Uninterruptible Power Supply (UPS) - Basics, Types

Uninterruptible Power Supply (UPS) - Introduction:   

Definition of UPS:
A Uninterruptible Power Supply [UPS] is a device that maintains a continuous supply of electric power to the equipment by supplying power from a separate source when the main power supply is not available. The UPS is normally inserted between the commercial utility mains and the critical loads. When a power failure occurs, the UPS will immediately switch from utility power to its own power source.

The Uninterruptible Power Supply (UPS) is used in applications where loss of the mains supply could be disastrous, like hospital operating theatres or intensive care units.


Types of UPS:
UPS systems are generally classified as

  1. Static UPS systems, which use power electronic converters with semiconductor devices
  2. Rotary (or dynamic) UPS systems, which use electromechanical engines such as motors and generators
  3. The combination of both static and rotary UPS systems is often called Hybrid UPS systems

Static UPS Systems:
Static Uninterruptible Power Supply systems are based on power electronic devices. The continuous development of devices like IGBTs allows high frequency operation, which results in a fast transient response and low total harmonic distortion (THD) in the output voltage.

Solid state (Static) UPS system consists of several major elements like Rectifier& Battery charger, inverter, static transfer switch, battery bank.

All UPS systems have at least one large, low-frequency, magnetic component, usually a transformer. In early days these transformers are large and heavy. Now a days smaller size, less expensive high frequency magnetics are used. The main advantages of higher frequencies are that acoustic noise can be reduced, and flicker components become smaller.

All UPS systems use an internal battery that produces AC power via an inverter. How and when this inverter comes into play largely determines the effectiveness of the UPS.
According to the international standards, Static Uninterruptible Power Supply systems can be classified into three main categories:

  1. Off-line UPS
  2. On-line UPS
  3. Line Interactive UPS

                 In this post, we are going to see above mentioned three types of Static Uninterruptible Power Supply Systems in brief.

Off-line UPS:
Offline UPS

  • In the case of the off-line system, in normal operation power is supplied directly from ac mains. 
  • In the event of mains failure, a transfer switch disconnects the power line and connects the inverter to the load. 
  • When the mains power is restored, the load is reconnected to the power line.

 On-line UPS:

  • In the case of on-line systems, the rectifier-inverter combination supplies the load power from the ac mains during normal operation.
  •  In the event of mains failure, the battery automatically supplies the dc link to the inverter and there is no time delay involved. 
  • When the rectifier-inverter system fail, the load could be transferred to ac mains using a transfer switch.
  • The Online Uninterruptible Power Supply is discussed in separate post. Click here to know more about Online UPS System.

Line Interactive UPS:

  • In line interactive UPS system, When the mains supply is present, the static switch is ON. The static switch connects load to mains supply through inductor L.
  • The batteries are charged through the charger block.
  • When main power supply is turned off, the mains static switch is open. Consequently the inverter turns ON and provides power to the load.
  • When the main power is available, the Charger/Inverter block operates as a charger and when the main supply is not available it works as an Inverter.

Read More:
Comparison between Online and Offline Uninterruptible Power Supply (UPS)

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11 Responses

  1. Shiva says:

    Way of your explanation is super and simple. It is easy to understand. Thank You

  2. mishra says:

    Easy to understand

  3. Dear Friend, totally your contents and expressions are quite understandable and easy to guage.Thanks a lot. says:

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  4. Sajith says:

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  5. bhavani says:

    its very useful and easily understandable.thank u!

  6. Siddu says:

    Easy way understanding

  7. Sharan says:

    Thank you

  8. viji says:

    For a computer lab, which type of UPS is more suitable- SMPS powered or isolation transformer type

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