:

  • A line interactive UPS system has all the features of off-line UPS systems. In addition to that it has buck and boost feature. ie, it allows the load to be supplied from the mains by providing a low and high voltage mode of operation.
  • Without this feature, the UPS would often be running on battery, and so when a full power failure occurred, the battery would be partially discharged, and the autonomy time may be insufficient for a proper shutdown. 
  • The line interactive UPS systems are perfectly suitable for the situations where the supply is prone to sags and surges. 
  • Both line interactive UPS and Off line UPS systems normally draw high crest factor currents from the utility. It is because of the fact that the load is either connected directly to the mains(Offline UPS) or connected to the mains through the transformer(Line interactive UPS).
  • In contrast to this, the Online UPS systems normally draw low crest factor currents from the utility. It is because of the fact that this system contains Power Factor Corrector (PFC) which has the capability to draw undistorted (low crest factor) currents which minimize the stress to the power grid.

Related Posts:
Uninterruptible Power Supply : Introduction, Basics, Classification
Comparison of Online and Offline Uninterruptible Power Supply

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