Offline UPS Systems Tutorial

Offline UPS Systems

UPS Introduction:

The "Uninterruptable Power Supply" is also called as Standby Power Supply, since they stand by whilst the main power supply is working.
These power supplies usually consists of inverters operating from DC storage source (like battery).
In many applications like major computer installations, process control in chemical plants, hospital intensive care units continuous power supply is highly essential.
In these applications, temporary power failure can cause a great inconvenience and economic losses.  The UPS plays an important role in these situations.

There are various types of Uninterruptible Power Supplies available. Click here to know more about the Basics and Various types of UPS system. This post will explain about the Offline UPS system only. 

The following figure shows the configuration of an offline UPS system. It is also known as Line-Preferred UPS or Passive Standby. The Offline UPS System is the simplest form of back up power systems.

Offline UPS System Operation:

  • The Offline UPS systems normally operate off-line and the load is normally powered by the utility line.
  • When the utility power levels goes beyond acceptable limits, or fails, the load is transferred from the utility line to the UPS. 
  • The actual transfer time is very fast, but the detection time somehow long and therefore the offline UPS is not as reliable as an online UPS.

Advantages, Disadvantages of Offline UPS System:

  • The major advantage of the off-line UPS systems is lower cost, smaller size and weight and higher efficiency. 
  • It has higher efficiency because most of the time the UPS system is offline and the load is powered by the utility.
  • The disadvantage of the Offline UPS system is that switching to the inverter is required when the load is most vulnerable (ie, upon failure of the normal power source)
  • These kind of systems provide no line conditioning or voltage regulation and provide only limited surge and spike protection. During the sustained low voltage periods(brownouts) the Offline UPS can inaccurately detect a blackout and prematurely switch onto battery. 

Working of Offline UPS System:

  • The Offline UPS System consists of a Battery set, charger, and Power Semiconductor Switch. The switch connects the main power supply to the load and to the batteries. Thus the battery set remain charged during normal operation.
  • But when the main power supply fails to provide required power (or) if it meets abnormal function, then the static switch connects the load to the inverter in order to supply the energy from the batteries (stored energy operation).
  • The transfer time from the normal operation to the stored energy operation is generally less than 10 ms. This short time interval does not affect typical computer loads.
  • With this configuration, the UPS simply transfers utility power through to the load when either a power failure, sag, or spike occurs; at the same time, the UPS switches the load onto battery power and disconnects the utility power until it returns to an acceptable level.
  • Offline UPS systems are commonly rated at 600 VA for small personal computers and home applications.

Read Related Topics:
Online UPS Systems Tutorial
Line Interactive UPS Systems Tutorial
Uninterruptible Power Supply : Introduction, Basics, Classification
Comparison of Online and Offline Uninterruptible Power Supply

Do you know about Capacitance?

Every system of electrical conductors possesses capacitance. For example, there is capacitance between the conductors of overhead transmission lines and also between the wires of a telephone cable. In these examples, the capacitance is undesirable but has to be accepted, minimized or compensated for. There are other situations where capacitance is a desirable property.

Devices specially constructed to possess capacitance are called capacitors (also known as condensers). The simplest form a capacitor consists of two plates that are separated by an insulating material known as a dielectric . A capacitor has the ability to store a quantity of static electricity.

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