Online UPS Systems Tutorial

Online UPS Systems

This post will explain about the Online UPS systems. The Uninterruptible Power Supply Systems are generally classified as

  1. Static UPS systems
  2. Rotary (or dynamic) UPS systems
  3. Hybrid UPS systems

The Static UPS systems can be classified into three main categories:

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

The following figure depicts the configuration of an online UPS system. It is also known as “double conversion UPS system”.


Online UPS System Components:

  • As shown in the above figure, the major elements of online UPS are (i) Rectifier and Battery charger (ii) Inverter (iii) Static transfer switch (iv) Logic and control system (v) Battery bank (vi) Diagnostics and communication blocks.
  • In medium & large capacity online UPS systems good amount of power semiconductors like Power MOSFETs, GTOs, MCT, Darlington pairs are used.
  • As they are handling high voltage and current, they are mounted on speicially designed heatsinks. They play as a control elements in the rectifier blocks and the inverter.

Operation of Online UPS System:

  • During normal or even abnormal line conditions, the inverter supplies energy from the mains through the rectifier, which charges the batteries continuously. In addition to that it can also provide power factor correction.
  • When the line fails, the inverter still supplies energy to the loads from the batteries.
  • As a consequence, no transfer time exists during the transition from normal to stored energy modes.
  • In general, Online UPS system is the most reliable UPS configuration due to its simplicity (only three elements), and the continuous charge of the batteries, which means that they are always ready for the next power outage.
  • This kind of UPS provides total independence between input and output voltage amplitude and frequency. So high output voltage quality can be obtained.
  • When an overload occurs, the bypass switch connects the load directly to the utility mains, in order to guarantee the continuous supply of the load, thereby avoiding the damage to the UPS module (bypass operation).
  • In this situation, the output voltage must be synchronized with the utility phase, otherwise the bypass operation will not be allowed.
  • Typical efficiency of the online ups systems are up to 94%, which is limited due to the double conversion effect.
  • Online UPS systems  are typically used in environments with sensitive equipment or environments.
  • Almost all commercial UPS units of 5 kVA and above are Online UPS Systems.

Features of Online UPS:

  • A well designed online UPS protects against blackouts, surges, sags, spikes, transients, noise and brownouts.
  • Previously the price of on-line ups systems is more than other ups systems. Now a days the price reduces drastically.
  • Since they continuously regenerate clean AC power, they provide the highest level of protection available, regardless fo utility line condition.
  • These kind of UPS have much more complex designs than off-line or hybrid types, while the price, weight and volume are higher. 
  • The inverter of an online ups system supplies continuous power to the critical load. Under conditions of overload or loads with high in rush currents which is beyond the capacity of the inverter, the static bypass switch provides mains power to the load.

Advantages of On-line UPS System:

  • No switching involved
  • 100 percent line conditioning and regulation
  • Good sustained brownout protection
  • Typically sinusoidal output
  • Power factor correction and higher reliability

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

Thanks for reading about online ups system….

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3 Responses

  1. Prashant Kshirsagar says:

    Great article written. Indeed a helpful

  2. Rahul Menon says:

    Very helpful article. Keep up the good work.

  3. rajkumar says:

    very use full thanks

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